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Pure and Applied Chemistry

The Scientific Journal of IUPAC

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Volume 88, Issue 8


Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)

John H. Duffus
  • The Edinburgh Centre for Toxicology, 43 Mansionhouse Road, Edinburgh, Scotland EH9 2JD, United Kingdom of Great Britain and Northern Ireland
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Michael Schwenk / Douglas M. Templeton
  • Corresponding author
  • Department of Laboratory Medicine and Pathobiology, University of Toronto, 1 King’s College Circle, Toronto, Ont. M5S 1A8, Canada
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  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
Published Online: 2016-10-19 | DOI: https://doi.org/10.1515/pac-2015-1202


The primary objective of this glossary is to give clear definitions for those who contribute to studies relevant to these disciplines, or who must interpret them, but are not themselves reproductive physiologists or physicians. This applies especially to chemists who need to understand the literature of reproductive and teratogenic effects of substances without recourse to a multiplicity of other glossaries or dictionaries. The glossary includes terms related to basic and clinical reproductive biology and teratogenesis, insofar as they are necessary for a self-contained document, particularly terms related to diagnosing, measuring, and understanding the effects of substances on the embryo, the fetus, and on the male and female reproductive systems. The glossary consists of about 1200 primary alphabetical entries and includes Annexes of common abbreviations and examples of chemicals with known effects on human reproduction and development. The authors hope that toxicologists, pharmacologists, medical practitioners, risk assessors, and regulatory authorities are among the groups who will find this glossary helpful, in addition to chemists. In particular, the glossary should facilitate the worldwide use of chemical terminology in relation to occupational and environmental risk assessment.

Keywords: birth defects; chemical toxicology; developmental biology; reproductive health; teratogenesis; terminology

Article note:

This manuscript was prepared in the framework of IUPAC project 2013-001-2-700.


A major goal of IUPAC is to promote “regulation, standardization, or codification” globally in relevant areas of chemistry. To this end, the Division of Chemistry and Human Health (Division VII), recognizing the importance of toxicology to chemists, produced the Glossary of Terms Used in Toxicology, 2nd ed., in 2007 [1]. That glossary was intended to provide clear and concise definitions for a range of terms in toxicology and toxicokinetics, primarily for chemists who find themselves working in toxicology or requiring a working knowledge of the subject. It was also recognized that other scientists, regulators, and managers must, from time to time, interpret toxicological information, and it was hoped that the glossary would also provide them with ready access to internationally accepted definitions of relevant terms. A number of subspecialties have broadened the scope of toxicology; the Division expanded the collection of available definitions in 2009 with the publication of a Glossary of Terms Used in Ecotoxicology [2] and again in 2012 with a Glossary of Terms Used in Immunotoxicology [3]. A Glossary of Terms in Neurotoxicology [4] has special significance with respect to the present compilation, insofar as the developing nervous system is particularly susceptible to toxic substances.

Scientific terminology continues to evolve and definitions need periodic refinement. A searchable, electronic database updating and combining entries from the previous glossaries is both desirable and achievable, and a project to realize this is underway, but at its inception we realized that some areas of toxicology have been under-represented. Addressing this deficit will enhance the usefulness of the database. One area that has been under-represented in previous IUPAC glossaries is reproductive and developmental toxicology. The present document is an attempt to address this deficit. Intended to stand alone as an IUPAC Recommendation in the narrower field, it is also destined for integration into the revised, online Glossary of Terms Used in Toxicology, currently under construction.

In order to minimize the reader’s time in consulting additional texts, terms from the previously mentioned glossaries [1], [2], [3], [4] are included in the present text when it is felt that they are used with particular frequency in reproductive and developmental toxicology. By the nature of the subject, it is necessary to include a number of clinical and anatomical terms. We have also exercised judgment in deciding which terms from basic developmental biology should be included for the reader’s convenience. In the spirit of producing a document primarily useful for chemists and allied professionals, we have tried not to be over-inclusive in this regard, only including terms that may be encountered with reasonable frequency in the literature of reproductive and developmental toxicology. When a medical term is defined, we have tried to provide a brief, useful definition that is nevertheless accurate in terms of current medical understanding. A number of syndromes caused by gene mutations have been included, although the list is necessarily selective and by no means exhaustive. In many instances there is no evidence of these being the direct result of toxic or teratogenic effects, and the guiding principle has been to include more commonly mentioned syndromes (even when their actual occurrence may be rare) as examples of potentially teratogenic outcomes.

In general, commonly preferred or American spelling has been adopted for the main entry terms; thus, for example, disc (not disk), fiber (not fibre), masculinization (not masculinisation), and tumor (not tumour). Further, somewhat arbitrary decisions must be made in listing alternative forms of terms as the main entry (e.g., haploid instead of monoploid, semen instead of seminal fluid, and undescended testis instead of cryptorchism). We have generally tried to use the form we find to be in most common usage and cross-reference the lesser-used term if it also seems common: if a desired entry is not found under one construction, it should be sought under another.

Some definitions have been compiled from earlier sources, with or without modification, as indicated in the citation. When no citation is given, the term is newly defined. When a citation is given, the definition is more or less a quotation from the original. With the qualification “After [ref.]”, the general concept of the original has been retained with some rewording, often for consistency with IUPAC guidelines for glossaries. “Modified from” implies that a concept specific to the source is retained but given revised wording. When a citation is indented following a Note, it refers only to the Note.

This document has been put together with invaluable input from many colleagues and expert reviewers. Where flaws remain, they are the responsibility of the authors.


Early versions of this glossary were advanced with helpful comments from Dr. Raymond York (Manlius, NY), Dr. Alan Hoberman (Horsham, PA), Dr. Stephen Harris (San Diego, CA), Dr. Robert Parker (East Millstone, NJ), and Prof. Peter Wells (University of Toronto). Final versions were given helpful scrutiny by Prof. Rita Cornelis (University of Gent) and Prof. Mike Wiley (University of Toronto), as well as a number of anonymous reviewers to whom we are equally grateful.

Alphabetical entries

A/D ratio

Ratio of the adult toxic dose to the developmentally toxic dose.

abdominal cavity

Body cavity between the diaphragm and pelvis that contains the abdominal organs.

ablepharia (n)/ablepharous (adj)

Congenital absence of the eyelids.


Premature termination of pregnancy with the death of the embryo or fetus.

abortion, induced

Intentional termination of a pregnancy with death of the embryo or fetus.

abortion, spontaneous


Non-intentional termination of pregnancy before the embryo or fetus has developed to the stage of independent viability, or in humans before the 20th week of gestation.

accessory rib

Rib arising from a cervical vertebra (cervical rib), or supernumerary rib arising from a thoracic or lumbar vertebra.

accessory sex gland

Any gland, other than a gonad, associated with the genital tract, such as the bulbourethral gland and prostate.

accessory sex organ

secondary sex organ

Organ or structure other than the gonads that matures at puberty and assists indirectly in sexual reproduction by nurturing and transporting gametes.

  • Note 1:

    In the human female the accessory sex organs include the Fallopian tubes, uterus, vagina, and the external genitalia.

  • Note 2:

    In the human male, the accessory sex organs include the epididymis, vas deferens, ejaculatory duct, urethra, seminal vesicles, bulbourethral glands, prostate, and penis.


Hydrolytic enzyme (EC that acts on ganglioside GM2, producing N-acetyl-D-galactosamine and ganglioside GM3.

  • Note:

    Deficiency of this enzyme is associated with Tay-Sachs disease.

achondroplasia (n)/achondroplastic (adj)

Inherited disorder where ossification of cartilage is retarded, especially affecting growth of long bones, resulting in very short limbs and a comparatively large head. Type of dwarfism.

  • Note:

    Achondroplasia results from a mutation in the fibroblast growth factor receptor FGFR3 gene, increasing its activity in suppressing endochondral ossification.


Opposite term: alkalosis

Abnormal increase in hydronium ion activity (decrease in pH below the reference interval measured in arterial blood) usually caused either by an accumulation of carbon dioxide or acidic metabolites, or by a depletion of alkaline reserve (i.e., bicarbonate).

  • Note 1:

    In humans, the blood pH is tightly regulated within a range of 7.35 to 7.45.

  • Note 2:

    Acidosis may occur as a result of the accumulation of ketones in uncontrolled diabetes mellitus (diabetic acidosis) or after calorie deprivation (starvation acidosis), or with accumulation of keto-acids at the expense of bicarbonate (metabolic acidosis). Suppression of respiration can produce a respiratory acidosis.

  • Note 3:

    Some intoxications can produce metabolic and (or) respiratory acidosis.


  1. Small sac-like cavity in a gland, surrounded by secretory cells.

  2. Terminal region of the airways of the lung where gas exchange occurs.

acoustic meatus

Either of two passages in the ear, one leading to the tympanic membrane (external acoustic meatus), and one for passage of nerves and blood vessels (internal acoustic meatus).



Rare congenital disorder that occurs in the human fetus, in which the flat bones in the cranium are either completely or partially absent.




Type of cephalic disorder where the top of the skull is pointed or conical due to premature closure of the coronal suture plus any other suture.

  • Note:

    Acrocephaly should be differentiated from Crouzon syndrome, which involves the maxilla and mandible.



Abnormal enlargement of the hands, feet, and face in adults, caused by overproduction of growth hormone by the pituitary gland.

  • Note:

    Overproduction of growth hormone by the pituitary gland in children causes gigantism.


Organelle that develops over the anterior half of the head in the spermatozoon.

  • Note:

    The acrosome is a cap-like structure derived from the Golgi apparatus.

acrosome reaction

Process that occurs in the acrosome of the sperm as it contacts the egg, leading to structural changes that facilitate fusion.


Growth factor of the transforming growth factor β (TGF-β) superfamily, originally identified as a gonadal factor that stimulated secretion of follicle stimulating hormone, also involved in many aspects of development including mesodermal induction (see mesenchyme), hematopoiesis, and neuronal differentiation.


Irregular or absent estrous cycles.

Compare anestrus.


See adrenal 4 binding protein.




Congenital absence of fingers or toes.


Malignant tumor formed from glandular epithelial tissue or formed in a glandular pattern.


Congenital absence of skin.

adhesion factor

Substance contributing to selective cell-cell and cell-matrix binding.


Entrance or opening to some interior space or cavity.

adolescence (n)/adolescent (adj)

Stage of human development beginning with puberty and ending with adulthood.

adrenal 4 binding protein (Ad4BP)

Transcription factor that regulates the expression of the enzymes of steroid synthesis and is expressed primarily in steroidogenic cells.

adrenal gland

paranephric gland

suprarenal gland

Either of two small endocrine glands, one located above each kidney, consisting of a cortex, which secretes several steroid hormones, and a medulla, which secretes adrenaline and noradrenaline.




Catecholamine hormone secreted by the adrenal glands that increases heart rate, breathing rate, blood pressure, and carbohydrate metabolism.

See also noradrenaline.

adrenocortocotropic hormone (ACTH)

Hormone secreted by the pituitary gland and stimulating the adrenal cortex (see adrenal gland).

adrenogenital syndrome

See congenital adrenal hyperplasia.


Person or animal that is fully grown, developed and sexually mature.


State of being adult.

adult stem cell

See stem cell.

aganglionic megacolon, congenital

See Hirschsprung disease.


Absence or partial development of an organ or body part observed at birth.

age-specific birth rate

age-specific fecundity

age-specific fertility rate

Mean number of offspring born to a female in a specific age class in a given year, expressed per 1000 females in that age class.


aggregation chimera

Organism made by combining cells from two embryos of different genotypes.

After [6]


Congenital absence orpartial absence of the lowerjaw.

See also macrognathia, otocephaly, synotia.


Opposite term: antagonist

Substance, naturally occurring or otherwise, that binds to cell receptors that normally respond to a naturally occurring substance, and producing an effect similar to the natural substance.

  • Note 1:

    A partial agonist activates a receptor but does not cause as much of a physiological change as does a full agonist.

  • Note 2:

    A co-agonist works together with other co-agonists to produce a desired effect.

After [1]

alar plate

Part of the dorsal side of neural tube in the embryo, involved in general somatic and visceral sensory communication.




Congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes, due to an absence of or defect in tyrosinase, a copper-containing enzyme involved in the production of melanin.

Compare leucism.

alcohol-related neurodevelopmental disabilities (ARND)

Spectrum of functional neurologic (behavioral) defects resulting from exposure in utero to alcohol.

See also fetal alcohol syndrome.


Opposite term: acidosis

Abnormal decrease in hydronium ion activity (increase in pH above the reference interval of 7.35 to 7.45 in humans) measured in the arterial blood.

  • Note:

    Common causes of alkalosis include a decrease in CO2, deficiency of chloride, and decreased bicarbonate.

allantois (n)/allantoic (adj)

Extra-embryonic membrane formed early in development as an outpouching of the yolk sac into the area of the future umbilical cord.

  • Note:

    Blood vessels of the allantois become the umbilical artery and umbilical veins.


One of several alternate forms of a gene that occur at the same relative position (locus) on homologous chromosomes.

  • Note:

    Paired alleles become separated during meiosis and can be recombined following fusion of gametes.



Hairlessness; absence or thinning of hair from areas of skin where it is usually present.


alopecia, neonatal

See neonatal occipital alopecia.

alpha-fetoprotein (AFP)


Protein coded by the AFP gene and produced by fetal tissues, an abnormally high amount of which in the amniotic fluid or maternal serum may indicate a neural tube defect, or some other loss of structural integrity in the fetus.


See 5α-reductase.

alveolar period

alveolar phase

Phase in lung development beginning in utero (about 32 to 36 weeks in the human fetus) and lasting until about 8 years of age.

  • Note:

    In this phase, the terminal alveolar saccules subdivide several more times, giving rise to the mature alveoli.


Any of the many terminal air sacs of the airways in the lungs necessary for rapid gaseous exchange with the blood.


Congenital absence of one or both mammary glands.



  1. Pertaining to or characterized by hermaphroditism.

  2. Denoting sexual characteristics common to both sexes, e.g., pubic hair.


Lacking one or more limbs or having a shrunken or deformed limb as a birth defect.

See also phocomelia.


Epithelial cell that deposits enamel during tooth development.

amelogenesis imperfecta

hereditary yellow, brown, or grey tooth enamel

Developmental disorder of the teeth in which they are covered with thin, abnormal enamel resulting from defective structure or processing of enamel proteins.



Absence or abnormal stoppage of menstruation.

Ames test

Method for assessing mutagenicity performed in vitro using mutant (see mutation) strains of the bacterium Salmonella typhimurium that cannot grow in a given histidine-deficient medium

  • Note 1:

    Mutagens cause reverse mutations that enable the bacterium to grow on the deficient medium.

  • Note 2:

    The test can be carried out in the presence of differentially centrifuged liver homogenates, providing enzymes that catalyze the metabolic transformation of mutagen precursors to active derivatives.



Transabdominal procedure in which amniotic fluid is sampled by means of a needle inserted into the amniotic sac.

  • Note:

    This procedure is used to screen for infections and abnormalities in the developing fetus.

amniochorionic membrane

Composite membrane, formed by fusion of the amnion, interiorly, with the outer chorion that surrounds the developing fetus.


Membrane that lines the amniotic cavity (amniotic sac) and encloses the embryo of a mammal, bird, or reptile.


Any vertebrate animal, such as a reptile, bird, or mammal, that possesses an amnion, chorion, and allantois during embryonic development.

Compare anamniote.

amniotic cavity

Fluid-filled space that surrounds the developing embryo of a mammal, bird, or reptile, bounded by the amniotic sac.

amniotic fluid

Fluid surrounding an embryo or fetus in the amniotic cavity.

amniotic sac

Membranous structure in amniotes, within which the embryo and fetus develop.

  • Note:

    It consists of a thin but tough, transparent pair of membranes, the amnion and chorion, that hold the developing embryo (and later fetus) until shortly before birth.

amphibian metamorphosis assay

Procedure in which a frog (often Xenopus laevis) is exposed to a substance starting at the tadpole stage, and the growth and development of the animals is studied.

See also FETAX.


Flask-like dilatation of a canal or duct.

anal canal

Terminal part of the large intestine between the rectum and anus.

  • Note:

    The anal canal arises as the dorsal cavity of the cloaca after division of the cloaca by the urorectal septum, the ventral cavity becoming the urogenital sinus.

See also anal membrane.

anal membrane

In the embryo, dorsal part of the cloacal membrane after its division of the urorectal septum.


anal pit

See proctodeum.


Any vertebrate animal, such as a fish or amphibian, that lacks an amnion, chorion, and allantois during embryonic development.

Compare amniote.


Stage of mitosis and meiosis in which the chromosomes move from the equatorial plate toward opposite ends ofthe nuclear spindle.

anaphase lag

Slowing or stopping of normal migration of chromosomes during anaphase, resulting in chromosomes being excluded from one of the daughter cells causing aneuploidy.




Generalized edema in the subcutaneous connective tissue.

  • Note:

    Anasarca may be congenital (whole body edema) caused by liver failure, renal failure, right-sided heart failure, or severe malnutrition with resultant protein deficiency.


Connection creating continuity between two tubular body structures, e.g. blood vessels or loops of bowel.

  • Note:

    An anastomosis may be a surgical construction (as when the two cut ends are joined following resection of a loop of bowel), may result from trauma, or may occur as a natural anatomic feature, typically involving blood vessels (as in an arteriovenous anastomosis bypassing a capillary bed; see arteriovenous shunt).

anatomic position, fetal

Usual positioning of the developing fetus with the back curved forward, head bent forward, and limbs drawn in towards the body.

  • Note:

    This position arises from the natural positioning of the embryo as the germ layers develop.

anchoring villus

Chorionic villus that is attached to the region of the endometrium (see also decidua) that interacts with the trophoblast during development of the placenta.



Sertoli cell tumor

Rare benign tumor of the testis histologically resembling the fetal testis: the epithelium contains Sertoli cells that may produce estrogen and cause feminization.



Substance, such as a naturally occurring steroid hormone, that binds to androgen receptors to activate the male accessory sex organs and induce male secondary sexual characteristics.

See also androgenic.


Producing masculine characteristics.

See also androgen.

androgen receptor (AR)

Nuclear receptor that is activated by binding of testosterone or dihydrotestosterone.

See also androgen, androgenic.


Steroid hormone produced in the adrenal glands and gonads as a common precursor of male (testosterone) and female (estrogen) sex hormones.


Decrease in the number of erythrocytes or total hemoglobin in the blood that results in a decrease in the oxygen-carrying capacity of the blood, sometimes inducing pallor and fatigue.


  1. Fetus lacking all or most of the neural tissues of brain.

  2. Anencephaly (see below).

anencephaly (n)/anencephalic (adj)


Congenital absence of a major portion of the brain and its meninges, skull, and scalp that results from abnormal embryonic development.


Interval of sexual inactivity between two periods of estrus.

aneugen (n)/aneugenic (adj)

Agent inducing aneuploidy.


Referring to a cell or organism with missing or extra chromosomes or parts of chromosomes, and thus an abnormal number of chromosomes that is not an exact multiple of the haploid number.


See also euploid, ploidy.


State of being aneuploid.


Abnormal bulging of the wall of an artery or a chamber of the heart.

  • Note:

    An aneurysm may present a risk of hemorrhage if it ruptures.


vasoformative cell

  1. Cell taking part in blood vessel formation.

  2. Primordial mesenchymal tissue from which embryonic blood cells and vascular endothelium are differentiated.


angiogenesis (n)/angiogenic (adj)

Development of new blood vessels in the embryo or from pre-existing vessels; formation of capillary networks.

Compare arteriogenesis, vasculogenesis.

angiotensin converting enzyme (ACE)

Enzyme (EC that converts angiotensin I to angiotensin II, causing blood vessels to constrict thus increasing blood pressure.

  • Note:

    ACE inhibitors are used clinically to lower blood pressure.


Congenital oral anomaly, in which the lingual frenulum (the membrane connecting the underside of the tongue to the floor of the mouth) is unusually short and thick.

  • Note:

    As a result of this, mobility of the tongue tip is decreased.


Congenital absence of all primary or permanent teeth.

anogenital distance (AGD)

Distance along the perineum between the anus and the base of the vagina or penis, or in the fetus between the anus and the base of the genital tubercle.

  • Note 1:

    The distance is relatively longer in males than females. This in part relates to dihydrotestosterone levels, and abnormal distances may indicate birth defects, feminization in males, etc.

  • Note 2:

    In some species where gender is not obvious at birth, it can be used for a tentative determination of the sex of the neonate.

anomaly, developmental

congenital anomaly

Deviation in structure or function arising in the embryo or fetus, due to genetic or other causes.

See also congenital malformation, variation.


Absence of nails.

  • Note:

    This rare disorder may be the result of a congenital defect of ectoderm, ichthyosis, severe infection, severe allergic contact dermatitis, self-inflicted trauma, Raynaud phenomenon, lichen planus, epidermolysis bullosa, or severe exfoliative diseases.


Congenital absence of one or both eyeballs.


Congenital absence of one or both testes.


Relating to the anus and rectum.


Congenital absence of the (outer ear) pinna, often with narrowing or absence of the ear canal.


Failure of the ovaries to produce, to facilitate maturation of, or to release ova.


Total absence of dioxygen.

  • Note:

    Sometimes anoxia is incorrectly used instead of hypoxia to mean a decreased dioxygen supply to the tissues.

After [1]

antagonist (in toxicology)

Opposite term: agonist

  1. Any substance that competes for effect with, or blocks the biological action of, another.

  2. At a cell receptor, substance that binds to the receptor without activating it, and prevents a response to the natural ligand (the agonist).


Opposite term: postmortem.

Before death.


Before birth.

antidiuretic hormone

See vasopressin.


Referring to inhibition of cell division by mitosis.

anti-Müllerian hormone (AMH)

Müllerian inhibiting factor (MIF)

Müllerian-inhibiting hormone (MIH)

Müllerian-inhibiting substance (MIS).

Protein that inhibits the development of the Müllerian ducts (paramesonephric ducts) in the male embryo.

  • Note:

    The Müllerian ducts would otherwise differentiate into the uterus and Fallopian tubes.


Cavity or chamber, often in bone, with specific meanings in some hollow organs (e.g., the pyloric end or gastric antrum of the stomach).

anus (n)/anal (adj)

External opening of the rectum to the body surface, controlled by the anal sphincter.

See also imperforate anus.

aorta (n)/aortic (adj)

Great artery arising from the left ventricle, being the main trunk from which the systemic arterial system proceeds


aorta-gonad-mesonephros (AGM) region

Region of the vertebrate embryonic mesoderm that gives rise to the genitourinary tract and its blood supply.

  • Note:

    The AGM region is the first embryonic site for autonomous hematopoiesis and production of hematopoietic stem cells.

aortic arch

Curved portion of the aorta between its ascending portion exiting the heart and its descending portion that proceeds to the arteries of the thoracic and abdominal cavities and the lower body.

aortopulmonary septum

aorticopulmonary septum

Spiral septum that, during development, separates the truncus arteriosus into a ventral pulmonary trunk and the ascending aorta dorsally.

After [7]


Absence of the lens of the eye, occurring as a congenital defect or as a result of trauma or surgery.

  • Note:

    Aphakia causes a loss of visual accommodation, far sightedness (hyperopia), and a deep anterior chamber of the eye.

apoptosis (n)/apoptotic (adj)

Active process of programmed cell death, characterized by cell shrinkage, nuclear condensation, and fragmentation and loss of individual cells; usually involving activation of caspase enzymes and requiring energy provided by hydrolysis of ATP.

  • Note 1:

    Other factors trigger cell death with characteristics of apoptosis but independent of caspase activation, an example being release of apoptosis-inducing factor (AIF) from the mitochondrion. Here the term caspase-independent apoptosis is used.

  • Note 2:

    While ATP is generally necessary to sustain the apoptotic program, depletion of ATP during the course of apoptosis may cause cells to default to death by necrosis (see necrapoptosis, aponecrosis), or proceed to apoptotic death with features common to necrosis, sometimes called “late apoptosis”. Alternatively, in some circumstances apoptosis may proceed without ATP (“ATP-independent apoptosis”).

See also anoikis, autophagy, extrinsic pathway, intrinsic pathway, parthanatos.

appendicular skeleton

Bones and cartilage that support the appendages, including the bones of the shoulders, upper limbs, pelvis, and lower limbs.

See also axial skeleton.

appendix (in anatomy)

  • 3.

    Tube-shaped sac (“vermiform appendix”) attached to and opening into the upper end of the large intestine (cecum) in humans and some other mammals.

  • 4.

    Appendage, blind sac, or diverticulum.

See also epididymal appendix.


Extreme length and slenderness of the fingers or toes.

arachnoid mater

See arachnoid membrane.

arachnoid membrane

Weblike membrane that lies between the outer (and much thicker) dura mater and the deeper pia mater, and which covers the brain.

  • Note:

    The arachnoid membrane is separated from the pia mater by the subarachnoid space, in which the cerebrospinal fluid flows and is absorbed by the arachnoid villi.

arachnoid villus

arachnoid granulation

Small projection of the arachnoid membrane into some of the venous sinuses of the dura mater.

areola (n)/areolar (adj)

  1. A circular area of different color surrounding a central point, such as that surrounding the nipple of the breast, the part of the iris surrounding the pupil of the eye, or an area surrounding a vesicle.

  2. Any minute space or interstice in a tissue.

After [5]

arm bud

See limb bud.

Arnold-Chiari malformation

Chiari malformation

Congenital herniation of the brainstem and lower cerebellum through the foramen magnum into the cervical vertebral canal.

  • Note:

    This malformation is often associated with meningocele and spina bifida.


See also neural tube defect.


estrogen synthetase

Enzyme (EC of the cytochrome P450 superfamily that converts testosterone to 17β-estradiol and androstenedione to estrone.

  • Note:

    Inhibiting its action is an approach to the management of breast cancer.


See Sertoli-Leydig cell tumor


Increase in the diameter of arterial vessels that leads to the formation of large conductance arteries from pre-existing arterioles.

arteriovenous (AV) anastomosis

See anastomosis.

arteriovenous (AV) shunt

Connection between the arterial and venous sides of the circulation that bypasses the capillary beds.

artificial insemination

Introduction of semen into the cervix, uterus or Fallopian tubes by means other than the natural one.

See also insemination.

aryl hydrocarbon receptor (AHR)

Ligand-activated transcription factor involved in the regulation of biological responses to planar aromatic hydrocarbons.

  • Note 1:

    This receptor has been shown to regulate xenobiotic-metabolizing enzymes such as various forms of cytochrome P450 family 1 members.

  • Note 2:

    It also appears to play a role in cell proliferation and differentiation during vertebrate development, including hematopoiesis and development of the lymphoid and immune systems.

aryl hydrocarbon receptor nucleotide translocator protein (ARNT)

Protein coded for by a gene on chromosome 1q21 that forms a complex with ligand-bound aryl hydrocarbon receptor, resulting in translocation of the ligand-binding subunit to the nucleus.

  • Note:

    A t(1;12)(q21;p13) chromosomal translocation, which results in a translocated ETS leukemia (TEL)-ARNT fusion protein, is associated with acute myeloblastic leukemia.


  1. Total or partial collapse of the lung.

  2. Congenital condition characterized by the incomplete expansion of the lungs at birth.


Congenital absence of one or both nipples.


  1. Congenital absence of functioning thymus tissue.

  2. Absence of affect; suppressed emotion.



Congenital absence or abnormal narrowing of a normal opening or normally patent lumen.

atresia, follicular

Degeneration of those ovarian follicles that do not ovulate during the menstrual cycle.

atrial-septal defect (ASD)

Defect in the septum between the atria of the heart, due to failure of normal closure of the foramen ovale in the perinatal period.

atrichia congenita

Congenital baldness caused by an abnormality of the hairless gene.

  • Note:

    It may include loss of hair in early childhood, which never regrows.


Relating to the atrial and ventricular chambers of the heart.

atrioventricular bundle

See bundle of His


Each of the two upper chambers of the heart from which blood is passed to the ventricles.

  • Note:

    The right atrium receives deoxygenated blood from the veins of the body, the left atrium oxygenated blood from the pulmonary vein.


Wasting away of the body or of an organ or tissue, involving a decrease in size and (or) numbers of cells.


auditory tube

Eustachian tube

Narrow channel connecting the middle ear and the nasopharynx.



  1. See pinna of the ear.

  2. An appendage to the atrium of the heart.


Postmortem examination of the human organs and body tissue to determine cause of death or pathological condition.


Independent twin of a pair of conjoined twins; the other twin is aparasitic twin.

autosomal dominant mutation

Change in an autosomal gene capable of expression when carried by only one of a pair of homologous chromosomes.

autosomal recessive mutation

Change in an autosomal gene that produces an effect in the organism only when it is homozygous.

autosome (n)/autosomal (adj)

Any chromosome that is not a sex chromosome.

avascularity (n)/avascular (adj)

Absence of a blood supply.

axial skeleton

Bones of the body axis, including the skull, spinal column, ribs and sternum.

See also appendicular skeleton.


Absence of viable spermatozoa in the semen.

Barr body

See sex chromatin.

basal ganglia

basal nuclei

Complex structure at the base of the brain consisting of several groups of neurons, the caudate nucleus, the putamen, the globus pallidus, and the substantia nigra.

  • Note:

    These ganglia are involved in various functions, including voluntary motor movements and involuntary movements, such as tremors, bruxism (grinding the teeth), athetosis (involuntary writhing movements), and chorea (involuntary jerky movements).

basal lamina

See basement membrane.

basement membrane

basal lamina

Thin layer of connective tissue underlying an attached epithelial cell layer.

bell stage

Period in which the developing tooth takes on a bell shape in cross section, immediately preceding the “advanced bell stage”, in which the hard tissues (dentin and enamel) form the crown of the tooth.

bicornate uterus

Uterus that is divided into two lateral horns as a result of imperfect fusion of the paired embryonic tubes from which the uterus is formed.

  • Note:

    In humans it is a uterine malformation, but in some mammalian species, including rodents and pigs, it is normal.

bilaminar embryo

bilaminar blastoderm

Early blastula having only two of the three primary germ layers that it will ultimately have; the two layers present are the cells of the epiblast that will give rise to the ectoderm and those of the hypoblast that will become the endoderm during gastrulation – both attached to a basement membrane before the mesoderm has formed.


Start of life as a physically separate being, e.g., the emergence of a baby from the body of its mother, or the hatching of an egg.

birth defect

congenital defect

Physical or biochemical abnormality that is present at birth, and that may either be inherited or be the result of environmental influence.

Compare congenital malformation.


See ambisexual.


Mass of cells from which an organ or a body part develops, either in normal development or in the regeneration of a lost body part.

After [7]


cleavage cavity

segmentation cavity

Fluid-filled cavity in the blastula of a developing embryo.


Modified blastula that is characteristic of placental mammals.

  • Note:

    It has an outer cell layer, known as the trophoblast, which participates in the development of the placenta, and an inner mass of cells in the blastocoel, which develops into the embryo.


Any of the cells formed by cleavage of a fertilized egg.



blastodermic vesicle

Hollow mass of cells formed after a zygote has undergone approximately six cell divisions.

See also blastocoel, blastocyst.


Process by which the early embryo transforms from the morula into the blastula.

See also blastocyst.

blood-testis barrier

blood-seminiferous tubule barrier

Sertoli cell barrier (SCB).

Occluding barrier, formed by the Sertoli cells of the seminiferous tubules, that separates the more mature cells of spermatogenesis from blood-borne products.

  • Note:

    The name “blood-testis barrier” is misleading in that it is not a blood-organ barrier in a strict sense, but is formed between Sertoli cells of the seminiferous tubule and, as such, isolates the further developed stages of germ cells from the blood. A more correct term is the “Sertoli cell barrier” (SCB).

After [7]

body cavity

See coelom.

bone age

Average age at which children reach a given stage of bone maturation, denoting the stage of skeletal development of an individual fetus or child.

  • Note:

    A child’s current height and bone age can be used to predict adult height.

bone morphogenetic protein (BMP)

bone morphogenic protein

Any of a family of growth factors involved in bone and cartilage formation and, more generally, in orchestrating tissue architecture through morphogenetic signals.

  • Note:

    These proteins are also considered to be metabologens.

bone morphogenetic protein-4 (BMP4)

bone morphogenic protein-4 (BMP4)

Bone morphogenetic protein member of the transforming growth factor β (TGF-β) superfamily.

  • Note:

    Amongst the multiple functions of BMP4 is a role in early embryonic differentiation, where it is secreted from the notochord and acts with sonic hedgehog protein to establish a dorsal-ventral axis.

See also Spemann organizer.



Congenital abnormal shortness of fingers and toes.


Of, relating to, or resembling the gills of a fish or the homologous embryonic structures and their derivatives in higher animals.

branchial arch

gill arch

  1. In embryology, one of several arches, resembling the gill arches of a fish, found in the embryo of a higher vertebrate; these arches develop into structures of the ear and neck.

  2. In biology, one of several bony or cartilaginous arches located on either side of the pharynx and supporting the gills in fish and amphibians.

branchial cyst, congenital

branchiogenic cyst

Congenital cyst arising in the lateral aspect of the neck, from epithelial remnants of a pharyngeal groove.


Shortness of the neck.

bronchopulmonary segment

Largest subdivision of a lobe of the lung with its air supply from a major branch of the bronchus and having its own arterial blood supply.

buccopharyngeal membrane

oropharyngeal membrane

Membrane present in fetal life that separates the nasal cavities from the pharynx.


Small protuberance resembling the bud of a plant and having the potential for growth and differentiation.

bulb, olfactory

Region of the frontal lobe of the brain, receiving input from neurons of the nasal mucosa and involved in the sense of smell.

bulbourethral gland

Cowper’s gland

One of two small glands located on each side of, and inferior to, the prostate, draining to the urethra. Bulbourethral glands secrete a fluid component of the seminal fluid

bulbus cordis

Outflow tract of the embryonic heart between the primitive ventricle and the aorta.


bundle of His

atrioventricular bundle

Band of specialized cardiac muscle fibers connecting the atria with the ventricles of the heart.

  • Note:

    These muscle fibers conduct the electrical impulse that regulates the heartbeat from the right atrium to the ventricles.

After [5]


Padlike fluid-filled sac or sac-like cavity, especially one reducing friction at a joint.

Caesarian section

Caesarian delivery

Surgical operation for delivering a child by opening the mother’s abdominal wall and uterus.


In biology and medicine, a relatively narrow tubular passage or channel.


Disease resulting from the development of a malignant tumor.


capacitation (of sperm)

Sum of biochemical changes undergone by mammalian spermatozoa in the female genital tract that enables them to penetrate and fertilize (see fertilization) an egg.


caput epididymis

globus major

head of epididymis

Upper and larger extremity of the epididymis.

carcinogenesis (n)/carcinogen(et)ic (adj)

Induction, by chemical, physical, or biological agents, of malignant neoplasms and thus of cancer.



Pertaining to the heart.

cardiac jelly

Gelatinous substance, present between the endothelium and myocardium of the heart in early embryos, that develops into the connective tissue of the endocardium.


Development of the heart in the embryo.


  1. Originating in the heart; describing anything caused by normal or abnormal function of the heart.

  2. Pertaining to cardiogenesis.


Pertaining to the heart and blood vessels.

cartilage (n)/cartilaginous (adj)

Specialized, fibrous connective tissue present in adults, forming the temporary skeleton in the embryo, providing a model in which the bones develop, and constituting a part of the organism’s growth mechanism.


Removal of the testicles or ovaries.

See also sterilization.


Partial or complete opacity (clouding) of the lens of the eye.


Any one of a group of bioactive molecules that contains a catechol (1,2-dihydroxyphenyl) moiety and usually affects the sympathetic nervous system.

  • Note:

    The common catecholamines are dopamine, adrenaline, and noradrenaline.

cauda epididymis

globus minor

Tail of the epididymis that opens into the ductus deferens; part of the reservoir of spermatozoa.


Situated more toward the cauda, or tail, than some specified reference point; toward the inferior (in humans) or posterior (in animals) end of the body.


cavitation (in biology)

Formation of a cavity, as in formation of the amnion in mammalian development.


Pouch connected to the junction of the small and large intestines.

See also appendix, vermiform.

central nervous system (CNS)

Part of an animal’s nervous system that exerts control over the rest of the nervous system; in vertebrates, the brain and spinal cord protected within the dorsal body cavity (cranial and spinal cavities).


Constricted region of a chromosome that joins the two chromatids to each other and attaches to spindle fibers in mitosis and meiosis.

centromere, acrocentric

Having the centromere very close to one end.

cerebral hemisphere

Right or left half of the brain in sagittal section.

cerebral palsy

spastic paralysis

Condition marked by lack of muscle control, resulting from brain damage before, at, or shortly after birth.

cerebrospinal fluid (CSF)

Clear, colorless extracellular fluid that is found in the brain and spinal cord, filling the ventricles and subarachnoid spaces.

  • Note:

    The fluid acts as a cushion, providing mechanical and immunological protection to the brain, and plays an important role in the homeostasis and metabolism of the central nervous system.

cervical rib

costa cervicalis

Supernumerary rib arising from a cervical vertebra.


cervix uteri

uterine cervix

Narrow lower end of the uterus that opens into the vagina.

checkpoint pathway

Type of pathway used in intracellular signaling, activated in response to a cell’s own internal imbalance or to errors in its synthetic activities.

  • Note:

    Activation of checkpoint pathways leads to a delay in certain synthetic processes until other processes are complete, thereby averting damage.


See cleft lip.

Chernoff-Kavlock assay

Group of testing methods for assessing parturition, postnatal growth, and viability of prenatally exposed litters of test animals.

Chiari malformation

See Arnold-Chiari malformation.

chimera (n)/chimeric (adj)

  1. Animal consisting of genetically different cells derived from two (or more) different zygotes.

  2. Substance, such as an antibody, created from the proteins or genes of two different species.


Formation of cartilage; transformation into cartilage.


Cartilage-producing mesenchymal progenitor cell, capable of proliferating and maturing into a chondrocyte or osteoblast.

chondrocyte (n)/chondrocytic (adj)

Any one of the cells embedded in the lacunae of the cartilage matrix.

After [5]


Secreted protein that dorsalizes early vertebrate embryonic tissues by binding to ventralizing transforming growth factor β (TGF-β)-like bone morphogenetic proteins, sequestering them in latent complexes.

chorioallantoic placenta

Placenta developed from the allantois and chorion, establishing a nutritive and excretory connection between the blood of the fetus and that of the mother.


  1. In human embryology, the cellular, outermost extra-embryonic membrane, composed of trophoblast lined with mesoderm.

    • Note:

      The chorion develops villi, becomes vascularized by allantoic vessels, and forms the fetal part of the placenta.

  2. In mammalian embryology, the cellular, outer extra embryonic membrane, not necessarily developing villi.

  3. In biology, the noncellular membrane covering eggs of various animals, e.g., fish and insects.

After [5]

chorionic gonadotropin

Hormone secreted by the chorionic villi of the placenta in mammals, especially human chorionic gonadotropin.

  • Note 1:

    Chorionic gonadotropin promotes the secretion of progesterone by the corpus luteum.

  • Note 2:

    Human chorionic gonadotropin is the hormone that is detected by pregnancy tests.

chorionic sac

Outermost membranous sac that encloses the embryo in higher vertebrates (reptiles, birds, and mammals), lined by chorion.

chorionic villus

Any of the tiny extensions from the chorion that contain fetal blood vessels and combine with the uterine tissue to form the placenta.


Inflammation of the choroid layer behind the retina of the eye.

choroid plexus

Infoldings of blood vessels of the pia mater, projecting into the cerebral ventricles, covered with ependymal cells, and secreting cerebrospinal fluid.

chromaffin cell

Cell of the adrenal medulla that secretes epinephrine and norepinephrine, and contains granules that are readily stained with chromate salts.


One of a pair of chromosomes arising by duplication during mitosis or by pairing during meiosis, and joined together at the centromere.

  • Note:

    Sister chromatid refers to either one of the joined pair arising from the same chromosome by duplication, and non-sister chromatid refers to either of the joined homologous chromosomes of maternal or paternal origin arising during meiosis.

chromosomal abnormality

Abnormality in the number or structure of chromosomes.

chromosomal translocation

See translocation, chromosomal.


  1. Self-replicating structure consisting of DNA complexed with various proteins and involved in the storage and transmission of genetic information.

  2. Physical structure that contains the genes.

After [1]

chromosome, acrocentric

See centromere, acrocentric.

chromosome deletion

Loss of a chromosome or part of a chromosome.

chromosome ring

Abnormal chromosome in which both ends have been lost and the two broken ends have reunited to form a ring-shaped structure.

See also chromosome deletion.

clastogen (n)/clastogenic (adj)

Agent causing chromosome breakage and (or) the consequent gain, loss, or rearrangement of pieces of chromosomes.


cleavage (in embryology)

First few divisions of a fertilized egg.

  • Note:

    There is little or no growth during these divisions and the cytoplasm is cleaved into smaller and smaller units with individual biochemistry that contributes to subsequent cell differentiation.

See also blastocyst, blastomere, blastula, gastrulation, holoblastic, inner cell mass, meroblastic, trophoblast, and zygote.

cleavage, meroblastic

Partial cleavage of the egg, occurring in some animals, such as reptiles and birds, whose eggs have a large amount of yolk, resulting in unequal blastomeres and only part of the egg progressing to further cell division; the yolk mass remains and has nutritive value.

cleavage, holoblastic

Cleavage producing separate, equal blastomeres.

cleft (n)


Gap in soft tissue, bone, or both.

cleft (adj)

Split, divided, or partly divided into two.

cleft lip


hare lip

Congenital malformation consisting of one or more clefts in the upper lip, the result of failed closure in the embryo of the maxillary and median nasal processes.

See also cleft palate.


cleft palate

Congenital fissure along the midline of the hard palate, that may extend into the soft palate.

cleft sternum

Rare congenital malformation resulting from defective fusion in the embryo of paired mesodermal bands in the ventral midline.

clitoral hood

See prepuce.


Erectile body in the female genitalia, homologous with the penis in the male.

cloaca (in zoology)

Common cavity at the end of the digestive tract in vertebrates (apart from most mammals) into which are released both excretory and genital products.

cloacal membrane

Membrane that covers the embryonic cloaca during the development of urinary and reproductive organs in those vertebrates where the cloaca occurs.

clover disease


Condition caused by the continued ingestion of low but toxic levels of estrogens.

  • Note 1:

    The most important occurrence is in farm animals pastured on plants containing phytoestrogens. The signs are those related to endometrial hyperplasia and vaginal tumefaction (swelling and puffiness), including long-term infertility and rectal prolapse, especially in pigs; uterine prolapse, especially in ewes; and feminization of males that have undergone castration.

  • Note 2:

    Dogs are particularly susceptible to the myelotoxic effects of estrogens and high dose or prolonged administration causes severe bone marrow depression with thrombocytopenia, followed by leukopenia and anemia.

After [9]



Deformed foot that is twisted out of shape or position, usually congenital (congenital talipes equinovarus (CTEV)).


Narrowing or constriction of a short section of a blood vessel, commonly of the aorta.


Spiral cavity of the inner ear containing the organ of Corti that transduces sound into nerve impulses.


Body cavity, found in many animals, lined with mesodermal epithelium and containing the digestive tract and other visceral organs

  • Note:

    The principal cavities of the trunk arise from the intraembryonic coelom.


Rich in the fibrous connective tissue protein, collagen.

collembolan reproduction test

Procedure in which collembolans (small soil-inhabiting insects) are exposed to soil that has been treated with a test substance, and adult mortality and reproductive output are studied.


Any defect resulting from incomplete closure of the retinal fissure.

  • Note:

    The defect may be congenital, pathological, or artificial.

combined repeated dose toxicity study

Procedure to evaluate both general systemic toxicity, with an emphasis on neurological endpoints, and notably reproductive effects and developmental progression in rodents. Groups of males and females are administered a test substance in graded doses prior to mating, during the mating period, and subsequently (up to two weeks post-mating in males and four days post-delivery in females).


  1. Complication of labor in twin births in which there is attempted simultaneous expulsion of both twins, so that the lower part of the mother’s pelvis is filled and further descent through the birth canal is prevented.

  2. In embryology, process during which blastomeres change their shape and align themselves tightly against each other to form the compact morula.

conal growth hypothesis

Hypothesis that explains transposition of the great arteries of the heart by failure of the aortopulmonary septum to follow a spiral course during partitioning of the bulbus cordis and truncus arteriosus in the process of forming the aorta and pulmonary trunk.


Formation of a viable zygote by the union of a spermatozoon and an ovum.

See also fertilization.


Embryo and associated membranes in the uterus, especially during the early stages of pregnancy.


Present from birth, as of a disease or physical abnormality.

congenital adrenal hyperplasia

adrenogenital syndrome

Group of disorders caused by hyperplasia of the adrenal cortex or by malignant tumors, resulting in excess secretion of adrenocortical androgenic hormones, and characterized by masculinization of women, feminization of men, or precocious puberty in the male.

  • Note:

    The condition is associated with a decrease in the blood level of cortisol and an increase in the level of androgens in both sexes, most commonly as a result of 21-hydroxylase deficiency.

congenital malformation

Malformation existing at birth, or developing during the first month of life, regardless of the cause.

See also birth defect.

conjoined twin

One of a pair of identical twins fused together with varying degrees of union and of residual duplication of organs.

After [7]

See also parasitic twin.

connective tissue

Extracellular matrix of fibrous proteins and glycoproteins, with associated cells such as fibroblasts, that fills the spaces between and within organs and tissues.

  • Note 1:

    Connective tissue provides the organs and tissues with structural and metabolic support.

  • Note 2:

    Specialized connective tissues also include bone, cartilage, blood components, and adipose tissue.


Anything that prevents, or reduces the likelihood of pregnancy.


Sudden, violent, irregular movement of the body caused by involuntary contraction of muscles and associated with brain disorders, such as epilepsy, fever in children, or with drug or alcohol abuse.

corpus cavernosum

Either of the two columns of erectile tissue forming the body of the clitoris (corpus cavernosum clitoridis) or penis (corpus cavernosum penis).

After [5]

corpus luteum

Glandular mass of yellowish tissue in the ovary, formed by a Graffian ovarian follicle that has matured and released its oocyte.

After [5]

  • Note:

    Observed in animal reproductive testing to calculate preimplantation loss.

cortex (in anatomy)

Outer layer of an organ such as the kidney, the cerebellum, or the adrenal gland.

See also cortex, cerebral.

cortex, cerebral

Outer layer of the cerebrum, composed of folded grey matter, playing an important role in consciousness.

Cowper’s gland

See bulbourethral gland.

cranial placodes

Thickenings in the surface ectoderm of the embryo associated with future eye and ear regions.

cranial suture

Line where the bony plates of the skull are joined together by fibrous bands of tissue, easily felt in the newborn before closure by ossification.

See also fontanelle.


Pertaining to the cranium and the face.


Neural tube defect in which both the cranium and vertebral column remain open.


Developmental failure of the cranial sutures to close completely, especially at the occiput, usually leading to grossly defective development of the brain.


Premature closure of the cranial sutures.


cranium (n)/cranial (adj)

Bony structure surrounding the brain, excluding the bones of the face.


Bacterial system in which the Cre protein mediates DNA recombination between specific DNA sequences known as lox-P sites.

  • Note:

    This system is used in mammalian cells to delete (or invert) a stretch of DNA by flanking it with lox-P sites and then exposing the cell to Cre protein at some predetermined time.


Developmental disorder caused by deficiency of thyroid hormone, characterized by severe mental retardation and stunted physical growth, sometimes resulting from maternal iodine deficiency.

cri du chat syndrome

Lejeune syndrome

5p – syndrome

Hereditary congenital syndrome due to deletion of the short arm of chromosome 5.

  • Note:

    This syndrome is characterized by hypertelorism, microcephaly, severe mental deficiency, and a plaintive cat-like cry.

After [5]


One of the inward projections or folds of the inner membrane of a mitochondrion.

critical period

Specific time in the development of a biological system when it is particularly vulnerable to injury or misdirection.

Crouzon syndrome

branchial arch syndrome

Group of autosomal dominant genetic diseases characterized by midfacial hypoplasia, craniosynostosis, exophthalmos, and a shortened head.

  • Note 1:

    This affects the first branchial arch (pharyngeal arch), which is the precursor of the maxilla and mandible.

  • Note 2:

    This syndrome is thought to be caused by a genetic mutation of the FGFR3 gene, located on chromosome 10.

crypt (in anatomy)

Small tubular gland, pit, or depression.


Failure of the eyelids to develop with a fissure between them, usually with defective formation of the eyeballs.


See undescended testis.

cumulus oophorus

Mass of follicular cells surrounding the oocyte in the Graafian follicle.


Bluish discoloration of the skin and mucous membranes due to excessive concentration of deoxygenated haemoglobin, owing to poor circulation or inadequate oxygenation of the blood.

cycle, reproductive

Cycle of physiological changes in the female reproductive organs, from the time of fertilization of the oocyte through gestation and parturition.





Rare form of holoprosencephaly, a congenital disorder characterized by a single orbital fossa due to the failure of the embryonic prosencephalon to divide the orbits of the eye correctly into two cavities.

cyst (n)/cystic (adj)

  1. In an animal or plant, thin-walled hollow organ or cavity containing a liquid secretion.

  2. Sac, vesicle, or bladder.

cytochrome P450 (CYP)

Member of a superfamily of heme-containing monooxygenases involved in xenobiotic metabolism, cholesterol biosynthesis, and steroidogenesis, in eukaryotic organisms found mainly in the endoplasmic reticulum and inner mitochondrial membrane of cells.

  • Note:

    “P450” refers to the observation that a solution of this enzyme exposed to carbon monoxide strongly absorbs light at a wavelength of 450 nm compared with the unexposed solution (a difference spectrum caused by a thiolate in the axial position of the heme opposite to the carbon monoxide ligand).


Branch of genetics in which the structure and function of chromosomes and of other cell constituents concerned with heritable properties and their expression are studied.


Type of Herpes virus that usually produces very mild symptoms in an infected person but may cause severe neurological damage in people with weakened immune systems and in the newborn.


Inner cellular layer of the trophectoderm (trophoblast); part of the mammalian placenta.

Daphnia magna reproduction test

Procedure in which young female Daphnia are exposed to a substance added to the water and numbers of living offspring and surviving parent organisms are recorded.


Modified endometrial layer that lines the uterus during pregnancy and is shed with the afterbirth.

decidual cell

Enlarged, ovoid, connective tissue cell appearing in the endometrium during pregnancy.

decidual cell response technique

Biological test method in which pseudopregnant rats undergo a surgical treatment of the uterus to induce uterine differentiation and proliferation, resulting in massive tissue growth that mimics the response of the uterus during normal blastocyst implantation.

  • Note:

    Measurement of decidual growth during chemical treatment can be used to assess both hormonal status and uterine function. Uterine weight is a sensitive measure of the success of the decidual cell response.


Changes in response to progesterone that include the eosinophilic proliferation around arterioles after ovulation or progesterone action on endometrium.

  • Note:

    Decidualization increases glandular epithelial secretion, stimulates glycogen accumulation in stromal cell cytoplasm, and promotes stromal vascularity (spiral arterioles) and edema.


Swelling of a uterine crypt produced by its reaction to an implanted embryo.


decussate (v)/decussation (n)

Pertaining to two or more things that cross or intersect each other to form an X.

See also chiasma.

defeminized gonadotropin secretion

Gonadotropin secretion without female sex hormones.



Describing a male animal from which the testicles have been removed.


Layer of the skin deep under the epidermis, consisting of a bed of vascularized connective tissue and containing the nerves and organs of sensation, the hair roots, and sebaceous and sweat glands.

After [5]


Failure of synapsis due to separation of homologous chromosomes after initial pairing in meiosis.

developmental biology

Study of biological development from fertilization to adulthood, usually with an emphasis on the prenatal period and regulation of morphogenesis, and neural development on through adolescence.

  • Note:

    Common model organisms include Caenorhabditis elegans, Xenopus, Drosophila, zebrafish (Danio rerio), chick, and mouse.

developmental neurotoxicity study

Procedure in which female rodents are exposed to a test substance from the time of implantation through all of lactation; offspring may be exposed during the preweaning period, either directly or through milk, and are studied with regard to neurologic and behavioral abnormalities during postnatal development and until adulthood.

developmental susceptibility gene

Any gene that encodes a gene product that can be altered by environmental agents to cause disturbance of normal development.

developmental toxicology

Study of adverse effects on the developing organism that result from exposure of either parent to an agent or substance prior to conception, of the mother and fetus during prenatal development, and of the child from birth until the time development is completed.

  • Note:

    Functional brain development continues past the time of sexual maturation.


Congenital defect in which the apex of the heart is directed toward the right side of the thorax (isolated dextrocardia), or the heart is found in a mirror image position on the right side of the body (dextrocardia situs inversus).


Displacement to the right.

Di George syndrome (DGS)

congenital thymic aplasia

conotruncal anomaly face syndrome

DiGeorge anomaly

Shprintzen syndrome

Strong syndrome

thymic hypoplasia

velocardiofacial syndrome

Condition caused by the deletion of a small piece of chromosome 22 near the middle of the chromosome at a location designated 22q11.2.

  • Note:

    Characteristic signs and symptoms may include birth defects such as congenital heart disease; defects in the palate, most commonly related to neuromuscular problems with closure (velopharyngeal insufficiency); learning disabilities; mild deviations in facial features; and recurrent infections.


Dome-shaped muscular membranous partition separating the thoracic cavity from the abdominal cavity in mammals.

  • Note:

    The diaphragm plays a major role in breathing, as its contraction increases the volume of the thorax and so inflates the lungs.


Hereditary predisposition to a particular medical condition.


Posterior part of the forebrain (prosencephalon), containing the hypothalamus and other thalamic components and enclosing the third ventricle.

Compare telencephalon.

diethylstilbestrol (DES)


Synthetic nonsteroidal substance with estrogenic activity.

  • Note 1:

    DES was formerly given to reduce the risk of complications and loss of pregnancy but was shown to cause a rare vaginal tumor in female offspring who had been exposed to this drug in utero.

  • Note 2:

    DES also has effects on the reproductive organs of male progeny exposed in utero.


Cell or nucleus containing two complete sets of chromosomes, one from each parent, or an organism composed of such cells.

Compare haploid.

disc, embryonic


germinal disc

Bilayer plate of cells in the blastocyst from which the mammalian embryo develops.


See WnT.


Entrance of two spermatozoa into one egg.

Compare monospermy, polyspermy.


Disturbance that interrupts a process or disorganizes a structure.

distal (in anatomy)

Opposite term: proximal.

Situated away from the centre of the body or from the point of attachment.


Outpouching of a hollow structure in the body, that may be acquired (as in colonic diverticulitis) or be present at birth (e.g., Meckel diverticulum or Zenker diverticulum).


Of a gene, determining the phenotype even when present only in one copy or inherited from only one parent.

Compare recessive.


Opposite term: ventral.

On or relating to the upper side or back of an organism.

Down syndrome

See trisomy 21.

drug metabolizing enzyme (DME)

Any enzyme that metabolizes xenobiotics.

  • Note:

    Most if not all these enzymes also metabolize endogenous substrates.


Vessel for transporting fluids such as lymph, or glandular secretions such as tears or bile.

duct, excurrent

See excurrent.

ductus arteriosis

Fetal blood vessel connecting the pulmonary artery to the proximal descending aorta.

  • Note:

    This vessel allows most of the blood from the right ventricle to bypass the fetus’s fluid-filled non-functioning lungs. Upon closure at birth, it becomes the ligamentum arteriosum.

See also patent ductus arteriosis.

dung fly test

Dipterian dung fly test

Procedure using dung flies of the order Diptera where a test substance is mixed with bovine feces to which eggs of the fly are added; number, sex, and morphology are determined when the last adult has emerged.


First portion of the small intestine, between the stomach and the jejunum, where bile and pancreatic digestive juices enter the intestinal tract.

dura mater

See meninx.


Defective or abnormal development of an organ.


Dysgenesis of the gonads.


Difficult and painful menstruation.


dysmorphia (n)/dysmorphic (adj)


Abnormality of shape of a body part or organ, often referring to a birth defect.


Production of dysmorphia.


Abnormal development of an organ or tissue identified by morphological examination.



Defect in the normal ossification of fetal cartilage.

early life stage (ELS) test

Toxicity test using an organism in an early life stage such as the embryo or larva, noting that this stage is often the most sensitive part of the species life cycle.

After [2]

earthworm reproduction test

Procedure in which adult worms (Eisenia sp.) are exposed to a test substance mixed into the soil; number, weight and behaviour of the adult worms are studied after four weeks and the number of juveniles hatched is counted after a further four weeks.


Acute, life-threatening complication of pregnancy characterized by tonic-clonic seizures (convulsions), usually in a woman who has developed pre-eclampsia.


Study of the influence of hereditary factors on the response of individuals or populations to environmental factors.


In an embryo, outermost layer that develops into the epidermis of the skin and the nervous system.

See also bilaminar embryo, epiblast.

ectopia (n)/ectopic (adj)


Abnormal location or malpositioning of an organ or body part, usually congenital.

ectopic expression

Expression of a gene in a tissue in which it is not normally expressed.


ectopic pregnancy

Complication of pregnancy in which the embryo implants (see implantation) and develops outside the cavity of the uterus.


Congenital absence or deficiency of one or more of the central digits of the hand or foot.


Congenital absence or deficiency of one or more of the central digits of the hand or foot with fusion of the existing ones.



Presence of abnormally large amounts of fluid in body cavities and intercellular spaces of tissues.


efferent duct

efferent ductule

Seminal duct leading from the testis to the head of the epididymis.


Female gamete after completion of the second meiotic division.

  • Note:

    The term is often loosely applied to secondary oocytes and early embryos.

See also ovum, zygote.


Expulsion of semen from the genital ducts and urethra, usually resulting from a reflex process during sexual stimulation

ejaculatory duct

Passage formed by the junction of the duct of the seminal vesicles and the ductus deferens through which semen enters the urethra.



Means of introducing molecules into cells by transiently permeabilizing their membranes with brief electric field pulses.


embryo (n)/embryonic (adj)

  1. Stage in the developing mammal at which the characteristic organs and organ systems are being formed:for humans, this involves the stages of development from the second to the eighth week post-conception (inclusive), or to the end of organogenesis.

  2. In birds, the stage of development from the fertilization of the ovum up to hatching.

  3. In plants, the stage of development within the seed.



Development and growth of an embryo in the period from formation of the bilaminar embryo to the beginning of the fetal period (in humans, the second through the eighth week after conception).

embryoid body

Structure resembling an embryo which is formed by embryonic stem cells or teratocarcinoma cells when they are removed from a growth-promoting medium.

embryonic induction

Process whereby the development of one group of cells, called the competent region, is altered by an inducing factor from another group, called a signaling center or organizer.

embryo transfer

Process of implanting a fertilized ovum into a uterus.

embryo transport rate analysis

Method to evaluate the potential for early embryonic loss in rodents caused by accelerated or retarded arrival of embryos into the uterus.


Science of the origin and development of the organism from fertilization of the oocyte to the beginning of fetal life (in humans, the end of the eighth week).

  • Note:

    In common usage this term includes all the stages of prenatal life.

After [7]

embryonic period

Period from fertilization to the end of major organogenesis.

See also embryo, embryogenesis.


embryonic stem (ES) cell

Undifferentiated, pluripotent cell from an early stage of the pre-implantation embryo (inner cell mass of the blastocyst).

  • Note:

    Embryonic stem cells have the potential to proliferate and differentiate into various cell types of the body.


Inflammation of the brain.


Congenital protrusion of brain tissue through a fissure or defect in the skull.

endocardial cushion

atrioventricular canal cushion

Subset of cells in the primordial tube-like heart that is essential to the development of the ventricular septum and the atrioventricular valves of the heart.

  • Note:

    Endocardial cushion defects may cause various types of heart malformation.


Endothelial lining of the heart chambers, also containing small blood vessels and a few bundles of smooth muscle, continuous with the endothelium of the great blood vessels.

endochondral ossification

Formation and growth of bone tissue (especially of the long bones) during fetal development of the mammalian skeletal system, and in repair of bone fractures, that takes place in the presence of cartilage.

  • Note:

    Endochondral ossification is one of two mechanisms for bone formation, the other being intramembrous ossification.


Pertaining to hormones or to glands that secrete hormones directly into the bloodstream.


endocrine disrupter

endocrine modifier

Exogenous substance that, at some dose, alters function(s) of the endocrine system and consequently causes adverse health effects in an intact organism, its progeny, or (sub)populations.

After [1]


Uptake of material into a cell by invagination of the plasma membrane and its internalization in a membrane-deliminated vesicle.

After [1]

endoderm (n)/endodermal (adj)

Innermost of the three primary germ layers of an embryo, developing into the gastrointestinal tract, the lungs, and associated structures.

See also bilaminar embryo, hypoblast.



Inner lining of the uterus that is shed during menstruation.


endothelium (n)/endothelial (adj)

Layer of flat cells that line the blood vessels, lymphatic vessels, and the chambers of the heart.

environmental factor

Physical, chemical, or biological agent or condition in the environment that has the potential to affect development and health of an organism, either in a positive or a negative manner.


Located above or on the dorsal side of an axis.


Primitive ectoderm of the early embryo.



visceral pericardium

Inner layer of the pericardium, a sac of fibrous tissue that surrounds and lies upon the heart and the base of the great vessels.


Study of the distribution and determinants of health-related states or events in specified populations and the application of this study for the control of health problems.



In vertebrates, protective outer stratified squamous layer of the skin consisting mainly of keratinocytes.

epididymis (n)/epididymal (adj)

Tightly-coiled, thin-walled tube that conducts sperm from the testis to the vas deferens.


epididymal appendix

appendix epididymidis

Cystic structure sometimes found on the head of the epididymis.

  • Note:

    The epididymal appendix represents a remnant of the mesonephric duct.


epigenesis (n)/epigenetic adj)

Phenotypic change in an organism brought about by alteration in the expression of genetic information without any change in the genomic sequence itself.

  • Note:

    Common examples include changes in nucleotide base methylation and changes in histone acetylation. Changes of this type may become heritable.



Chronic neurological disorder marked by sudden recurrent episodes of sensory disturbance, loss of consciousness, or convulsions, associated with abnormal electrical activity in the brain.


See adrenaline.

epiphyseal plate

The disk of hyaline cartilage between the metaphysis and the epiphysis of an immature long bone that permits the bone to grow longer.



Malformation in which the urethra does not develop to full length, opening typically on the dorsum of the penis (and rarely, in females, between the clitoris and labia); frequently associated with exstrophy of the bladder.


Situation in which the phenotypic expression of one gene obscures the phenotypic effects of another gene.


epithelium (n)/epithelial (adj)

Sheet of one or more layers of cells covering the internal and external surfaces of the body and hollow organs.


epithelial-to-mesenchymal transition (EMT)

Sequence of events, where epithelial cells detach, migrate, and become mesenchymal stem cells that can differentiate.

  • Note:

    This transition plays a role in embryogenesis, wound healing, and cancer.

See also mesenchymal-to-epithelial transition.

epoxide hydrolase

epoxide hydratase

Detoxification enzyme (EC, located mainly in the endoplasmic reticulum, that hydrolyzes epoxides, converting them to metabolites that can be more rapidly excreted.


erectile dysfunction

Inability of a male to have or to maintain an erection.


See hemolytic disease of the newborn.



Steroid sex hormone, involved in many regulatory processes, but most prominently in the development of female sex organs, the menstrual cycle, and pregnancy.


Hormonally active metabolite of estradiol, eliminated in urine, especially during pregnancy.


Any substance, natural or synthetic, that exerts the hormonal effects of the natural estrogen, estradiol, usually by interaction with the estrogen receptor.

  • Note:

    Phytoestrogens are substances that occur in plants and have estrogenic activity.

estrogen activity assay, in vitro

  1. Procedure in which a tumor cell line is exposed to a test substance and activation of the estrogen receptor is studied by a reporter molecule in order to assess the estrogen-sensitivity of the tumor.

  2. Test using estrogen-sensitive cells to evaluate estrogenic activity of a substance or environmental sample.

Compare uterotrophic assay.


Behaving like an estrogen.


Exhibiting hormonal activity similar to that of estrogens.


See clover disease.

estrous cycle

Sequence of recurring physiological uterine, ovarian, and other changes, induced by reproductive hormones in higher female animals, resulting in readiness for insemination. This cycle starts in adulthood (at menarche), transiently discontinues during pregnancy, and disappears at the menopause.




Recurrent period of sexual receptivity and arousal in the female of most mammals except the higher primates, during which conception is possible.

estrus syndrome

Persistent estrus caused by elevated estrogenic activity, either due to endogenous estrogens (e.g., in polycystic ovary syndrome) or due to exogenous substances.

  • Note:

    Estrus syndrome is associated with infertility.


Referring to a cell or organism with chromosomes present in an integral multiple of the haploid number.

  • Note:

    A human cell with the normal number of 46 chromosomes, an integral multiple of the monoploid number, 23, is thus euploid However, a human with abnormal, but integral, multiples of this full set (e.g., 69 chromosomes) would also be considered as euploid.


Subclass of mammals having a placenta through which the fetus is nourished.


herniation of intestines

Protrusion of omentum and/or intestine through a defect or weakness in the abdominal wall while the skin remains intact.

After [1]

See also omphalocele.


Specific induction of a tissue within a developing embryo that results from the action of a single hormone or other chemical (the evocator).


Pertaining to a vessel (or opening) conveying fluid outwards.

  • Note:

    The male excurrent ducts are those involved in the transport, enrichment and ejaculation of semen.



Exposed brain resulting from failure of the neural tube to close and absence of the top of the skull.

  • Note:

    In humans this is followed by degeneration of the brain, resulting in anencephaly.


exfoliation (n)/exfoliative (adj)


Detachment and shedding of superficial cells of an epithelium or tissue surface, especially of the horny layer of the epidermis.



Relating to or denoting glands that secrete through ducts opening on to superficial epithelium (e.g., the intestinal tract) rather than into the blood.



Hernia in which bowel or omentum protrudes through the abdominal wall under the skin at the umbilicus.

After [7]



Marked protrusion of the eyeballs, usually resulting from an increased volume of the orbital contents.

After [8]

expressed sequence tag (EST)

Partial or full complementary DNA sequence that can serve as a marker for a region of the genome that encodes an expressed product.


expression, gene

Translation of a gene into a gene product.


Congenital malformation in which a hollow organ has its interior exposed through eversion; most commonly observed in the urinary bladder (ectopia vesicae).

external genitalia

Genitalia visible outside the body.

extracellular matrix (ECM)

Mesh of molecules, secreted by cells into the surrounding extracellular space.

  • Note:

    Cells may adhere to the matrix and this can provide them with structural and biochemical support.

extraembryonic membrane

One of the membranes that surrounds the embryo, which include the yolk sac, allantois, amnion, and chorion.

After [5]

extraembryonic mesoderm

Cells outside the embryo that, although derived from the zygote, are not part of the embryo proper and contribute to the fetal membranes (yolk sac, allantois, amnion, and chorion).

extrinsic pathway

Apoptotic pathway of cell death initiated upon occupancy of so-called death receptors at the cell surface with their ligands, such as Fas/CD95 and tumor necrosis factor-α, and involving activation of caspase 8.

  • Note:

    This pathway converges with the intrinsic pathway at the level of cleavage of procaspase 3 by either caspase 8 (extrinsic pathway) or caspase 9 (intrinsic pathway).

F1 generation

Indicating ‘first filial’ generation, denoting offspring from genotypically (see genotype) distinct parents.

  • Note:

    F2 denotes offspring resulting from mating (inbreeding) of individuals from F1. The term may be extended to F3 and beyond.

F2 generation

See F1 generation.

facial cleft

Congenital anomaly resulting from the failure of facial structures in the embryo to fuse properly.

See also orofacial cleft.


In medicine, facial expression of an individual that is typical of a particular syndrome, disease, or condition.

Fallopian tube


uterine tube


One of a pair of tubes, on either side of the upper or outer extremity of the ovary, that provides a path by which an ovum travels from the ovary to the fundus of the uterus.

  • Note:

    Each tube is largely enveloped by its expanded infundibulum, where, if the ovum is fertilized in the tube, it will implant (see implantation) as a zygote.

Modified from [7]



Process of fertilization.



Ability to produce offspring within a given period of time.

Quantity of reproductive output.

  • Note 1:

    The potential reproductive capacity of an organism or population may be measured by the number of gametes.

  • Note 2:

    Fecundity is controlled by both genetic and environmental factors, and is a major determinant of fitness.

After [1]


Development of female characteristics as part of normal female maturation, or, in a male, pathologically as a result of endocrine imbalance.

fertility, female

Ability to conceive (see conception).

Production of live offspring.

fertility, male

Ability to induce conception.

  • Note:

    Sperm count and motility are important contributing factors.

Fertility Assessment by Continuous Breeding (FACB)

Reproductive Assessment by Continuous Breeding (RACB)

Reproductive toxicity test in rodents in which female animals are repeatedly mated and frequency and size of litter and other parameters are measured; second or later generation fertility is also studied.


In reproductive biology, union of male and female gametes to form a zygote from which an embryo can develop.

See also conception.

fetal alcohol syndrome (FAS)

Condition developing in the fetus and resulting in congenital abnormalities, caused by alcohol intake by the mother during pregnancy, typically characterized by decreased cognitive development, stunted growth and a characteristic facies.

  • Note:

    FAS is the most serious form of fetal alcohol spectrum disorder (FASD), a range of milder forms that may show reduced skills in learning, social development, memory, and control of emotions.

fetal period

See fetus.

fetal pole

Thickening on the margin of the yolk sac of a fetus during pregnancy.

  • Note:

    Appearance of the fetal pole is used as the earliest sign of a baby in ultrasound examination.


Assay for teratogenicity using embryos of the frog, Xenopus laevis.

  • Note:

    The term is derived from the first letters of “frog embryo teratogenesis assay Xenopus”.

After [2]


Procedure in which a fetus may be directly observed in utero, using a fiber-optic endoscope (fetoscope) introduced through a small incision in the abdomen under local anesthesia.

  • Note:

    Photographs may be taken, and amniotic fluid, fetal cells, or blood may be sampled for prenatal diagnosis of many congenital anomalies or genetic defects.

After [8]

fetus (n)/fetal (adj)

Young mammal within the uterus of the mother, from the visible completion of characteristic organogenesis until birth.

  • Note:

    In humans, this period is usually defined as from the third month after fertilization until birth; prior to this the young mammal is referred to as an embryo.





Flat fibrous tissue cell, with stellate or spindle shape, associated with the formation of collagen fibers and intercellular extracellular matrix.

  • Note:

    This cell may develop into a number of precursor cells, such as collagenoblasts, osteoblasts, and chondroblasts.

fibroblast growth factor (FGF)

Any member of a family of more than twenty protein growth factors involved in angiogenesis, wound healing, and, in early embryonic development, induction of mesoderm, patterning and morphogenesis, and development of the limbs and brain.

See also sonic hedgehog.

fish embryo test (FET)

Fish embryo acute toxicity test

Procedure in which fish (often zebrafish, Danio rerio) eggs and embryos are exposed to a test chemical and subsequent development or death of the animals is studied.

fish reproduction assay

Procedure in which sexually mature male and spawning female fish are exposed to a test substance and multiple endpoints, including egg production, endocrine activity, vitellogenin, and gonadal histopathology are measured.

fish sexual development test

Procedure in which fish are exposed from the time of fertilization of the egg until sexual differentiation is completed and markers of possible endocrine disruptor activity are studied, including vitellogenin concentration, gonadal histopathology, sex ratios, and the occurrence of intersex.


Permanent abnormal passage from an internal organ to the body surface or between two internal organs or structures.


Ability to survive and reproduce.

flexure, hepatic

flexure, right colic

Bend between the ascending colon and the transverse colon.


Alternative spelling of fetus found in British English usage.



  1. Ridge or margin apparently formed by the doubling back of a lamina.

    • Note:

      In anatomy, used as a common identifier, as, for example, in nail f., neural f., tail f., transverse f., and urogenital f.

  2. In the embryo, a transient elevation or reduplication of tissue in the form of a lamina.

After [7]

folic acid

Water-soluble B vitamin involved in breakdown of carbohydrates, releasing energy and promoting biosynthesis.

  • Note:

    Folic acid deficiency may lead to a range of serious abnormalities of the developing fetus, including neural tube defects.

follicle (n)/follicular (adj)

Small, secretory sac, such as the dental follicles that enclose the teeth before eruption or the hair follicles within the epidermis.

Fluid- or colloid-filled ball of cells in some glands such as the thyroid gland and the ovaries.


follicle-stimulating hormone (FSH)

Hormone secreted by the anterior pituitary that stimulates the Graafian follicles of the ovary and subsequent follicular maturation, and in the male contributes to inducing spermatogenesis.

After [7]

follicle-stimulating hormone releasing hormone (FSH-RH)


Hormone, released by the hypothalamus, inducing the secretion of follicle-stimulating hormone and luteinizing hormone by the pituitary gland.


See follicle.

follicular atresia

Degeneration and resorption of an ovarian follicle before it reaches maturity and ruptures.

follicular phase

First half of the human menstrual cycle, during which one or more follicles in the ovary may mature, culminating in ovulation.


Any of several soft spots in specific locations on the skull of the newborn where the bones have not yet fused and the brain is covered by skin and a tough membrane.

See also cranial suture.

foramen (pl. foramina)

Opening, hole.

  • Note 1:

    The skull has several such openings that act as passageways for structures, notably nerves and blood vessels.

  • Note 2:

    The vertebral column is perforated by the vertebral foramina through which nerves from the spinal cord exit to the periphery.

foramen magnum

Large foramen in the base of the skull, through which the spinal cord passes.

foramen ovale

In the fetal heart, opening that allows shunting of blood between the right and left atria and normally closes at birth.

One of the larger foramina in the base of the skull.


See prosencephalon.


See prepuce.

forward genetics

Approach to genetic investigation in which the aim is to identify the gene that governs a particular function.

  • Note:

    Mutant phenotypes indicate the responsible gene and co-inherited genetic markers indicate the region of the genome where it occurs.

See also reverse genetics.


In anatomy, a depression or hollow, often in a bone.

fossa ovalis

Oval depression in the heart, on the lower part of the septum of the right atrium.

  • Note:

    The fossa ovalis is a remnant of the foramen ovale.

After [7]

frontonasal dysplasia (FND)

frontonasal dysostosis

frontonasal malformation

median cleft face syndrome

Tessier cleft numbers 0/14 (See Tessier classification)

Congenital malformation of the midface in which the nose has a flat, wide appearance, the eyes may be wide-set, and there is a groove of varying size, running down the middle of the face across the nose.

functional genomics

Development and implementation of technologies to characterize the mechanisms through which genes and their products function and interact with each other and with the environment.



Part of a hollow organ (e.g., the uterus) that is furthest from its opening.

gall bladder

Pear-shaped muscular sac underneath the liver that stores bile secreted by the liver before its release into the duodenum.


Reproductive cell (e.g., sperm or egg) containing a haploid set of chromosomes.


gamete intrafallopian transfer (GIFT)

Technique of assisted reproduction by placing eggs and sperm into a woman’s Fallopian tubes in order that fertilization may occur there.

  • Note:

    The embryo is expected to travel through the Fallopian tube and implant in the uterus as it would have done had natural fertilization occurred.


Development of male or female gametes to maturity.


Congenital defect characterized by a fissure in the anterior abdominal wall through which the small and (or) large intestine protrude.


Embryonic stage that develops from two to three germ layers during gastrulation.


Stage of embryo development in which the two-layered blastula transforms into the gastrula by developing ectoderm and endoderm, as well as a third layer (mesoderm), through the movement of specific cells.


Length of DNA or RNA (in viruses) that encodes a functional product, which may be a polypeptide or a ribonucleic acid.

  • Note:

    A gene is the fundamental unit of heredity.


gene product

Both the messenger RNA resulting from transcription of a gene and the proteins and peptides translated from that mRNA.

gene silencing

Prevention of the expression of a specific gene.

  • Note:

    Gene silencing may occur by natural genetic or epigenetic processes, or by experimental intervention, as in gene targeting knockout mutation, or by RNA interference.

gene targeting

Use of homologous recombination to change a gene, e.g., to delete a gene, remove exons, add a gene, or introduce point mutations.


Of or relating to human or animal reproduction.

Of or relating to the genital organs.

Of or relating to the final stage of psychosexual maturation.

Genital organ.


Plural of genital.

Externally visible sex organs.

genital fold

genital ridge

genitourinary ridge

labioscrotal swelling

urogenital ridge

Embryonic structure that will develop into the penis and scrotum in males or the labia in females.

genital organ

Organ of reproduction or generation, both external and internal to the body.

genital tract

reproductive tract

genital duct

Genital passages of the urogenital apparatus.

  • Note:

    In females, the tract runs from the ovaries to the vulva, in males from the testicles to the external urethral orifice of the penis.

genital tubercle

Primordium of the penis or clitoris.


See urogenital.


Complete set of chromosomal and extrachromosomal genes of an organism, a cell, an organelle (e.g., mitochondria or chloroplasts), or a virus, i.e., the complete DNA component of an organism (or the complete RNA component of an RNA virus).

After [1]


  1. Science of using DNA-and RNA-based technologies to demonstrate alterations in gene expression.

  2. In toxicology, method providing information on the consequences for gene expression of interactions of the organism with environmental stress, xenobiotics, etc.



Capable of causing a change to the structure of the genome.



Genetic constitution of an organism as revealed by genetic or molecular analysis; the complete set of genes possessed by a particular organism, cell, organelle, or virus.


germ cell

See germ line cell.

germ cell gene mutation assay

Procedure in which mutations induced by an agent are studied in germ line cells or somatic stem cells from a transgenic animal (rodent), containing reporter genes for detection of various mutations.

germ line cell

germ cell

Gamete or an embryonic cell that can develop into a gamete.

germ layer

Any of three distinct layers of cells (endoderm, ectoderm, and mesoderm) that become recognizable as the embryo develops.



Period between conception and birth during which an embryo or fetus is carried in the uterus of a female mammal.

gestation period

Time from conception to birth, during which a fetus develops.


Condition of abnormal size or overgrowth of the entire body or of any of its parts.

  • Note:

    Often caused by increased levels of growth hormone.

See also acromegaly.


Organized aggregation of cells functioning as a secretory or excretory organ.



Any small rounded mass or gland-like body.

glans clitoridis

Rounded mass of sensitive erectile tissue that forms the head of the clitoris.

glans penis

Rounded mass of sensitive erectile tissue at the distal end (head) of the penis, analogous to the glans clitoridis.


Disease of the eye characterized by increased intraocular pressure, resulting in atrophy of the optic nerve.

After [7].


Tuft or a cluster, as of a plexus of capillary blood vessels or nerve fibers (e.g., capillaries of the filtration apparatus of the kidney).



Any of a group of steroid hormones, such as cortisone, produced by the adrenal cortex, inhibiting inflammation, and mediating a response to stress that alters protein, fat, and carbohydrate metabolism.


Biochemical breakdown of glucose into pyruvic acid with the production of ATP.

  • Note:

    If the resulting pyruvate is not used efficiently in aerobic metabolism, it may then be converted to ethanol (fermentation) or lactic acid (anaerobic glycolysis).

gonad (n)/gonadal (adj)

Organ in animals that produces gametes, i.e., the testis or the ovary.



Glycopeptide hormone, produced by the fetal placenta, that maintains the function of the corpus luteum during the first few weeks of pregnancy.

gonadotropin-releasing hormone (GnRH, GRH)


luteinizing hormone-releasing hormone (LHRH)

Any factor from the hypothalamus that stimulates the anterior pituitary to release both follicle-stimulating hormone and luteinizing hormone.

Modified from [7]

Graafian follicle

vesicular ovarian follicle

Mature ovarian follicle within which an oocyte attains full size and which ruptures during ovulation to release the ovum.


White blood cell, distinct from a lymphocyte, containing cytoplasmic granules.


Pertaining to cells of the cumulus oophorus.


grey matter

Regions of the brain and spinal cord that are made up primarily of cell bodies and dendrites of nerve cells rather than of myelinated axons.

After [7]

Compare white matter.

growth factor

Naturally occurring or genetically engineered protein or steroid hormone that binds to a receptor to regulate cell growth, proliferation, or differentiation.

growth hormone (GH)


somatotropic hormone (STH)

Peptide hormone secreted by the anterior pituitary that stimulates growth, energy metabolism, and cell proliferation.

  • Note:

    Human growth hormone (hGH) isolated from cadavers or produced by recombinant technology is used clinically to treat deficiency of the hormone and other growth disorders, and inappropriately to enhance athletic performance.

growth hormone releasing factor (GHRF)

Substance produced in the hypothalamus that regulates release of growth hormone by the anterior pituitary.


Fetal mesenchyme ligament that passes through the anterior abdominal wall and connects the lower pole of each developing gonad with the developing scrotum in men, and with the developing labia majora in women.

  • Note:

    In men, it guides descent of the testes into the scrotum.



State in which a cell contains only one set of chromosomes.



See cleft lip.


Pertaining to blood or blood vessels.

Situated, in common with the heart and major blood vessels, on the ventral side of the body with respect to the vertebral column or its embryonic precursors.


Benign tumor formed by a vascular malformation present at birth or developing during life, in which proliferation of blood vessels leads to a vascular tangle.

  • Note 1:

    Hemangiomas can occur anywhere in the body but are most frequently noticed in the skin and subcutaneous tissues; most hemangiomas present at birth undergo spontaneous regression.

  • Note 2:

    Hemangioma in the brain (central nervous system cavernous hemangioma) can be accompanied with multiple neurological symptoms, ranging from headache to seizures.



Development of each of the types of blood cell from common precursor cells, located mainly in the bone marrow.


Congenital partial absence of all or part of the distal half of a limb.


Congenital defect of the vertebral column in which one side of a vertebra fails to develop completely due to failure of the chondrification center to form on that side.

  • Note:

    The resulting wedge-shaped vertebra can cause an angle in the spine leading to kyphosis, scoliosis, or lordosis.

After [7]

hemolytic disease of the newborn

erythroblastosis fetalis


Severe form of anemia in a fetus or newborn infant caused by incompatibility with the mother’s blood type, typically when the mother is rhesus factor negative (Rh -ve) and produces antibodies which attack (Rh +ve) fetal blood through the placenta.


Enlargement of the liver and spleen.


Having the reproductive organs and many of the secondary sex characteristics of both sexes.

  • Note:

    Pseudohermaphroditism is a state in which the person is of an unambiguous gonadal sex (possessing either testes or ovaries) but has ambiguous external genitalia.


State of being hermaphrodite.

See also pseudohermaphrodite.


Protrusion of an organ or structure through an abnormal opening in connective tissue or in the muscle wall of the cavity in which it is normally enclosed.

hernia, hiatal

hernia, hiatus

Hernia of a part of the stomach through a rupture of the diaphragm at the esophageal opening.


Family of viruses including herpes simplex types 1 and 2, and herpes zoster (also called varicella zoster).

  • Note:

    Herpes viruses cause several infections, all characterized by blisters and ulcers, including chickenpox, shingles, genital herpes, and cold sores or fever blisters.


Hershberger bioassay

Short-term in vivo procedure in which the androgenic or antiandrogenic effects of a test substance are studied in castrated, peripubertal male rodents, as indicated by changes in the weights of androgen-dependent tissues.

hexosaminidase A

See β-N-acetylhexosaminidase.


Structural component of the limbic system of the vertebrate brain involved in short-term and long-term memory and spatial navigation.

Hirschsprung syndrome

aganglionic megacolon

Absence of nerve ganglia throughout all or part of the gastrointestinal tract, causing dysregulation of intestinal motility.


Presence of excessive bodily and facial hair.

After [7]


(formerly arhinencephaly)

Birth defect in which the embryonic forebrain fails to divide completely to form the cerebral hemispheres.

  • Note:

    This results in varying degrees of mental impairment and abnormal development of eye, nose, and lip.


Any of a class of closely similar DNA sequences occurring in various genes and involved in regulating pattern formation, segmentation, and morphogenesis during embryonic development in many species.

See also homeosis.

homeobox gene

Gene containing a homeobox sequence.

homeosis (n)/homeotic (adj)

Replacement of part of one segment of an insect or other segmented animal by a structure characteristic of a different segment, especially through mutation.


  1. Having the same relationship, relative position, or structure.

  2. (Of organs) Similar in position, structure, and evolutionary origin but not necessarily in function.

  3. (Of chromosomes) Pairing at meiosis and having the same structural features and pattern of genes.

  4. (Of a series of chemical compounds) Having the same functional group(s) but differing in structure by a fixed group of atoms.

homologous recombination

Crossing over between two similar or identical strands of DNA, resulting in exchange of corresponding stretches of DNA between two sister chromosomes.

honey bee larval toxicity test

Procedure in which larvae of the honey bee (Apis mellifera) are exposed to a test chemical in the diet and subsequent mortality is recorded daily up to the 72-h LD50.

hormone (n)/hormonal (adj)

Substance formed in one organ or part of the body and carried in the blood to another organ or part where it selectively alters functional activity.


human chorionic gonadotropin (HCG)

See chorionic gonadotropin.

Human Genome Project (HGP)

International research project that started in 1990 with the goal of determining the base-pair sequence of the human genome. The project was declared complete in 2003.

  • Note:

    HGP also involved studying the genome of a number of organisms other than humans, including insects, fish, plants, and other mammals.

hydatidiform mole

Vesicular or polycystic mass resulting from the proliferation of the trophoblast, with hydropic degeneration and avascularity of the chorionic villi.

  • Note:

    The abnormal tissue typically results from gene expression from paternally derived chromosomes and a loss of maternal chromosomes.



Rare condition in which the brain’s cerebral hemispheres are replaced by sacs filled with cerebrospinal fluid.



Accumulation of serous fluid in a sacculated cavity; specifically, such an accumulation in the space of the tunica vaginalis testis, or in a separate pocket along the spermatic cord.

After [7]


Accumulation of excess cerebrospinal fluid within the ventricles of the brain; head enlargement and brain damage may occur.



Microcephaly with excess cerebrospinal fluid.


Swelling of the funnel-shaped part of the kidney, where urine is collected to enter the ureter.

  • Note:

    Hydronephrosis may be a congenital deformity or the consequence of an obstruction in the ureter.

hydrops (n)/hydropic (adj)

Gross edema of the entire body, with severe anemia, occurring in hemolytic disease of the newborn.




Dilation of a ureter with fluid.


See steroid 21-monooxygenase.

hygroma (n)/hygromous (adj)


Accumulation of fluid in a sac, cyst, or bursa.



Thin, membranous fold of highly variable appearance that, before its rupture, partly occludes the opening of the vagina.

hyoid arch

Second pharyngeal or branchial arch of the developing embryo, from which different tissues around the neck, ear, and face will develop.


Ventral to the long axis of the body.


Extension of a limb or part of a limb beyond the normal limit.


Flexion of a limb or part of a limb beyond the normal limit.


Abnormal multiplication or increase in the number of normal cells in a tissue or organ.



Abnormally wide space between two organs, especially referring to the eyes.


See hirsutism.


Enlargement of an organ or tissue as a result of the increase in the size of its cells.


Innermost of the three primary germ layers, adjacent to the blastocyst cavity, that develops into the endoderm.



Inadequate functioning of the testes or ovaries as manifested by deficiencies in gametogenesis or in secretion of gonadal hormones.

  • Note:

    Primary hypogonadism refers to a defect that is inherent in the gonad while secondary hypogonadism refers to a defect lying outside the gonad, often an endocrine effect.


See pituitary.


Underdevelopment or atrophy of a tissue or organ.


Birth defect in which the urethra opens on the underside of the penis, or into the vagina.


Region at the base of the brain containing specialized nerve cells that help activate, control, and integrate peripheral autonomic mechanisms, endocrine activities, and some somatic functions such as body temperature, sleep, and appetite.


Abnormally low dioxygen content or tension.

Deficiency of dioxygen in the inspired air, in blood, or in tissues, short of anoxia.


ichthyosis (n)/ichthyotic (adj)

fish-skin disease

fish-scale disease


Congenital disorder of keratinization, characterized by dryness and scaling of the skin, often associated with other defects and with abnormalities of lipid metabolism.

After [7]

identical twins

monozygotic twins

Twins resulting from one zygote that, at an early stage of embryonic development, separated into two independently growing cell aggregations giving rise to two individuals of the same sex and identical genetic constitution.



Relating to the ileum and the cecum.


Third portion of the small intestine, between the jejunum and the cecum.

iliac artery

hypogastric artery

Main artery of the pelvis.

imperforate anus

anal atresia

Congenital absence of an anal opening due to the persistence of epithelial plug (persistence of the anal membrane) or to complete absence of the anal canal.




Embedding of the early embryo in the lining of the uterus.



Pseudohermaphroditic condition in female gastropods (snails) manifested by the development (imposition) of male characteristics such as a penis or vas deferens.

  • Note:

    Quantitation of imposex in the dog whelk (Nucella lapillus) has been used to monitor pollution by the antifouling agent tributyltin oxide (TBTO) in marine environments.



See imposex.

imprinting (in genetics)

Differential expression of a gene, depending on whether it was transmitted through the sperm or the egg.

After [12]

  • Note:

    Imprinting is an epigenetic modulation of gene expression, thought to be regulated by attachment of methyl groups to the DNA, and by chromatin structure.

inbred strain

Strain of an animal that has been inbred by brother-sister matings for more than 20 generations, and consequently all the individuals of the strain are more than 98% genetically identical.

After [11]

index, female fertility

In rodents, number of pregnant females divided by the number of females mated, multiplied by 100.

  • Note:

    This index measures the female’s ability to become pregnant and may be used as a general indicator of fertility.

index, gestation

In rodents, number of females with live born offspring divided by number of females with evidence of pregnancy, multiplied by 100.

index, male fertility

In rodents, number of males impregnating females divided by number of males mated, multiplied by 100.

  • Note:

    This index measures the male’s ability to produce sperm that are capable of impregnating a female, assuming all mated females are fertile.

index, tubular fertility

Percentage of seminiferous tubules containing identifiable spermatogonia.

induced pluripotent stem (iPS) cell

Pluripotent stem cell that is generated from an adult cell by genetic reprogramming.


  1. Process of stimulating and determining morphogenetic differentiation in a developing embryo through the action of chemical substances transmitted from one embryonic part to another.

    See also evocation.

  2. Increased expression of a protein or set of proteins triggered by an endogenous mediator or a xenobiotic.


See infertility.





Persistent inability of either a male or a female to achieve conception or to produce offspring.


Anatomical funnel-shaped opening, often referring to the hollow stalk that connects the hypothalamus and the posterior pituitary gland, or to the portion of the Fallopian tube that receives the ovum.

inguinal canal

Passage in the lower anterior abdominal wall, which in men conveys the spermatic cord and in women the round ligament of the uterus.


Any of a group of peptide hormones secreted by the follicular granulosa cells of the ovary and the Sertoli cells of the testis, inhibiting follicle stimulating hormone secretion by the anterior pituitary.


Malformation producing a cranial defect at the occiput, with the brain exposed, often combined with a cervical rachischisis and retroflexion.

After [7]


Twins conjoined at the occiput.


Delivery of sperm into the female reproductive system for the purpose or with the result of causing pregnancy.


Any of a family of cell membrane glycoproteins that mediate cell-to-cell and cell-to-extracellular matrix interactions.

intermediate filament (IF)

Any of a group of fibrous proteins (including keratin fibres, neurofilaments, desmin, and vimentin) that make up part of the cytoskeleton of most eukaryotic cells; so named because, at about 10 nm diameter, they are intermediate in thickness between actin filaments and microtubules.

  • Note:

    Most types of intermediate filaments are cytoplasmic, but one type, the lamins, is nuclear.



Condition of having both male and female characteristics: thus, the state of being intermediate between the sexes.


intracytoplasmic sperm injection (ICSI)

Infertility treatment in which the sperm is injected through the membrane of the egg into its cytoplasm.


intramembrous ossification

Creation of bone tissue during fetal development of jawed vertebrates, as well as during the healing of bone fractures; the tissue forms from mesenchymal stem cells residing in an extracellular matrix devoid of collagen.

  • Note:

    Intramembrous ossification is one of two mechanisms of bone formation, the other being endochondral ossification.


Within the uterus.

intrauterine growth restriction (IUGR)

intrauterine growth retardation

Subnormal, poor growth of the fetus.

  • Note:

    Growth restriction can be due to maternal or fetal causes, as well as to malnutrition and toxic exposures.

intrinsic pathway

Apoptotic pathway of cell death initiated from within the cell by signals initiated by mitochondrial damage and (or) DNA damage, a defective cell cycle, detachment from the extracellular matrix, hypoxia, loss of cell survival factors, or other types of severe cell stress; characterized by the activation of caspase 9.

  • Note 1:

    This pathway involves the release of pro-apoptotic proteins from the mitochondria.

  • Note 2:

    The intrinsic and extrinsic pathways converge with cleavage of procaspase 3 by either caspase 8 (extrinsic pathway) or caspase 9 (intrinsic pathway).


See ostium.

in utero

See intrauterine.

inversion, chromosomal

Rearrangement of a chromosome in which, after breakage at two points, a segment is reversed, resulting in a change in sequence of nucleotides.

in vitro fertilization (IVF)

Fertilization outside the body, used as a treatment for infertility.



Local deficiency of blood supply, and hence of dioxygen, to an organ or tissue, caused by constriction or obstruction of the blood vessels.

After [1]


Vitamin A-like medication (13-cis retinoic acid) used, among other things, for acne treatment.

  • Note:

    Intake during pregnancy has a high teratogenic risk.


Second part of the small intestine between the duodenum and ileum.


Dissolution of a cell nucleus, especially during mitosis and meiosis but also following necrosis.

See also karyorrhexis.


Irreversible fragmentation of the nucleus of a dying cell following karyolysis, whereby its chromatin is distributed irregularly throughout the cytoplasm.


  1. Characterization of the number, form, and size of the chromosomes of an individual or a species.

  2. Photomicrograph of an individual’s chromosomes, arranged according to a standard classification for the species.


Fibrous protein forming the main structural constituent of hair, feathers, hoofs, claws, horns, etc.



Replacement of the outer epithelial layer of cells, especially in skin, with keratin-rich structures.


Acidosis, as in diabetes, pregnancy, or starvation, accompanied by the accumulation of ketone bodies in body tissues and fluids.

ketone body

Ketone that is an intermediate product of the breakdown of fats in the body; any of three compounds, acetoacetic acid, acetone, and (or) β-hydroxybutanoic acid, found in excess in blood and urine of persons with metabolic disorders.

  • Note:

    Ketone bodies tend to accumulate in the blood and urine of individuals affected by starvation or uncontrolled diabetes mellitus.

Klinefelter syndrome

Syndrome in males resulting from a genetic defect in which an extra female X chromosome (thus an XXY genotype) is present in a male, characterized by small testes, long legs, enlarged breasts, reduced sperm production, and mental retardation.

knockout mutation

null mutation

Mutation that leads to the loss of function of a particular gene.




Rearward curvature or convexity of the spine in excess of normal, resulting in a protuberant upper back.


Physical efforts of expulsion of the fetus and the placenta from the uterus during birth.


Secretion of milk from the mammary gland, usually to feed the infant during the neonatal period.

lactation period

Time following pregnancy during which the mammary glands secrete milk.

lamina propria

Layer of connective tissue, underlying the epithelium of a mucus membrane, containing capillaries and lymphatic vessels, as well as fibroblasts and cells of the immune system.


Part of the respiratory tract between the pharynx and the trachea, having walls of cartilage and muscle and containing the vocal cords.


Disease caused by infection with the bacterium Mycobacterium leprae, often affecting the skin and nerves and causing body parts to become deformed.

  • Note:

    Thalidomide is used as a therapeutic drug and thalidomide-associated embryopathy occurs in regions where leprosy is endemic.


Malformation characterized by an abnormally tall, narrow cranium.



Abnormally slender digits.


Lack of pigmentation in the skin, hair, or feathers as a result of a failure of pigment cells to develop, or to migrate to those locations from their origin in the neural crest, in the embryo.

  • Note:

    Unlike albinism, where the defect is in the production of melanin pigment only, animals with leucism usually have pigment in the eye, as retinal pigment cells do not derive from the neural crest.


Abnormally low concentration of white cells (leukocytes) in the blood.


Normal position of the heart in the left hemithorax with the apex pointed to the left, and transposition of other viscera.

See also dextrocardia, situs inversus.

Leydig cell

Cell in the testis that produces androgens, mainly testosterone, in the presence of luteinizing hormone.

ligamentum arteriosum

Fibrous remnant of the ductus arteriosus linking the aortic arch and the top of the left pulmonary artery.

limb bud

Embryonic outgrowth of mesoderm covered in ectoderm that will give rise to one of the forelimbs or hindlimbs.

limit test

Acute toxicity test in which, if no ill effects occur at a preselected maximum dose, no further testing at greater exposure levels is required.


linkage analysis

Technique that studies patterns of heredity in high-risk families, in order to locate a disease-causing gene mutation by identifying co-inherited traits, usually by producing a LOD score.

linkage disequilibrium

Occurrence in members of a population of combinations of linked genes in non-random proportions, implying that the genes are close enough together on a chromosome to make it unlikely that they will be separated by recombination (see homologous recombination) during meiosis.


Disorder of fat metabolism characterized by the accumulation of abnormal levels of certain lipids in the body.


Multiple offspring produced by a mammal from one pregnancy.

lobster-claw deformity

split hand

cleft hand

See ectrodactyly.

locus (in genetics); (pl. loci)

Specific location of a gene or DNA sequence on a chromosome.

LOD score

logarithm (base 10) of odds

Ratio of the likelihood of two or more loci remaining together when chromosomes recombine (true linkage) to the likelihood of this occurring by chance alone.

  • Note:

    An LOD score of 3 or greater is considered indicative of genetic linkage with odds greater than 1000:1.

See also linkage analysis.


Forward (ventral) curvature of the spine in the lumbar and cervical regions.

  • Note 1:

    Some degree of lumbar curvature is normal. Excessive curvature (lumbar hyperlordosis) is sometimes called swayback.

  • Note 2:

    This posture is also associated with a rodent being in heat.

lumbar puncture

spinal tap

Piercing into the subarachnoid space of the lumbar region in order to obtain spinal fluid for diagnostic purposes or to inject a drug.

luteal phase (menstrual cycle)

Phase of the human menstrual cycle, usually lasting 14 days, beginning with formation of the corpus luteum and ending, in the absence of fertilization, with the onset of menstruation.

Modified from [5]

luteinizing hormone (LH)

Hormone made by the pituitary gland that acts on the ovary to control egg maturation and to trigger ovulation. The same hormone acts in the testes to trigger production of testosterone.

luteinizing hormone-releasing hormone (LHRH)

See gonadotropin-releasing hormone.


Transparent fluid, containing chiefly lymphocytes, that bathes the tissues and drains through a system of vessels (the lymphatic system) into the venous bloodstream through the thoracic duct.


Relating to lymph or its secretion.


Relating to or denoting tissue responsible for producing lymphocytes and antibodies.

  • Note:

    Lymphoid tissue occurs throughout the body in lymph nodes, thymus, tonsils, and spleen.

Lyon hypothesis

Lyon law



Hypothesis (named after MaryLyon) that random inactivation of all but one X chromosome occurs in each female mammalian cell in early development, thus explaining why the effect of the X chromosome on phenotype is the same in males with one copy as in females with two copies.

  • Note 1:

    The Barr body, visible in some female cells, is an inactivated X chromosome.

  • Note 2:

    This phenomenon leads to mosaicism for X-linked genes in the female, since the paternal X chromosome is inactivated in some cells and the maternal one in others.

  • Note 3:

    The Lyon hypothesis was designated the Lyon law by the European Molecular Biology Organization (EMBO) in 2011.

lysosomal storage disease

One of a group of rare inherited metabolic disorders resulting from a defect in lysosomal function, commonly a deficiency of enzymes required for the metabolism of lipids, glycoproteins, or mucopolysaccharides, resulting in their accumulation in the cell.




Congenital disorder characterized by an abnormally large head and (or) brain.


Congenital disorder characterized by an abnormally enlarged tongue.

  • Note:

    This is often seen in Down syndrome.



Congenital disorder characterized by enlargement or elongation of the jaw.



Congenital disorder characterized by an abnormally large limb or limbs.


Congenital disorder characterized by an abnormally large body or body part.


Congenital disorder characterized by an abnormally large size of the mouth.

male efferent duct

See efferent duct.


Structural defect as a result of abnormal development.


Opposite term: benign.

  1. Occurring in a severe form, tending to become progressively worse, resistant to treatment, and likely to result in death.

  2. In cancer, cells showing both uncontrolled growth and a tendency to invade and destroy other tissues.

After [1]

mammary gland

Milk-producing organ in female mammals.

Marfan syndrome

Congenital connective tissue disorder associated with mutation of the fibrillin-1 gene FBN1.


Development of male characteristics, such as facial hair, either as part of normal male maturation, or, pathologically, by people of either sex as a result of hormonal imbalance.

maternal serum alpha-fetoprotein (MSAFP)

Protein made in the fetus that normally leaks, in small amounts, into the mother’s circulation.


  • Note:

    If there is an abnormal opening in the fetus, such as a neural tube defect, larger amounts appear in the mother’s serum, providing a screening test for such fetal anomalies.


Relating to the maxilla (upper jaw).

Meckel diverticulum

ileal diverticulum

Congenital pouch in the ileum resulting from incomplete closure of the yolk sac.


First intestinal discharges of the newborn infant, greenish in color and consisting of epithelial cells, mucus, and bile.


median teratogenic concentration (TC)

Median concentration resulting in developmental malformations for 50% of exposed test animals within a predetermined time, e.g., 96 h.



Region in the middle of the thorax between the two lungs, containing the regional vessels, trachea, esophagus, bronchi, lymph nodes, and heart.


  1. medulla oblongata

    Lower part of the brain stem continuous with the spinal cord, containing neural centers regulating the autonomic functions of breathing, heart rate, and blood pressure.

  2. Middle (central) part of an organ, as in adrenal medulla, renal medulla, or the medulla of a lymph node.


Condition of extreme dilation of the colon that can be congenital (as in Hirschsprung disease) or acquired (as when children refuse to defecate).

  • Note:

    Aganglionic megacolon is a condition, where parts of the colon have abnormal motor activity, resulting in spasms and massive distention of the colon proximal to the spasm.



Disorder characterized by an abnormally large fingers and (or) toes.


Process of “reductive” cell division, occurring during the production of gametes, by means of which each daughter nucleus receives half the number of chromosomes characteristic of the somatic cells of the species.


  • Note:

    Not to be confused with miosis.


  1. In anatomy, thin layer of tissue separating or connecting structures or organs.

  2. In cell biology, phospholipid-based bilayer structure, surrounding and isolating cells and organelles.


First menstrual cycle of a woman.

Mendelian gene

Gene located in a chromosome that obeys the laws of Mendelian inheritance.


Mendelian inheritance

Inheritance in which stable and indivisible characteristics are controlled entirely or overwhelmingly by a single genetic locus and transmitted over many generations.


Mendelian trait

Phenotype that shows a pattern of Mendelian inheritance.

meninx (s)/meninges (pl)

Any of the three membranes (the dura mater, arachnoid, and pia mater) that line the skull and vertebral canal and enclose the brain and spinal cord.


Herniation of the membranes of the brain or spinal cord (see meninx) through a defect in the cranium or spinal column.

After [7]



Birth defect following failure of the neural tube to close, resulting in protrusion of a sac of nerve tissue and its covering membranes.



Cessation of reproductive capability in a woman, marked by declining ovarian function and an end to menses.

menstrual cycle

The period, normally lasting 28 days in the human female, during which an ovum matures, is ovulated, and enters the uterus through the Fallopian tubes. If fertilization occurs, the cycle is interrupted by pregnancy; otherwise, the cycle ends with menstruation.



Cyclic endometrial shedding and discharge of a bloody fluid from the uterus during the menstrual cycle.



Congenital absence of a part of a limb, resulting in a shrunken and deformed extremity.

See also amelia, hemimelia, phocomelia.

After [7]



Embryonic migratory cells derived from the neural crest of the head that contribute to the formation of the meninges (see meninx) and become pigment cells.




Part of the vertebrate brain that develops from the middle section of the embryonic neural tube.

mesenchymal-to-epithelial transition (MET)

Process in which motile mesenchymal cells undergo a development to planar, polarized fixed cells, forming epithelia.

  • Note:

    This transition occurs in both normal development and tumor metastasis.

See also epithelial-to-mesenchymal transition.

mesenchyme (n)/mesenchymal (adj)

Meshwork of embryonic connective tissue in the mesoderm, from which are formed the bone, muscular, and connective tissues of the body and also the urogenital system, blood vessels, and lymph vessels.



Membranous sheet attaching various organs to the body wall, especially the peritoneal fold attaching the intestine to the dorsal body wall.


mesoderm (n)/mesodermal (adj)

embryonic mesoderm

Middle layer of cells in the embryo, lying between the ectoderm and the endoderm. It includes the following tissues:

  1. Lateral mesoderm: peripheral portion of intraembryonic mesoderm.

  2. Intermediate mesoderm: origin of the nephrogenic cord.

  3. Parietal (somatic) mesoderm: cell source for the formation of the lateral and ventral body wall.

  4. Visceral (branchial, pharyngeal, splanchnic) mesoderm:inner layer of lateral mesoderm that, with the endoderm, provides the cells from which the gut and lungs and their coverings arise.

See also mesenchyme.


Part of the embryonic mesentery that is attached to the early stomach.

mesonephric duct

Wolffian duct

Embryonic duct of the mesonephros, which in the male becomes the vas deferens and in the female becomes vestigial.


Middle part of the embryonic kidney in vertebrates, becoming the adult kidney in fishes and amphibians and the epididymis in reptiles, birds, and mammals.

See also pronephros, metanephros


Single layer of flattened cells forming an epithelium that lines serous cavities such as the peritoneum, pleura, and pericardium.

After [7]

messenger RNA (mRNA)

RNA that results from transcription of genes coding for polypeptides.


Morphogen, including bone morphogenetic protein, that affects metabolism and homeostasis.


One of a family of low-molecular mass proteins that binds metals such as zinc, copper, cadmium and mercury.


Primordium of the permanent kidney, developing later than, and caudal to, the mesonephros.



Stage of mitosis or meiosis in which the chromosomes become aligned on the equatorial plate of the cell, separating the centromeres.


Abnormal transformation of an adult, fully differentiated tissue of one kind into a differentiated tissue of another kind.



Attachment of a methyl group to a molecule.

  • Note:

    Methylation of DNA on cytosine bases is an epigenetic event that alters gene expression.


Congenital occurrence of an abnormally small head.


Congenital occurrence of abnormally small hands.


Congenital occurrence of an abnormally small tongue.


Congenital occurrence of abnormally small jaws, especially effecting the mandible.

micromass culture

Laboratory technique in which dispersed cells from an embryonic organ or tissue, such as the brain, limb bud, or cartilage, are allowed to reaggregate in culture.

After [12]

micronucleus test

  • 1.

    Test for mutagenicity in which animals are treated with a test agent, after which time the frequency of micronucleated cells is determined.

  • Note:

    If a test group shows significantly increased levels of micronucleated cells compared to a control group, the chemical is considered capable of inducing chromosomal damage.


  • 2.

    Procedure to detect clastogenic or aneugenic agents by microscopic examination of chromosomes.

  • Note:

    The test can be performed in vivo in rodents, or in cell culture (in vitro mammalian cell micronucleus test).


Abnormally small penis.


Congenital occurrence of abnormally small eyeballs.

microsatellite (in genetics)

short tandem repeat (STR)

Non-coding segment of DNA consisting of short nucleotide sequences (2–6 base pairs), typically occurring in 10–100 consecutive (tandem) repeats.

  • Note 1:

    The number of repeats varies between members of any given species.

  • Note 2:

    Microsatellites are used as markers in determining genetic diversity, in identifying important genetic traits, in forensic science, in population studies, and in determining paternity.


Spherical vesicle, prepared by tissue fractionation methods, that is rich in membranes of the endoplasmatic reticulum

  • Note:

    The microsomal fraction obtained in this way is often used as a source of monooxygenase enzymes in drug metabolism studies.

After [1]


Congenital occurrence of an abnormally small opening of the mouth.



Middle section of the digestive tract in a vertebrate embryo, from which the ileum, jejunum, and portions of the duodenum and colon develop.

Minamata disease

Neurological disease caused by methylmercury, first seen in subjects ingesting contaminated fish from Minamata Bay in Japan.



variable number tandem repeat (VNTR)

Non-coding segment of DNA, found throughout the genome, usually near the ends of chromosomes, that consists of tandem repeats of sequences of about 10–100 base pairs.

  • Note:

    These VNTRs are useful for genetic research and analysis.

Compare microsatellites.


Lay term for spontaneous abortion.

mitosis (n)/mitotic (adj)

Process by which a cell nucleus divides into two daughter nuclei, each normally having the same genetic complement as the parent cell: nuclear division is usually followed by cell division.


molecular epidemiology

Use in epidemiological studies of techniques of molecular biology, such as DNA profiling and genetic analysis, with the aim of detecting genetic patterns characteristic of susceptible populations or of groups at risk of disease.


Congenital absence of one eye.


Disorder in which body cells have only one pair instead of the normal two pairs of a particular chromosome.

monozygotic twin

See identical twin.

morphogen (n)/morphogenetic (adj)

Any of various signalling factors in embryonic tissue that influences the movement and organization of cells during morphogenesis by forming a concentration gradient.


Shaping of an organism during embryological development by differentiation of cells, tissues, organs, and organ systems, according to the genetic program of the organism, and influenced by morphogens and environmental conditions.

After [15]


Pertaining to structure or form.


Early multi-celled stage of the embryo from which the blastocyst is formed.



Condition in which an individual or an organism that develops from a single zygote has two or more cell populations that differ in genetic constitution.

  • Note:

    Mosaicism is seen in humans in Down syndrome, Turner syndrome and Kleinfelter syndrome.

After [8]

motility, sperm

Energy-dependent forward movement of the sperm, allowing it to reach and penetrate the egg.

  • Note:

    Decreased sperm motility is an indicator of reduced fertility.


See mucous membrane.

mucous membrane

Layer of epithelial tissue with a thin underlying layer of connective tissue (the lamina propria) that lines many body cavities, including the gut, respiratory passages, and tubular organs; it secretes, and is covered in, mucus.

mucus (n)/mucous (adj)

Slippery viscous fluid, consisting largely of water and glycoproteins, secreted by mucous membranes.

Müllerian duct

See paramesonephric duct.

Müllerian inhibiting factor (MIF)

See anti-Müllerian hormone.

Müllerian-inhibiting hormone (MIH)

See anti-Müllerian hormone.

Müllerian inhibiting substance (MIS)

See anti-Müllerian hormone.

multicotyledonary placentation

Formation of a placenta with many lobes.


multifactorial inheritance

Transmission from parents to offspring of a trait that is determined by multiple genetic and environmental factors, each with a small effect.


multigenerational study

  1. Animal test of reproductive toxicity in which two to three generations of the test organism are exposed to the substance being assessed.

  2. Animal test of reproductive toxicity in which only one generation is exposed and effects on subsequent generations are assessed.

After [1]

mutagen (n)/mutagenic (adj)

Agent that can induce heritable changes (mutations) in the genotype of a cell, as a consequence of alterations in, or loss of, genetic material.



Any relatively stable heritable change in genetic material that may be a chemical transformation of an individual gene (gene or point mutation), altering its function; or a rearrangement, gain, or loss of part of a chromosome, which may be microscopically visible (chromosomal mutation).

  • Note:

    A mutation can be either germinal and inherited by subsequent generations, or somatic and passed through cell lineage by cell division.


mutation (in immunology)

See somatic hypermutation.


Very small infectious microorganism, related to bacteria, but without a cell wall.

  • Note:

    Genital mycoplasmas may lead to spontaneous preterm labor or perinatal morbidity and mortality.


Part of the brain in the embyo that gives rise to the medulla oblongata.

myelination (n)/myelinated (adj)

The acquisition, development, or formation of a myelin sheath around a nerve fiber.

myeloblast (n)/myeloblastic (adj)

Immature bone marrow cell that is a precursor of the granulocyte series.


Harming bone marrow or any of its components.


Mononucleate cell that is committed to differentiate into muscle.



Inflammation of the heart muscle.


Muscle of the heart.


Muscular wall of the uterus.


Inflammation of muscle.


In the embryo, that part of a somite that develops into skeletal muscle.


Relating to the nose.


Rate of birth, the number of newborn individuals per unit time.

  • Note:

    Natality and mortality graphs together form a life table.



See umbilicus.


Postmortem examination of the organs and body tissue of a non-human animal to determine cause of death or pathological condition.

Compare autopsy.

necrosis (n)/necrotic (adj)

Sum of morphological changes resulting from cell death by lysis and (or) enzymatic degradation, usually accompanied by inflammation and affecting groups of cells in a tissue.

  • Note:

    Distinct from apoptosis, autophagy, and other modes of cell death.


neonate (n)/neonatal (adj)

Newborn animal or human infant during the first four weeks of postnatal life.

  • Note:

    For statistical purposes, some scientists have defined the period as the first seven days of human postnatal life. The precise definition varies from species to species.

After [1]


New and abnormal formation of tissue as a consequence of growth by cell proliferation, which may continue after the initial stimulus that initiated the proliferation has ceased, and which may develop into a tumor.


  1. Pertaining to a nerve or to the nervous system.

  2. Pertaining to the dorsal side of the vertebral bodies or to their precursors.

Compare hemal.

After [7]

neural arch

  1. Dorsal bony covering of the spinal cord.

    Syn. vertebral arch.

  2. Cartilaginous structures surrounding the embryonic spinal cord.

neural crest

Band of cells on either side of the neural tube.

  • Note:

    Cells from the region of the neural crest migrate to form parts of the nervous system, face, skin, and heart.

After [12]

neural plate

Area in the middle of the early embryo that rolls up to form the neural tube.

After [12]

neural tube

Embryonic tubular structure that becomes the brain and spinal cord.

After [12]

neural tube defect

Failure of the neural tube to close properly during gastrulation.

See also anencephaly, spina bifida.


Pertaining to the function of the nervous system as it relates to behavior.



Cell derived from neural stem cell that will differentiate into a neuron.

See also neuroblastoma.


Malignant neoplasm consisting of poorly differentiated embryonic nerve cells (neuroblasts).


Central region of the early embryonic ectoderm that will give rise to the brain and spinal cord, together with the neural crest cells that will form the peripheral nervous system.

neuroendocrine (system)

Combined cooperating network of the nervous system and hormonal systems.


Pertaining to the nervous and endocrine systems in anatomical or functional relationship.


Posterior lobe of the pituitary gland, involved in the storage and secretion of oxytocin and vasopressin.

neuron (n)/neuronal (adj)

Nerve cell.


Opening at either the cranial (caudal) or anterior (rostral) end of the neural tube before it completes closure in the early embryonic stage.


One of several protein growth factors that regulates development, maintenance, and function of the vertebrate nervous system.


Formation of the embryonic neural plate and its rolling up into the neural tube.


Projection on the apex of the breast through which the ducts of the milk-producing glands open.

After [7]

nodal (protein)

Growth factor of the transforming growth factor β (TGF-β) superfamily, coded by a gene designated NODAL, that plays crucial roles in embryogenesis, particularly in signalling from the primitive streak to the mesoderm to establish left-right assymetry as well as aspects of stem cell differentiation.

  • Note:

    The name derives from expression of the gene in the primitive node.

noggin (NOG)

Signaling protein released by the notochord, important in somite patterning and nervous system development.

See also Spemann organizer.


Failure of chromosomes to separate during meiosis, resulting in an uneven distribution of chromosomes in the two gametes (24 and 22 in humans).




Catecholamine hormone acting as a postganglionic adrenergic mediator at α- andβ-adrenergic receptors.

  • Note 1:

    Noradrenaline is also stored in, and released from, chromaffin granules in the adrenal medulla (see adrenal gland).

  • Note 2:

    Noradrenaline has strong vasoconstrictive effects.


See noradrenaline.


Rod-shaped structure of cells derived from mesoderm, lying ventral to the neural tube and defining the primitive axis of the embryo.

nuclear spindle

mitotic spindle

Structure, formed from microtubules, that draws the newly duplicated chromosomes apart during mitosis and meiosis.

nuclear type I receptor

Steroid hormone receptor, found in the cytosol, that upon ligand binding translocates into the nucleus and interacts with DNA as a homodimer to drive specific gene transcription.

Compare nuclear type II receptor.

nuclear type II receptor

Steroid hormone receptor, found in the nucleus, that upon ligand binding interacts with DNA as a heterodimer (usually with the retinoid X receptor, RXR) to drive specific gene transcription.

Compare nuclear type I receptor.


Never having been pregnant.

Compare nulliparous.


Never having given birth to a live infant.

Compare nulligravida.

occiput (n)/occipital (adj)

Back part of the head or skull.


Dentin-forming cell of the pulp cavity of a tooth, arising from the mesenchyme of the neural crest.


Child or children of a person, or the young of an animal, in relation to the parent(s).


Congenital occurrence of fewer than the usual number of digits, resulting in humans having fewer than five fingers or toes on a hand or foot.


Congenital absence of some of the teeth.


Infrequent or scanty menstruation.




Subnormal concentration of sperm in the male ejaculate (see ejaculation).


Fold of peritoneal membrane passing between the stomach and another abdominal organ.

After [7]

See also omentum, greater; omentum, lesser.

omentum, greater

Omentum passing from the greater curvature of the stomach in front of the small intestine, folding back on itself to fuse into four layers of peritoneal membrane, and ascending to the transverse colon.

  • Note:

    The greater omentum functions in fat deposition, contains islands of macrophages that contribute to immune function, and can delimit areas of traumatic tissue damage or infection.

omentum, lesser

Double-layer of peritoneal membrane (omentum) passing between the lesser curvature of the stomach and the duodenum.



Congenital herniation of abdominal viscera into the base of the umbilical cord.

Compare gastroschisis.


Lesser developed of two monozygotic twins that failed to separate completely during embryogenesis.

  • Note:

    The omphalosite is joined to the other twin (the autosite) by the umbilical vessels, receives its blood supply from the placenta, and is incapable of survival after separation from the placenta.

one-generation reprotox study

Procedure in which male rodents are dosed with a test substance for at least one spermatogenic cycle, and females for two estrous cycles, and then further exposed during mating, pregnancy, and nursing; adverse effects on reproduction, parturition, lactation and postnatal growth are studied.

Compare extended one-generation reprotox study.

See also two-generation reprotox study.



Immature precursor of the ovum resident in the ovary.

oocyte, primary

Oocyte during growth phase before completion of the first meiotic division.

  • Note:

    The primary oocyte is diploid and becomes a secondary oocyte before birth.

oocyte, secondary

Oocyte in which the first meiotic division to a haploid cell has been completed.

Compare oocyte, primary.


Process of formation and development of an ovum.


See also oocyte; oocyte, primary; oocyte, secondary; oogonium.


Primitive germ cell that proliferates by mitosis and develops into a primary oocyte prior to birth.

After [7]

open reading frame (ORF)

DNA sequence that begins with an initiation codon and ends with a termination codon that codes for and is potentially translatable into polypeptide.


Relating to the eye or vision.

orbit (in anatomy)

eye socket

Cavity in the skull of a vertebrate that contains the eye.

organ of Corti

spiral organ

Specialized collection of epithelial hair cells in the inner ear, involved in hearing.


Formation and development of organs.

orofacial cleft

Failure of the lip or palate to fuse properly.

See also facial cleft.

oropharyngeal membrane

See buccopharyngeal membrane.


See osteogenesis.


Fibroblast-derived bone-forming cell that produces a collagen type-I-rich matrix; this matrix calcifies to become bone.


Large multinucleate cell that resorbs bone, allowing for the deposition of new bone.

  • Note:

    Osteoclasts secrete enzymes and acids that dissolve the calcium phosphate matrix of old bone tissue.


Laying down of new bone tissue.


Opening into a vessel or cavity of the body.



Pertaining to, or located near the ear.


Congenital anomaly characterized by the absence or extreme underdevelopment of the lower jaw, producing closeness of the ears below the face.

See also agnathia, synotia.

ovarian cycle

Sequence of events occurring in the ovary involved in ovulation.

  • Note 1:

    It consists of the follicular phase with maturation of the ovarian follicle under control of follicle stimulating hormone (FSH), ovulation with release of the secondary oocyte, and the luteal phase in which FSH and luteinizing hormone drive formation of the progesterone-producing corpus luteum.

  • Note 2:

    Together with the uterine cycle, it comprises the menstrual cycle.

ovarian follicle

Cavity in the ovary containing a maturing ovum, at any stage of development, surrounded by its encasing cells.

After [5]

ovary (n)/ovarian (adj)

One of the paired female reproductive glands containing the ova (see ovum).



Abnormal gonad in which both ovarian and testicular tissues are present, and thus a form of hermaphroditism.

After [7]


Release of a mature ovum from the ovary into the Fallopian tube.


Mature female sex cell, capable of undergoing fertilization.

See also egg, gamete, oocyte.


Peptide hormone secreted by the neurohypophysis that stimulates contraction of the myometrium during labor and secretion of milk during lactation.

  • Note:

    Oxytocin is used clinically in the induction of labor and in the management of postpartum hemorrhage.

palate (n)/palatine (adj)

Roof of the mouth, separating the oral and nasal cavities.

  • Note 1:

    In embryonic development an anterior, primary palate is distinguished from a posterior, secondary palate.

  • Note 2:

    The embryonic primary palate gives rise to an anterior bony part called the hard palate, and a posterior muscular part called the soft palate.

See also orofacial cleft.

palatine raphe

Medial central ridge of the palate.


See cleft palate.

Papanicolaou smear

Papanicolaou stain

Pap smear

Pap test

cervical smear

Examination by light microscopy of a sample of cells scraped from the uterine cervix, used to screen for cervical cancer.


Type of signaling in which a cell secretes into the intercellular space a molecular messenger that diffuses and binds to receptors on nearby target cells, producing a signal in those cells.

After [3]

parafollicular cell

C cell

Neuroendocrine cell that migrates into the thyroid gland during embryonic development and secretes the hormone calcitonin.

paramesonephric duct

Müllerian duct

One of two paired embryonic ducts of mesodermal origin that will become the Fallopian tubes, uterus, cervix and upper part of the vagina in the female, and will regress in the male.


Connective tissue of the floor of the female pelvis that lies in front of the uterine cervix and separates it from the bladder.

parasitic twin

Smaller of unequal conjoined twins.


See also autosite.

paraxial mesoderm

somitic mesoderm

Area of mesoderm in the embryo that forms during neurulation, lies along both sides of the neural tube, and gives rise to the somites.

parenchymal cell

Distinguishing or specific type of cell of an organ or gland, contained in, or supported by, the connective tissue framework provided by the stromal cells.

After [7]


Relating to or denoting the wall of the body or of a body cavity or hollow structure.

parietal cell, gastric

Acid-producing cell in the stomach.



Type of nonsexual reproduction in which an unfertilized ovum develops into an embryo.


Process of giving birth by a maternal organism.

patent ductus arteriosus

Congenital disorder wherein the ductus arteriosus fails to close after birth.

  • Note:

    This may lead to failure to thrive and increased breathing in early life.

pelvic kidney

ectopic kidney

Congenital abnormality in which a kidney develops and remains in the pelvic area.

  • Note:

    Pelvic kidney is generally asymptomatic but may produce complications.


Serous membranous sac that surrounds the heart, consisting of an inner visceral layer (the epicardium), and an outer parietal pericardium that is attached to the sternum and diaphragm.


Relating to the period from shortly before to shortly after birth, in humans usually from the 20th to the 29th week of gestation to 1 to 4 weeks after birth.

After [1]


Muscular structure of the lower pelvis, between the legs and extending from the anus to the pubic symphysis, or alternatively including the anus and vagina in the female and the region from the anus to the base of the scrotum in the male.

peritoneal cavity

Potential space between the two layers (visceral and parietal) of the peritoneum.

peritoneal membrane

See peritoneum.

peritoneum (n)/peritoneal (adj)

Thin double layer of mesothelium and irregular connective tissue that lines the abdominal cavity and covers most of the abdominal organs.

  • Note:

    The two layers of the peritoneum consist of the parietal peritoneum that lines the abdominal wall and the visceral peritoneum that covers the abdominal organs.

See also peritoneal cavity.

phage display

Method that enables the presentation of large peptide libraries on the surface of phage particles, from which proteins with desired functional properties can be selected rapidly.

After [11]

pharyngeal groove

One of several paired grooves in the embryonic endoderm, lateral to the corresponding pharyngeal pouch in the ectoderm.

pharyngeal pouch

branchial pouch

One of several paired evaginations in the embryonic endoderm that develop into epithelial tissues and organs such as the thymus and thyroid gland.

After [7]

pharynx (n)/pharyngeal (adj)

Part of the digestive tube lying between the esophagus (below) and the mouth and nasal cavity (above and anterior).

  • Note:

    The pharynx is subdivided into the nasopharynx lying at the base of the nasal cavity, the oropharynx behind the mouth, and the laryngopharynx posterior to the larynx.

phenotype (n)/phenotypic (adj)

Observable characteristics or traits of an organism, including those at a biochemical level, resulting from interaction of its genotype with epigenetic and environmental factors.

phenylketonuria (PKU)

Autosomal recessive disorder resulting from mutations that impair the function of the enzyme phenylalanine 4-monooxygenase (EC

  • Note 1:

    The resulting failure of phenylalanine metabolism in the liver leads to its accumulation, causing mental retardation, seizures, and other neurological disorders in untreated individuals.

  • Note 2:

    The excess phenylalanine is metabolized to phenyl pyruvate (“phenylketone”) that is detected in the urine and is used for screening newborns.


Groove in the midline of the upper lip below the nose.


Congenital abnormality in which one or more of the hands or feet is attached to an underdeveloped limb, and therefore too close to the body.

  • Note:

    This very rare disorder is associated with prenatal exposure to thalidomide.


Non-steroidal natural product from a plant that, because of a structural similarity to 17β-estradiol can exert mild estrogenic effects, or antagonize them.

  • Note 1:

    Examples include isoflavones and coumestans.

  • Note 2:

    While foods containing phyto-estrogens are often promoted as beneficial, medical benefits are not well established.

pia mater

See meninx.

piebaldism (n)/piebald (adj)

Patchy absence of pigmentation of the hair or skin or, depending on the species, of feathers, scales, or other body surfaces.

pineal gland


Pea-sized conical mass of tissue behind the third ventricle of the brain, secreting the hormone-like substance melatonin in some mammals.


  1. Externally visible portion of the ear.

    See also auricle.

  2. Feather or fin.



pituitary gland

Small endocrine gland sitting in a bony cavity (the sella turcica) at the base of the brain and connected to the hypothalamus.

  • Note:

    The pituitary is structurally and functionally divided into anterior and posterior lobes. The anterior lobe secretes growth hormone (somatotropin), thyroid-stimulating hormone (TSH), adrenocortocotropic hormone (ACTH), prolactin (PRL), luteinizing hormone (LH), and follicle-stimulating hormone (FSH). The posterior lobe develops from the hypothalamus and secretes oxytocin and antidiuretic hormone (ADH) (also called arginine vasopressin (AVP)). An intermediate lobe (indistinct in humans) secretes melanocyte-stimulating hormone (MSH).


Organ of exchange of nutrients and waste products between mother and fetus, having parts derived from both.

See also placental barrier.

placental barrier

placental membrane

Multilayered membrane of fetal tissue within the placenta that separates the maternal blood from the fetal blood and allows selective passage of substances between the two.

  • Note:

    This membrane blocks only a few xenobiotics.

placental circulation

Circulation of blood through the placenta during intrauterine life, serving the needs of the fetus for oxygenation, nutrition, and elimination of waste.

After [7]

placental insufficiency

Inadequate blood flow through the placenta to maintain the needs of the fetus for oxygenation and metabolic balance.

See also placental circulation.

placental membrane

See placental barrier.

placental transfer

Delivery of substances from the maternal circulation to the fetus, across the placental barrier.

pleiotropism (n)/pleiotropic (adj)

  1. Having more than one effect.

  2. Relating to a gene, expression of which gives rise to multiple phenotypic traits.

pleura (n)/pleural (adj)

Each of a pair of serous membranes lining the thorax and enveloping the lungs in humans and other mammals.

  • Note:

    The pleural membrane covering the lungs and extending into the fissures of the lobes is called the visceral pleura and lacks a nerve supply. That lining the chest cavity, mediastinum, and diaphragm is called the parietal pleura; it is innervated and sensitive to pain.

pleural cavity

Potential space between the pleural membranes lining the chest cavity (parietal pleura) and covering the lungs (visceral pleura).

  • Note:

    In pathological conditions, fluid may accumulate in the pleural cavity, between the visceral and parietal pleura.


Number of haploid sets of chromosomes in a cell.


Able to differentiate into a variety of cell types.

  • Note:

    Examples are the ovum and embryonic stem cells.


Composed of many cysts.


Congenital occurrence of supernumerary fingers or toes.

polymorphism (n)/polymorphic (adj)

  • 1.

    Occurrence of a gene in more than one distinct nucleotide sequence in a population, resulting from mutation(s) and potentially producing gene products with different levels of function.

  • Note:

    It is sometimes considered that the least frequent of polymorphic gene sequences should be present in at least 1% of the population; otherwise the variant is a mutation rather than a polymorphism.

  • 2.

    Occurrence of different phenotypes of a single trait.


Occurrence of more than two complete sets of chromosomes.

See also ploidy.


Occurrence of a cyst in the substance of the brain that usually communicates with one of the lateral ventricles.

  • Note:

    These abnormal cavities may result from brain tissue destruction or maldevelopment.


Occurring after the early embryo embeds in the lining of the uterus.


Opposite term: antemortem.

After death.


Referring to events in the life of the mother or fetus following birth.


Period following the birth of an infant, often considered to be about six weeks in the human.

predatory mite test

Procedure in which adult females of the predatory mite (Hypoaspis (Geolaelaps) aculeifer, considered representative of soil fauna) are exposed to a test substance in artificial soil and the number of surviving females and of juveniles is determined.


Pathological condition in pregnancy with symptoms of high blood pressure, kidney dysfunction and edema.

See also eclampsia.


Fertilized ovum up to 14 days of age and before implantation.

pregnancy (n)/pregnant (adj)

State of a female between conception and the termination of gestation.


Occurring before the early embryo embeds in the lining of the uterus.

premature birth

Delivery before the expected length of gestation for the species; in humans taken as birth before a gestational age of 37 weeks.



Referring to events in the life of the mother or fetus preceding birth.

prenatal development study

Procedure in which a substance is administered to pregnant animals (usually rodents or rabbits) from the time of implantation until closure of the hard palate. The animals are killed one to two days before a scheduled delivery, whereupon uterine contents and the fetuses are evaluated for abnormalities.

preoptic area

preoptic region

Region of the anterior hypothalamus primarily involved in regulating body temperature.


Stage of human development before the onset of puberty.

Compare prepubescence.


Period before pubescence.

  • Note:

    Whereas prepubertal refers to any time before puberty, prepubescence refers less specifically to a time closely preceding early puberty.

See also pubescent.


Skin protecting the glans penis in the male (foreskin) or the clitoris in the female (clitoral hood).

preputial separation

Separation of the prepuce from the glans penis.


See subthalamus.

primary sex organ

Gonads, which form gametes; ovaries in the female and testes in the male.

See also accessory sex organ.

primitive axis

Thickening of the embryonic tissue extending forward from the primitive streak.

primitive groove

Shallow depression in the primitive streak that extends anteriorly to communicate with the yolk sac.

primitive node

primitive knot

Group of cells at the anterior end of the primitive streak, involved in secreting chemical signals that regulate differentiation of the germ layers during gastrulation.

primitive pit

Depression in the primitive node contiguous with the notochord.

primitive streak

Structure forming in the blastula during early embryogenesis that establishes bilateral symmetry and determines the site of gastrulation and germ layer formation.

primordial follicle

Early stage of the oocyte in which it is surrounded by a single layer of follicular cells.

See also follicle.

primordial germ cell

Germ cell in the earliest stages of development.


Organ or tissue at its earliest recognizable stage of development.


anal pit

Ectodermal depression of the caudal end of the embryo, where later the anus is formed.




Steroidal hormone produced in the ovary by the corpus luteum.

  • Note:

    Progesterone is involved in sexual development and the ovarian cycle, and essential for maintenance of pregnancy.


Synthetic variant of progesterone, often used in oral contraceptive pills or to suppress hyperplasia of the endometrium.



Member of a family of hormones, based on the 21-carbon pregnane skeleton, involved in maintaining gestation and regulating the ovarian cycle.

See also progesterone, progestin.


Hormone released from the anterior pituitary gland that stimulates milk production after childbirth.


Falling or slipping out of place of an organ or other body part.

  • Note:

    Uterine prolapse occurs when the ligaments holding the uterus weaken and the uterus slips down into, or even protrudes from, the vagina.


Collection of cells in the vertebrate embryo that represent the earliest stage of development of the kidney.


Haploid nucleus formed by either the head of the sperm or the nucleus of the oocyte after fertilization but before their nuclei fuse to form the diploid zygotic nucleus.


See pharynx.


First phase of meiosis or mitosis in which thickening and orientation of the chromosomes occurs.



Most forward part of the brain that includes most of the cerebral cortex, the thalamus and hypothalamus, and the basal ganglia.


Male exocrine gland whose secretion contributes alkaline fluid to the ejaculate (see ejaculation).

  • Note:

    The alkaline environment facilitates sperm transport and prolongs the survival of sperm in the acidic environment of the vagina.


Small, basic (arginine-rich) protein involved in sperm maturation and DNA packaging during spermatogenesis.

protein kinase

Any enzyme catalyzing protein phosphorylation, often for the purpose of signal transduction.


Global analysis of gene expression using a variety of techniques to identify and characterize proteins.

  • Note:

    In toxicology, proteomics can be used to study changes caused by exposure to substances and to determine if changes in mRNA expression correlate with changes in protein expression:the analysis may also show changes in post-translational modification, which cannot be distinguished by mRNA analysis alone.


proximal (in anatomy)

Nearer to an anatomical point of origin or attachment, or the midline of the body.

Opposite term: distal.


See pseudopregnancy.


Organism possessing either male (testes) or female (ovaries) gonads, but with ambiguous external genitalia.

Compare hermaphrodite.



false pregnancy

Development of signs of pregnancy in the absence of an embryo.

  • Note:

    This is commonly called pseudocyesis in humans and pseudopregnancy in other mammals.

psychomotor retardation

Delayed development of both cognitive and motor function.


Process in which an individual undergoes sexual development, including the onset of gametogenesis, changes in hormonal secretions, secondary sexual characteristics, and reproductive competence.


Being in the early stage of puberty.


Relating to the region of the most ventral bone of the pelvis (the pubic bone), and thus the region around the genital area, normally showing hair growth (pubic hair) in the adult.

pubic symphysis

Cartilaginous joint between the pubic bones forming the front of the pelvis.


Pertaining to the lungs.

pulmonary artery

Large vessel that transports blood from the heart to the lungs.

pulmonary valve stenosis

Condition in which the flow of blood from the heart to the lungs is slowed by a deformity on or near the pulmonary valve that controls the blood flow from the right heart to the lungs.


  1. (n) Young or newborn dog, rat or seal.

  2. (v) Give birth to a pup.

pyloric stenosis

Narrowing of the sphincter (the pylorus) between the stomach and duodenum.


Initial awareness of fetal movement by the pregnant mother, usually occurring early in the second trimester.


Failure of the neural tube to close in utero, resulting in a birth defect that leaves a fissure in the vertebral column with part of the spinal cord exposed.


Molecular component in or on a cell that is specifically recognized by and binds a molecular structure (ligand), leading to physiological signal transduction or the mediation of an effect.

receptor, nuclear

Receptor that encounters its ligand in the cytosol and translocates to the nucleus, where it binds DNA and acts as a factor regulating gene transcription.


In genetics, relating to or denoting heritable characteristics controlled by genes which are expressed in offspring only when inherited from both parents.

Compare dominant.

reciprocal mesenchymal-epithelial interaction

Process in tissue or organ development in which epithelium induces changes in the mesenchyme and the mesenchyme also induces changes in the epithelium, through paracrine signaling.

  • Note:

    An example occurs during development of the prostate, in which mesenchyme of the urogenital sinus influences differentiation and patterning of the prostatic epithelium, and the developing epithelium induces prostatic mesenchymal cells to differentiate into smooth muscle and other prostatic stromal cells.

See also epithelial-to-mesenchymal transition, mesenchymal-to-epithelial transition.

reciprocal translocation

Chromosomal translocation in which crossing over between two non-homologous chromosomes leads to each carrying genetic material from the other.

  • Note:

    If the exchange is equal (i.e., no genetic material is missing) it is called a balanced translocation, and may be without severe consequences for the developing embryo. If extra or missing genetic material results from the process, it is called an unbalanced translocation, and the consequences are often more harmful.

recombination, genetic

  1. Referring to new gene sequences, and thus to new heritable information, caused by gene crossovers, occurring naturally in either meiosis or mitosis, or constructed artificially for the purpose of introducing a new trait into an organism or cell line.

  2. In immunology, random scrambling of specific gene sequences to produce the vast array of possible immunoglobulin and T cell receptor sequences required to recognize, potentially, all possible antigens.

rectum (n)/rectal (adj)

Terminal part of the large intestine, ending at the anus.


Enzyme (EC that converts testosterone to 5α-dihydrotestosterone in peripheral tissues.

5α-reductase deficiency

Condition caused by a mutation of the 5α-reductase type II gene, with autosomal recessive inheritance.


Pertaining to the kidneys.

reproductive senescence

Progressive decline in reproductive capacity with age, marked by menopause in females.

reproductive toxicant

Agent that interferes with reproductive function, including sexual performance, fertility, and development of the fetus and embryo.

  • Note 1:

    Effects on fertility may include effects on sperm count and sperm viability, oogenesis and ovulation, placental function, lactation, male erectile function, genetic intactness, etc.

  • Note 2:

    Effects on development may include effects on embryo viability, fetal growth, morphogenesis, functional integrity, etc.

  • Note 3:

    Reproductive toxicants may show long-term effects not detectable before the second or later generations.

See also developmental toxicology.

reproductive toxicity test

Procedure based on specific guidelines in which adverse effects of a substance on reproduction and development are studied either in animals or by use of in vitro assays.


  1. Removal of mineralized tissue by natural processes, as in the breakdown of bone by osteoclasts or the disappearance of a tooth.

  2. Disintegration and assimilation of an embryo or fetus through a natural process involving lysis and removal of all the products of conception by cells of the maternal immune system.

  3. In multigestational pregnancies, death of one fetus followed by absorption of its tissue by another, sometimes referred to as a “vanishing twin”.

restriction fragment length polymorphism (RFLP)

Variations in the DNA nucleotide sequence that are indicated by the presence or absence of particular restriction sites in the DNA.

  • Note:

    RFLP analysis is a common method used for DNA profiling.

restriction site

Short DNA nucleotide sequence that is recognized by a sequence-specific enzyme (restriction enzyme) that cleaves the double stranded DNA.


Fibrous mesh or network.

rete testis

Rete in the testis that carries sperm from the seminiferous tubules.


Multicell-layer membrane in the back of the eye that is sensitive to light and transduces incoming optical information into signals that are necessary for vision.

retinoic acid

(2E,4E,6E,8E)-3,7-dimethyl-9-(2,6,6-trimethylcyclohex-1-en-1-yl)nona-2,4,6,8-tetraenoic acid

Acidic metabolite of Vitamin A.

  • Note 1:

    Retinoic acid binds to the retinoic acid receptor (RAR), which then acts as a transcription factor.

  • Note 2:

    The role of RAR in development (e.g., in regulating homeobox genes) accounts for the teratogenicity of related pharmaceuticals.


Member of a class of substances structurally related to vitamin A.

  • Note:

    Retinoids play roles in cell growth and differentiation, tumor suppression, immune function, and vision. At higher concentrations they cause multiple toxicities, including effects on the long bones, spleen and lymph nodes, and retina.


Noninflammatory degenerative disease of the retina.


  • Note:

    Retinopathy of prematurity (see premature birth) is a risk for preterm infants, possibly induced by oxygen therapy and related to disorganized retinal blood vessels.

retinoschisis, congenital

juvenile retinoschisis

X-linked retinoschisis

Congenital splitting of the retina into two layers.


Outside of and posterior to the peritoneum.

reverse genetics

Approach to elucidating the function of a gene by studying the effects on phenotype of expressed variations in a known DNA sequence.

  • Note:

    The term is intended to indicate the reverse of the approach (starting from a phenotype and working back to identify the gene) of forward genetics.

rhesus (Rh) factor

Antigen that occurs on the red blood cells of about 85 per cent of humans and can cause hemolytic disease of the newborn (erythroblastosis fetalis) and hemolytic transfusion reactions.

  • Note:

    The Rh factor was first identified in the blood of a rhesus monkey.



Posterior portion of the brain that includes the cerebellum, pons, and medulla.

RNA interference (RNAi)

Effect of small, often double-stranded RNA targeting a specific messenger RNA (mRNA), blocking its translation, and thereby silencing expression of the corresponding gene.

  • Note 1:

    Cells produce two types of small RNA, microRNA (miRNA) and small interfering RNA (siRNA), that are both involved in the regulation of gene expression.

  • Note 2:

    As an experimental technique, RNAi uses designed sequences to silence a gene of interest.

  • Note 3:

    Short hairpin RNAs (shRNA) are sequences, designed for improved effect in RNAi experiments, that have a base-paired stem representing the double-stranded portion and an intervening unpaired loop.

Robertsonian translocation

Chromosomal translocation in which chromosomes with acrocentric centromeres break the centromere to fuse the long arms into a new large chromosome and the short arms into another small one that may be lost, leaving a karyotype in the human of 45 chromosomes.


  1. Pertaining to, resembling, or having a rostrum or beak.

  2. Situated toward the beak (thus, oral and nasal region), which may mean superior (in relationships of areas of the spinal cord) or anterior or ventral (in relationships of brain areas).

    After [5]

  3. Closer to the head.

Compare caudal.


Smallest and weakest animal of a litter.

  • Note:

    A runt may be disadvantaged in competing with its siblings for resources and its mother’s attention.

sacral agenesis

caudal regression syndrome

Absence or significant underdevelopment of the lower part of the spine and the lower limbs.

  • Note:

    Sacral agenesis is associated with maternal diabetes.

After [12]


Vertical (longitudinal) plane dividing the body into right and left sections.

Salmonella test

See Ames test.


Surgical removal of the Fallopian tubes.

saturation mutagenesis

Mutagenesis screening in which a large number of mutations are introduced in a target area of the genome with the aim of identifying all the genes or their functions that are associated with that area.

scaphocephaly (n)/scaphocephalic (adj)

Elongated and narrow head with decrease of the parietal regions and conspicuous frontal and occipital protrusions; a type of craniosynostosis.

  • Note:

    Scaphocephaly is a result of the premature closure of the sagittal suture and is usually accompanied by mental retardation.


Abnormal lateral and rotational curvature of the vertebral spinal column.



Pouch of skin containing the testicles and their accessory tissues.

secondary sexual characteristic

Feature of the adult animal, appearing during puberty in humans and distinct between the sexes, although not part of the reproductive system.

  • Note:

    Examples include enlargement of the female breasts and sex-specific patterns of body hair, such as facial hair in the male and the male or female pubic escutcheon.

secondary spermatocyte

See spermatocyte.


Sudden change in neural activity, leading to convulsions and (or) changes in consciousness of varying degrees of severity.

See also epilepsy.

selector gene

Member of a group of genes that codes factors driving differentiation and regional patterning during development.

sella turcica

Depression on the upper surface of the sphenoid bone, accommodating the pituitary gland.

semen (n)/seminal (adj)

ejaculate (n)

seminal fluid

Fluid, containing spermatozoa, that is expelled from the penis during ejaculation.

seminal fluid

See semen.

seminal vesicle

One of a pair of glands of the male reproductive system that together produce a significant portion of the semen.

  • Note:

    Seminal vesicle secretion does not contain spermatozoa, but provides nutrients and an alkaline environment that prolongs their survival.

seminiferous tubule

One of numerous coiled tubes, found in the testis, in the walls of which spermatogenesis occurs.

sensitized strain

Model organism containing a mutation in a pathway (e.g., metabolic or signal transducing) that does not itself cause a change in phenotype, but makes the organism more sensitive to another change elsewhere in the pathway.

After [11]

septum (n)/septal (adj)

In anatomy, thin wall dividing two cavities or masses of softer tissue.


Containing, resembling, or secreting serum.

Sertoli cell

Somatic cell in the seminiferous tubule that supports spermatogenesis.

  • Note:

    Tight junctions between Sertoli cells contribute to the blood-testis barrier.

Sertoli-Leydig cell tumor

Ovarian tumor composed of both Sertoli and Leydig cells.

  • Note:

    The tumor may secrete androgens, thus causing masculinization.

sex chromatin

Barr body

Condensed mass of one inactivated X chromosome seen inside the nuclear membrane of an interphase cell.

  • Note:

    The Barr body is not seen in normal male cells (XY), but one body may be seen in normal female cells (XX). In chromosomal abnormalities, the number seen is one less than the number of X chromosomes.

See also Lyon hypothesis.

sex chromosome

One of the pair of chromosomes determining sex; in humans designated an X chromosome and a Y chromosome, with females having an XX genotype and males an XY genotype.

sex-determining region Y (SRY) protein

testis-determining factor (TDF)

Transcription factor coded by a gene on the Y chromosome that initiates development of the male sex organs (i.e., testicular differentiation) in the embryo.

sexual dimorphism

Phenotypic difference between the sexes of a species.

sexually dimorphic nucleus (SDN)

Compact area of large cells in the anterior hypothalamus, larger in men than women, and believed to influence sexual behaviour.

See also sexual dimorphism.

sexual maturation

Process of reaching the age or stage when an organism can reproduce sexually.

sexual maturity

Age or stage when an organism can reproduce sexually.

sexual reproduction

Creation of a new organism requiring combination of the genetic material from two different sexes.

Siamese twin

See conjoined twin.



One of two or more individuals with a common pair of parents.

signal transduction

Process whereby a signal arising outside the cell is converted through a series of intermediate chemical reactions inside the cell to produce a functional change in the cell.


single-nucleotide polymorphism (SNP)

Single base variation at a chromosomal locus, which exists stably within populations (typically defined as each variant form being present in at least 1 to 2 % of individuals).


sinoatrial node

sinuatrial node

Mass of specialized cardiac muscle cells (“pacemaker cells”) that spontaneously depolarize to initiate rhythmic contraction of the heart.


  1. Hollow or cavity.

  2. Channel for the passage of fluid, lacking the usual lining of a blood or lymphatic vessel; especially a dilatation for the passage of venous blood.

sinus venosus

venous sinus

Cavity at the caudal end of the developing embryonic heart where intra- and extra-embryonic veins meet.

  • Note:

    The sinus venosus develops into the portion of the right atrium in the adult heart that receives blood from the vena cava.


See RNA interference.

sister chromatid

See chromatid.

sister chromatid exchange (SCE)

Reciprocal exchange of chromatin between two replicated chromosomes that remain attached to each other until anaphase of mitosis.

  • Note:

    SCE is used as a measure of mutagenicity of substances that produce this effect.


situs inversus

Developmental anomaly in which major visceral organs are found in a mirrored position to their normal location.

See also dextrocardia.

small interfering RNA (siRNA) molecule

short interfering RNA

See RNA interference.


Pertaining to the body or describing cells that form the body other than germ line cells or undifferentiated stem cells.

somatic hypermutation (SHM)

Programmed process of mutation affecting the variable (V) regions of immunoglobulin (Ig) genes. SHM affects only individual immune cells, and the mutations are not transmitted to offspring.

  • Note 1:

    This process is part of the way the immune system adapts to new antigens.

  • Note 2:

    Mistargeted SMH is a likely mechanism in the development of B-cell lymphoma.


somatomammotropin, human chorionic

human placental lactogen (hPL)

Placental hormone that affects maternal metabolism and supports fetal growth by making more glucose and fatty acids available to the fetus.


Any of a group of peptide growth factors produced by the liver following stimulation by somatotropin.

  • Note:

    Somatomedin acts directly on cartilage cells to stimulate skeletal growth.


growth hormone

Hormone produced in the pituitary gland that stimulates the liver to produce somatomedin.


One of bilaterally paired, segmented masses of mesodermal tissue lying along the notochord that gives rise to the vertebrae and associated muscle and connective tissue.

sonic hedgehog

Protein morphogen produced in the notochord that plays a critical role in early development, including vertebrate organogenesis, brain development and limb formation.

  • Note:

    Production in the embryo is dependent upon secretion of fibroblast growth factors.

See also bone morphogenetic protein 4.


  1. (v) Release gametes. (Of mature adult fish, frogs, molluscs, crustacea, etc.).

  2. (n) Fertilized eggs of mature adult fish, etc.


  1. (n) Release of gametes (eggs or sperm) from mature adult fish, frogs, molluscs, crustacea, etc.

  2. (adj) Describing behavior related to the readiness of mature adult fish, etc. to release gametes.

Spemann organizer

Group of cells in the amphibian embryo that is important in orientation of surrounding cells and facilitates development of the central nervous system.

  • Note:

    Cells on the ventral side of the Xenopus blastula secrete factors such as bone morphogenic protein-4 (BMP-4) to signal the overlying ectoderm to become skin. The Spemann organizer blocks the action of BMP-4 by secreting chordin and noggin, allowing the ectoderm to develop into the brain and spinal cord by default.


See spermatozoon.

sperm banking

Storage of frozen donor sperm for use in artificial insemination.


Haploid cell in the late stage of developing into a spermatozoon, derived from the secondary spermatocyte.



Agent that kills spermatozoa.


Parent cell of a spermatid, derived by mitotic division from a spermatogonium.


  • Note:

    The primary spermatocyte gives rise to a pair of haploid secondary spermatocytes by meiosis.


Entire process by which spermatogonial stem cells divide and differentiate into spermatozoa.


spermatogonial chromosome aberration test

Procedure in which rodents are exposed to a test substance and chromosome aberrations are then studied microscopically in germ cells.


Primitive cell derived from mitosis of the germ cell that becomes the diploid primary spermatocyte.


sperm cell

Mature male gamete.


Release of mature spermatozoa from the Sertoli cells.


See spermatocide.


Maturation of an immature spermatid into a spermatozoon.

sphenoid bone


Irregular wing-shaped bone at the anterior base of the skull, forming part of the orbit, serving as an attachment site for muscles of mastication, and containing foramina for nerves and vessels of the head and neck.

  • Note:

    The complex structure, function, and development of the sphenoid bone have been linked with developmental disorders such as sphenoid dysplasia and cystic degeneration of the sphenoid.


Ringlike muscle that maintains closure of a body passage or orifice by contracting, and opens it by relaxing.

spina bifida

Failure of the neural tube to close, resulting in the absence of part of the vertebral arch at the midline of the spinal column and exposure of the spinal cord and its covering membranes.

spinal column


vertebral column


spinal cord

Portion of the central nervous system outside the brain and inside the vertebral column.

spiral arteriole

spiral artery

spiral endometrial artery

corkscrew-like artery

coiled artery of the uterus




Relating to the visceral organs, especially those of the abdomen.


Embryonic epithelial cell that develops into a neuroglial cell.


In anatomy, narrow connection with an organ or other structure.

After [7]

stalk, allantoic

Narrow connecting tube between the intra- and extra-embryonic parts of the allantois.

After [7]

stem cell

Multipotent cell, with mitotic potential, that may serve as a precursor for many kinds of differentiated cells.


  • Note:

    Embryonic stem cells give rise to the embryonic germ layers, whereas adult stem cells are involved in tissue regeneration, repair processes and possibly in carcinogenesis.

See also induced pluripotent stem cell.


Abnormal narrowing of a passage or opening in the body.

sterility (n)/sterile (adj)

  1. Infertility

  2. Asepsis.


  1. Process that makes an organism incapable of fertilization or reproduction, e.g., castration, vasectomy, or salpingectomy.

  2. Process that makes an object aseptic.

steroid (n)/steroidal (adj)

Naturally occurring compounds and synthetic analogues, based on the cyclopenta[a]phenanthrene carbon skeleton, partially or completely hydrogenated; there are usually methyl groups at C-10 and C-13, and often an alkyl group at C-17. By extension, one or more bond scissions, ring expansions, and/or ring contractions of the skeleton may have occurred.

After [16]

  • Note:

    Natural steroids are derived in biogenesis from triterpenoids; they play important roles as hormones (sex hormones), membrane components (cholesterol) and emulsifying agents (bile acids).

steroid 21-monooxygenase

Member (EC of the cytochrome P450 family of enzymes required for the synthesis of some steroid hormones including aldosterone and cortisol.

steroidogenesis assay

Procedure in which the effect of a test substance on steroid synthesis is studied, usually in a human adrenocortical carcinoma cell line.

  • Note:

    This assay is part of the OECD framework for the “Testing and Assessment of Endocrine Disrupting Chemicals”.


Birth of a baby without any signs of life, at a time variously taken as 20–28 weeks of pregnancy.

  • Note:

    The baby may have died during pregnancy (called intrauterine death), labor, or birth.

Compare miscarriage.

stress-response pathway

Signaling events induced when normal cell function or development is disrupted by environmental (physical or chemical) factors.

  • Note:

    Induction of stress pathways may lead to cellular repair and counteraction of stress effects, or to adverse effects.

stromal cell

Cell producing the connective tissue framework of an organ or other anatomical structure.

See also parenchymal cell.


Part of the diencephalon, of which the major part is the subthalamic nucleus, and which connects to the globus pallidus, part of the telencephalon.


Any long, narrow groove, furrow or shallow depression, specifically referring to one of the fissures on the surface of the brain.

After [7]


Fertilization of two or more ova released in the same period of ovulation, either by different males or by the same male in separate acts of intercourse.

  • Note:

    If there are two or more different fathers, the state may be called heteropaternal superfecundation. Rare in humans, heteropaternal superfecundation is common in some species, such as cats and dogs.


Presence of two fetuses in the uterus at different stages of development, resulting from fertilization of ova released in two successive periods of ovulation.

After [7]


Exceeding the usual number; thus, in anatomy, anomalies such as a sixth finger on one hand or a third nipple (in a human).

surfactant, pulmonary

Surface-active layer of phospholipids and proteins coating the pulmonary alveoli that increases lung compliance, facilitates lung expansion, and stabilizes alveolar volume.

  • Note:

    Lack of pulmonary surfactant is more common following premature birth and is a cause of infant respiratory distress syndrome.



Point-wise pairing of homologous chromosomes during the prophase of meiosis.

synaptonemal complex

Structure of filamentous proteins that forms between homologous chromosomes when they pair during meiosis; it may contribute to synapsis and genetic recombination.



Conjoined twins with two bodies and a fused head.


Outer syncitial layer of trophoblast cells that invades the endometrium during implantation.

  • Note:

    The syncytiotrophoblast is the site of synthesis of human chorionic gonadotropin and is involved in implantation.

syncytium (n)/syncytial (adj)

Referring to a large multinucleated mass of cellular contents that arises from the fusion of originally individual cells.


Congenital occurrence of fusion or webbing of the fingers or toes.


Group of signs and symptoms occurring together, having a common cause and defining a disease.




Congenital fusion of the orbits in the midline, which then contains one eyeball.


Congenital malformation characterized by the union or approximation of the ears in front of the neck, often accompanied by the absence or defective development of the lower jaw.


See also agnathia, megagnathia, otocephaly.

tail bud

In the vertebrate embryo, mass of proliferating cells at the caudal end that arises from the primitive node.


See clubfoot.

Tay-Sachs disease

infantile GM2 gangliosidosis

Autosomal recessive inheritance of hexosaminidase A deficiency leading to a lysosomal storage disease characterized by central and peripheral neuronal involvement and early death.


Floor of the midbrain.

  • Note 1:

    In the embryo this term refers more generally to the anterior portion of the neural tube.

  • Note 2:

    The tegmentum is the site of the nuclei of several cranial nerves.


Embryonic structure that develops into the cerebrum.


Final phase of both meiosis and mitosis in which distinct nuclei form in each daughter cell.


Malignant teratoma, occurring most often in the testis.

teratogen (n)/teratogenic (adj)

Chemical, physical, or biological agent that, when administered to a parent either prior to conception or before birth of the child, induces permanent structural malformations or birth defects in the offspring.

Modified from [1]

  • Note 1:

    Teratogens may act at vulnerable points in the development of gametes in parents or of organ development in the embryo and fetus.

  • Note 2:

    Their modes of action include mimicry of morphogens (thus interference with morphogenesis), modulation of genes and gene expression, and direct alterations in protein function.


Process of development of malformations or birth defects.

See also teratogen.


Study of how genes and teratogens interact to cause birth defects.

teratogenic index (TI)

Mortality of eggs expressed as a lethal concentration divided by the threshold concentration for production of abnormal embryos with nonheritable permanent structural malformations or defects following exposure to a teratogen.

  • Note:

    The TI is thought to reflect the developmental hazard of a contaminant.


Inherent ability to act as a teratogen.


Study of the causes, mechanisms, and manifestations of developmental deviation of either structure or function.


dermoid cyst

Benign germ cell-derived tumor containing embryonic elements of the three primary germ layers, such as skin, hair, and muscle, occurring most frequently in the ovary.

See also teratocarcinoma.

Tessier classification

Classification of bony and soft tissue facial clefts based on their anatomic location.

  • Note:

    They are numbered from 0–14, with 0 and 14 being in the midline.

testicular feminization

Type of male pseudohermaphroditism in which the individual has a male karyotype and testes present within the abdominal cavity, but develops external female genitalia and female secondary sexual characteristics.

  • Note 1:

    Testicular feminization is due to an androgen receptor mutation, causing the target tissues to be insensitive to the masculinizing effects of androgens.

  • Note 2:

    Clinical presentation is usually as a phenotypic female with primary amenorrhea.

testis (n)/testicular (adj)


Gonad of a male animal, involved in the secretion of androgens and the site of sperm production.


Androgenic steroid hormone produced primarily in the testis, responsible for the development of the male sexual organs and male secondary sexual characteristics.

  • Note:

    The ovary also secretes testosterone, but circulating levels in the female are much lower than in the male.



Broad-spectrum antibiotic of polycyclic polyketide structure.

  • Note:

    Tetracyclines are considered to be teratogens because they can deposit in, and cause discoloration of, the developing teeth, while also inhibiting the tooth development process.

tetralogy of Fallot

Set of four congenital cardiac defects involving an opening in the wall that should separate the right and left ventricles (ventricular septal defect), allowing the aorta to receive venous as well as arterial blood: including stenosis of the pulmonary artery, hypertrophy of the right ventricle, and displacement of the aorta. Right ventricular hypertrophy is the fourth part of the tetralogy, although it may also be a consequence of the other defects.

  • Note 1:

    The constellation of anatomical defects is understandable in terms of a flaw in the ordered sequence of events in the normal development of the heart and associated vessels.

  • Note 2:

    These anatomical defects are the most common cardiac cause of cyanosis in infants.


Either of two masses of grey matter, the largest part of the diencephalon, lying between the cerebral hemispheres on either side of the third ventricle, relaying sensory information and acting as a centre for pain perception.


Teratogenic indole-based drug with sedative and anti-angiogenic properties.

  • Note 1:

    The introduction of thalidomide for alleviating nausea in pregnancy in some countries in 1957 was soon linked to severe birth defects in the infant, with phocomelia being highly characteristic.

  • Note 2:

    Currently, it is used successfully to treat and prevent the moderate to severe skin lesions caused by leprosy, and is used together with dexamethasone to treat multiple myeloma.


Bilayer covering of the ovarian follicle (thus theca interna and externa), of which the theca interna is involved in production of androstenedione.

thoracic duct

Largest of the lymphatic vessels; it conducts lymph from its origin in the abdomen to the venous circulation at the junction of the left subclavian and jugular veins.


Abnormally low concentration of platelets (thrombocytes) in the blood.

  • Note:

    Thromocytopenia is an unwanted effect of some drugs and carries an increased risk of spontaneuous bleeding.


Pyramid-shaped organ in the thoracic or cervical region of mammals, composed of lymphatic tissue in which minute concentric bodies (thymic corpuscles, the remnants of epithelial structures) are found.

  • Note 1:

    Stem cells in the outer cortex of thymus develop into different kinds of T cells. Some migrate to the inner medulla and enter the bloodstream; those that do not may be destroyed to prevent autoimmune reactions.

  • Note 2:

    This organ is necessary for the development of thymus-derived lymphocytes (T cells) and is the source of several hormones involved in T-cell maturation, for example, thymosin, thymopoietin, thymulin, and thymocyte humoral factor.

  • Note 3:

    If a newborn’s thymus is removed, not enough T cells are produced, the spleen and lymph nodes have little tissue, and the immune system fails, causing a gradual, fatal wasting disease. Thymus removal in adults has little effect.


thyroglossal duct

Connection in the embryo between the site of initiation of development of the thyroid gland and its final location in the neck.

thyroid gland

Bilobar endocrine gland, located below the prominence of cartilage surrounding the larynx (the Adam’s apple), that produces and secretes the hormones triiodothyronine (T3), thyroxin (T4), and calcitonin.

thyroid tissue, accessory

Ectopic thyroid tissue arising from remnants of the thyroglossal duct.

thyroid stimulating hormone (TSH)

Pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3).


Small, rounded mass of tissue, especially of lymphoid tissue; generally used to designate one of the paired palatine tonsils.



  1. Capacity to cause injury to a living organism, defined with reference to the quantity of substance administered or absorbed, the way in which the substance is administered and distributed in time (single or repeated doses), the type and severity of injury, the time needed to produce the injury, the nature of the organism(s) affected, and other relevant conditions.

  2. Adverse effects of a substance on a living organism, defined as in 1.

  3. Measure of incompatibility of a substance with life that may be expressed as the reciprocal of the absolute value of median lethal dose (1/LD50) or concentration (1/LC50).



Infection with the parasite Toxoplasma gondii that, when acquired by the fetus, can lead to a variety of congenital abnormalities that may include hydrocephalus or microcephaly, mental retardation, and problems with vision and hearing.

  • Note:

    Infection of the fetus is from the mother who may acquire it from contact with feces of the domestic cat or from undercooked meat.


Thin-walled, cartilaginous tube descending from the larynx to the bronchi and carrying air to the lungs.

transcription factor

DNA-binding protein that is involved in regulating gene expression.


Describing an organism that is genetically changed by the addition or deletion of genetic material or whose existing genes are altered by gene targeting.

translocation, chromosomal

Rearrangement involving transfer or exchange of genetic material between non-homologous chromosomes that may occur during gametogenesis or in somatic cells, potentially leading to birth defects.

See also reciprocal translocation, Robertsonian translocation.

transplacental carcinogen

Substance that crosses the placenta and subsequently causes cancer in the child or young adult.

After [17]

transposition of the great arteries

transposition of the great vessels

Congenital cardiovascular malformation in which the position of the chief blood vessels of the heart (aorta and pulmonary artery) is reversed.

  • Note:

    Survival then depends on a crossflow of blood between the right and left sides of the heart, as through a ventricular septal defect.

After [5]

See also conal growth hypothesis.

Treacher Collins syndrome

Treacher Collins–Franceschetti syndrome

mandibulofacial dysostosis

Congenital deformity characterized by structural abnormalities of the head and face.

See also craniofacial.

  • Note:

    Autosomal dominant mutations have been identified in several genes involved in early development of bone and the tissues of the face, the most frequently implicated being TCOF1.

trilaminar embryo

trilaminary blastoderm

Disc-like embryonic stage in which differentiation into three layers (endoderm, mesoderm, and ectoderm) has occurred.

  • Note:

    In the human embryo this occurs in the third week, following the onset of gastrulation.


In pregnancy, one of three equal timed divisions of the normal human gestation period, each lasting approximately three months.

triple screen

triple test

Measurement of maternal serum levels of alpha-fetoprotein, human chorionic gonadotropin, and estriol, usually in the second trimester of pregnancy, that can serve as an indicator of risk for certain fetal chromosomal abnormalities and neural tube defects.


Presence of three haploid sets of chromosomes in the cell.

  • Note:

    The condition is fatal in fetal or early neonatal life.

See also diploid, euploid, ploidy.


Condition of having three homologous chromosomes in each somatic cell instead of the normal pair.

trisomy 8

Warkany syndrome 2

Presence of an extra chromosome 8.

  • Note:

    Complete trisomy 8 is lethal, but most affected individuals show mosaicism with craniofacial abnormalities, a short wide neck, multiple joint defects, and deep creases in the palms and soles.

trisomy 13

Patau syndrome

trisomy D

Presence of part or all of an extra chromosome 13 from nondisjunction during meiosis.

  • Note 1:

    This leads to multiple congenital abnormalities and mental retardation, and is usually fatal in early childhood.

  • Note 2:

    Patau syndrome can also occur with an extra partial chromosome 13 resulting from a Robertsonian translocation.

trisomy 18

Edwards syndrome

Presence of part or all of an extra chromosome 18 from nondisjunction during meiosis.

  • Note:

    This leads to multiple congenital abnormalities and mental retardation, and is usually fatal in early childhood.

trisomy 21

Down syndrome

Presence of an extra chromosome 21.

  • Note 1:

    This produces a characteristic constellation of physical abnormalities with delayed growth and mental development, but is compatible with life into adulthood.

  • Note 2:

    The formerly used terms mongoloid and mongolism are now considered offensive.


Outer layer of the blastocyst that contacts the endometrium, establishes nutrition for the embryo, and differentiates into the trophoblast.


Outer layer of the blastocyst that invades the endometrium and establishes nutrition for the embryo.

  • Note:

    Trophoblast cells do not form part of the embryo, but contribute to the development of the placenta.

truncus arteriosus

Arterial trunk, opening from the fetal heart and developing into the aorta and pulmonary artery.

tuberoinfundibular pathway

Group of dopaminergic neurons in the hypothalamus involved in regulating prolactin secretion from the anterior pituitary gland.



Any abnormal swelling or growth of tissue, whether benign or malignant.

Compare neoplasm.

tunica albuginea

Dense, collagenous fibrous coat surrounding an anatomical structure, and in particular surrounding the ovaries, testicles, and corpora cavernosa of the penis.

tunica vaginalis

Membranous sheath, derived from the peritoneum, surrounding the testis and epididymis.

Turner syndrome

Ullrich–Turner syndrome

Phenotypic female lacking one X chromosome, the genotype being designated XO.

  • Note:

    This results in a number of developmental abnormalities including short stature, webbed neck, sexual immaturity, and sterility.

two-generation reprotox study

Procedure in rodents wherein parents (P0 generation) and their offspring (F1 generation) are both exposed to a test substance and the second generation (F2) is examined for possible toxic effects.

See also one-generation reprotox study.


In anatomy, relating to the resonant cavity and membrane of the inner ear.


Imaging technique that creates a picture of internal body structures from differentially reflected ultrasound waves.

  • Note:

    By using this technique, the human embryo can be observed in utero as early as 5 1/2 weeks of gestation: fetal monitoring by this technique (obstetric ultrasound) is standard.


  1. Sound waves of frequency higher than the range audible to the human ear.

  2. Imaging technique that uses such sound waves to create a picture of internal body structures.

See also ultrasonography.



Relating to the umbilicus.

umbilical artery

Paired artery that, in the fetus, returns deoxygenated blood from each half of the fetal body to the placenta, via the umbilical cord.

  • Note:

    In the adult, part of the umbilical artery remains open as a branch of the internal iliac artery, and part ceases to be an artery and becomes the medial umbilical ligament.

umbilical cord

Cord-like structure connecting the fetus with the placenta and housing the umbilical artery and umbilical vein that carry nutrients from the mother and remove wastes from the fetus.

umbilical hernia

Protrusion of bowel or omentum through the abdominal wall at the umbilicus.

umbilical vein

left umbilical vein

Vessel within the umbilical cord, entering the fetus at the umbilicus and carrying oxygenated blood from the placenta.

umbilicus (n)/umbilical (adj)


belly button

Feature marking the point in the abdominal wall where the umbilical cord entered the fetus.

undescended testis

cryptorchid testis


Testis that has failed to descend completely from its developmental origin in the lower abdominal cavity into the scrotum.


Duct by which urine is conveyed out of the body from the bladder, and which in male vertebrates also conveys semen.



Concerning the urinary and genital organs.

urogenital sinus

Embryonic structure that forms in the ventral part of the cloaca when it separates from the anal canal to give rise to the genitourinary organs.

  • Note:

    A rare urogenital sinus anomaly occurs as a birth defect in the female when the urethra and vagina open into a common channel.

uterine crypt

Chamber on the interior surface of the uterus that serves as a site of embryo homing and implantation.

uterine cycle

Regular, periodic changes that occur in the uterus and, together with the ovarian cycle constitute the menstrual cycle necessary for female fertility.

  • Note 1:

    The uterine cycle consists of three phases:i) menstruation, ii) a proliferative phase in which the uterine wall thickens under the influence of estrogen, and iii) a secretory phase in which the corpus luteum is producing progesterone and the endometrium becomes receptive to implantation of the blastocyst.

  • Note 2:

    The three phases of the uterine cycle correspond to the three phases of the ovarian cycle.



Having an effect on the uterus.

  • Note:

    This usually refers to estrogen-like effects of uterine cell proliferation caused by some drugs and other substances with weak estrogen-mimetic properties.

uterotrophic assay

Procedure in which immature female rodents are treated for three days with a test substance. A resultant increase in uterine wet weight suggests an estrogenic activity of the substance.

uterus (n)/uterine (adj)


Hollow muscular organ of the female reproductive system that receives the fertilized ovum (see implantation) and supports the subsequent development of the fetus.


palatine uvula

Muscular projection from the posterior edge of the soft palate.

  • Note:

    The uvula contributes to the gag reflex and is involved in shaping some sounds of human speech.

vagina (n)/vaginal (adj)

Fibromuscular canal passing between the uterine cervix and the opening to the vulva.

  • Note:

    The vagina permits sexual intercourse and delivery of babies.

vaginal cornification

Conversion of the normal epithelium of the vagina to a keratinized squamous (flattened) epithelium.

  • Note:

    The appearance of keratinized (“cornified”) epithelial cells in a vaginal smear is an indication of increased estrogen levels.

vaginal patency

Referring to the opening of the vagina that usually occurs at sexual maturity and is maintained by hormonal influences pre-menopausally.

vaginal smear

Examination by light microscopy of a sample of a vaginal discharge, used in the diagnosis of a vaginal infection.

  • Note:

    Not to be confused with a Papanicolaou smear.

variation, developmental

Anatomical deviation that is not life-threatening.


De novo development of blood vessel endothelium in absence of preexisting vessels, e.g., during fetal development or in connection with tissue repair, initiated by migration of mesodermal precursor cells.

Compare angiogenesis, arteriogenesis.

vas deferens

ductus deferens

spermatic duct

Secretory duct of the testis running between the epididymis and the ejaculatory duct.


Surgical removal of all or part of each vas deferens, typically as a means of sterilization.


Pituitary hormone that acts to promote the retention of water by the kidneys and to increase blood pressure.

velopharyngeal insufficiency

velopharyngeal incompetence

Inability to achieve closure of the velopharyngeal sphincter (closure of the muscle of the soft palate) during speech, owing to muscular dysfunction, cleft palate, or other disorders.

  • Note:

    This insufficiency often causes speech problems by allowing air to escape through the nose instead of the mouth.

vena cava

Large vein returning deoxygenated blood to the right heart.

  • Note:

    The vena cava has two branches in humans, the inferior vena cava (carrying blood from the lower body) and the superior vena cava (carrying blood from the head, arms, and upper body).

venous shunt

See arteriovenous shunt.


Situated to the anterior side of the trunk; in humans, to the front of the body.

Opposite term: dorsal.

ventral thalamus

See subthalamus.

ventricular septum

Dividing wall between the two lower chambers (ventricles) of the heart.

ventricular septal defect (VSD)

Congenital defect with an opening in the wall separating the right and left ventricles of the heart (see ventricular septum).

  • Note:

    This cardiac anomaly allows reflux of blood back to the right ventricle during left ventricular contraction, producing varying degrees of cyanosis.

vertebra (n)/vertebral (adj)

One of the bony segments that together make up the bony column surrounding the spinal cord.

vertebral arch

See neural arch.


Denoting an animal that has as part of its nervous system a spinal cord that is surrounded by a bony vertebral column.

vesicle (n)/vesicular (adj)

Small fluid-filled sac, bladder-like structure, or blister.

  • Note:

    This term may refer to an anatomic structure, a subcellular organelle, or a liposome-related particle.


In anatomy, pertaining to a remnant of an embryonic or fetal structure persisting in the adult.


Ability to continue living.

  • Note:

    Fetal viability refers to the ability of the fetus to survive outside the uterus.


  1. Projection from the surface, typically of a mucous membrane.

  2. Elongated projection of the dermis into an epidermal space.

After [7]


Intermediate filament protein that is expressed in mesenchymal cells, e.g., fibroblasts, leukocytes, and blood vessel endothelial cells.

  • Note:

    Vimentin filaments support cell membranes, keep some organelles in a fixed place within the cytoplasm, and transmit membrane receptor signals to the nucleus.


Possession or acquisition of characteristics of a male body, especially by a female or prepubescent male.

After [7]

viscera (n)/visceral (adj)

Soft and hollow organs of the body, in vertebrates particularly those found in the thoracic and abdominal cavities.

  • Note:

    This gives rise to the distinction of the visceral and parietal serous membranes, located closest to the organ or to the body wall, respectively. For examples, see pericardium, peritoneal cavity and pleura.

vitellogenin (VTG)

Protein that forms part of the yolk of egg-laying vertebrates.

  • Note:

    Its expression in male fish is used in ecotoxicology as an indicator of exposure to environmental estrogenic endocrine disruptors.


External genitalia of the female.


Accustomize a young mammal to independence from its mother’s milk as a source of nutrition.

web (in anatomy)


Tissue or membrane present between adjacent structures where it is not usually found, e.g., between fingers or toes, or between the neck and shoulder, representing a congenital abnormaility.

Wharton’s jelly

Gelatinous substance that embeds the vessels of the umbilical cord.

  • Note:

    The jelly consists mainly of hydrated glycosaminoglycans such as hyaluronic acid and chondroitin sulfate.

white matter

substantia alba

Portion of the brain and spinal cord consisting mainly of myelinated axons (see myelination) and glial cells.

  • Note:

    Neurons of the white matter transmit signals between areas of grey matter within the cerebrum, to lower brain centers, and up and down the superficial aspect of the spinal cord.

whole embryo culture

Technique, used prior to embryo transfer during in vitro fertilization, in which embryos undergoing organogenesis are maintained in formulated medium outside the body.

  • Note:

    Whole embryo culture is a useful procedure for testing the teratogenic effects of a substance on organogenesis in rodent or chick embryos.

wild type

Of an allele or phenotype, the naturally occurring, unaltered, or most frequent; if several alleles occur, the most frequent one is considered to be the wild type.

Williams syndrome

elfin facies syndrome

Congenital disorder with characteristic facies (described as elf-like), short stature, outgoing personality and mild mental retardation; associated with contiguous gene deletions on chromosome 7.


Family of genes important in development, the proteins they encode, or the signal transduction pathways they determine.

  • Note 1:

    Wnt pathways are involved throughout embryonic development, regulating such processes as cytoskeletal dynamics, cell polarity, proliferation, migration, and body axis patterning.

  • Note 2:

    The proteins signal by binding to cell-surface G-protein coupled receptors of the Frizzled family that signal to members of the Dishevelled (Dsh) family of cytoplasmic phosphoproteins.

  • Note 3:

    Wnt is derived from Wingless-related integration site, originally identified in Drosophila.

Wolffian duct

See mesonephric duct.


See uterus.

X chromosome

Chromosome determining female sex in the absence of a Y chromosome.

See also sex chromosome.


Carried by a gene located on the X chromosome.


Compound with a chemical structure foreign to a given organism.

  • Note:

    Frequently restricted to man-made compounds.

Y chromosome

Chromosome determining male sex.

See also sex chromosome.



Nutritive material stored in the ovum for the nourishment of the embryo.

yolk sac

Fluid-filled, membrane-bound pouch on the ventral side of the early embryo that provides nourishment until the circulatory system develops.

yolk stalk

Narrow tubular support that connects the yolk sac to the middle of the digestive tract of an embryo.

zinc finger

Small tertiary protein structure in which parts of the protein, typically with cysteine and histidine residues, link to a divalent zinc ion, forming a loop or “finger” that attaches proteins to DNA.

  • Note:

    A zinc finger is present in many proteins, such as nucleases or transcription factors that regulate expression of eukaryotic genes.

zona pellucida

Acellular glycoprotein-rich membrane surrounding the mature ovum.


Sexual union of two cells with fusion of their nuclei.

zygote (n)/zygotic (adj)

  1. Cell such as a fertilized egg (ovum after fertilization) resulting from the fusion of two gametes.

  2. Cell obtained as a result of complete or partial fusion of cells produced by meiosis.


zygote intrafallopian transfer (ZIFT)

tubal embryo transfer

Introduction of a zygote fertilized in vitro into one of the Fallopian tubes.

See also in vitro fertilization.

Compare gamete intrafallopian transfer.

Membership of sponsoring body

Membership of the Committee of the Chemistry and Human Health Division during the preparation of this report (2014–2016) was as follows:

President: T. Perun; Vice President: R. Cornelis; Secretary: M. Schwenk; Titular Members: E. Differding, J. Fischer, V. Gubala, H. P. Illing, L. Johnston, H. Møller Johannessen, and W. Temple; Associate Members: V. Abbate, M. Kiilunen, Y. Martin, S. Mignani, and D. Rotella; National Representatives: S. Alihodžić, S. Bachurin, B. Haug, R. J.-R. Hwu, R. Leurs, N. Nahar, P. Ploypradith, A. Rahatgaonkar, G. B. Teh, and M. X. Wang.

Annex I – Abbreviations


– angiotensin converting enzyme


– adrenocortocotropic hormone


– adrenal 4 binding protein


– alpha-fetoprotein


– anogenital distance

AGM region

– aorta-gonad-mesonephros region


– aryl hydrocarbon receptor


– apoptosis-inducing factor


– anti-Müllerian hormone


– androgen receptor


– alcohol-related neurodevelopmental disabilities


– aryl hydrocarbon receptor nucleotide translocator protein


– atrial-septal defect


– arteriovenous


– bone morphogenetic protein


– central nervous system


– cerebrospinal fluid


– congenital talipes equinovarus


– diethylstilbestrol


– Di George syndrome


– drug metabolizing enzyme


– extracellular matrix

ELS test

– early life stage test


– epithelial-to-mesenchymal transition

ES cell

– embryonic stem cell


– expressed sequence tag


– Fertility Assessment by Continuous Breeding


– fetal alcohol syndrome


– fetal alcohol spectrum disorder


– fish embryo test


– frog embryo teratogenesis assay Xenopus


– frontonasal dysplasia


– follicle-stimulating hormone


– follicle-stimulating hormone releasing hormone


– fibroblast growth factor


– growth hormone


– growth hormone releasing factor


– gamete intrafallopian transfer


– gonadotropin-releasing hormone


– gonadotropin-releasing hormone


– human chorionic gonadotropin


– human growth hormone


– human placental lactogen


– Human Genome Project


– intracytoplasmic sperm injection


– intermediate filament


– intrauterine growth restriction (retardation)


– in vitro fertilization


– induced pluripotent stem cell


– median lethal concentration


– median lethal dose


– luteinizing hormone


– luteinizing hormone-releasing hormone

LOD score

– logarithm (base 10) of odds score


– mesenchymal-to-epithelial transition


– Müllerian inhibiting factor


– Müllerian-inhibiting hormone


– Müllerian-inhibiting substance


– messenger RNA


– microRNA


– maternal serum alpha-fetoprotein


– noggin


– open reading frame


– phenylketonuria


– Reproductive Assessment by Continuous Breeding


– retinoic acid receptor


– restriction fragment length polymorphism

Rh factor

– rhesus factor


– RNA interference


– retinoid X receptor


– sister chromatid exchange


– sexually dimorphic nucleus


– somatic hypermutation


– short hairpin RNA


– small interfering RNA


– single-nucleotide polymorphism

SRY protein

– sex-determining region Y protein


– somatotropic hormone


– short tandem repeat


– triiodothyronine


– thyroxin


– tributyltin oxide


– median teratogenic concentration


– testis-determining factor


– translocated ETS leukemia-ARNT fusion protein (where ARNT is defined above and ETS refers to the E twenty-six transformation-specific family of transcription factors)


– transforming growth factor β


– teratogenic index


– variable number tandem repeat


– ventricular septal defect


– vitellogenin


– zygote intrafallopian transfer

Annex II – List of some chemicals with important adverse effects on the reproductive system, embryo, and fetus.


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About the article

Received: 2015-12-04

Accepted: 2016-08-23

Published Online: 2016-10-19

Published in Print: 2016-08-01

Citation Information: Pure and Applied Chemistry, Volume 88, Issue 8, Pages 713–830, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2015-1202.

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