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Unless otherwise noted, SDB CoRe glossary terms were reprinted with permission of John Wiley & Sons, Inc. from Essential Developmental Biology, 3rd Edition, J.M.W. Slack, © 2012 by Jonathan Slack.


aboral: located opposite to or away from the mouth (source:

adaptive evolution: evolutionary changes produced by natural selection

adult stem cell: =tissue stem cell

allantois: extraembryonic structure of amniotes arising from posterior mesoderm

allele: genes with alternative DNA sequences that occupy the same locus in the genome

amnion: extraembryonic membrane characteristic of amniotes, consisting of ectoderm on the side facing the embryo and mesoderm on the side facing the chorion

animal cap: explanted region from the animal pole of an amphibian embryo, used to assay inducing factors

animal hemisphere: the upper hemisphere of an egg or oocyte; after fertilization usually bears the polar bodies

anlage: an embryonic area capable of forming a structure: the primordium, germ, or bud (source:

anterior: the head end of an animal

anteroposterior: the direction towards, or line joining, the anterior and posterior extremities of an anima

anther: The pollen-bearing part of the stamen (source:

apical ectodermal ridge: (=AER) epidermal ridge on limb bud, needed for distal outgrowth

apoptosis: =programmed cell death

AVE region: (=anterior visceral endoderm) a signalling centre in the mouse conceptus

axis: (1) a line or direction, as in anteroposterior axis. (2) the midline structures of a vertebrate embryo: notochord, somites and neural tube

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basement membrane: extracellular matrix layer underlying an epithelium

birth defects: (=congential defects) defects of a mammalian embryo apparent at the time of birth. These may arise from genetic causes, or chromosome abnormalities in the embryo, or teratological stimuli

blastocoel: fluid filled cavity in the center of a blastula

blastocyst: mammalian embryo after cavitation; differs from blastula because it contains differentiating extraembryonic structures

blastoderm: similar to a blastula but arranged as a sheet rather than a ball of cells

blastodisc: the embryo-forming portion of an egg with meroblastic cleavage usually appearing as a small disc on the upper surface of the yolk mass (source:

blastomeres: the large cells arising from the fertilized egg by cleavage divisions

blastopore: depression or slit through which cells move to the interior during gastrulation

blastula: early developmental stage; a ball of similar cells arising from repeated cleavage of a fertilized egg

body plan: the essential features of the whole body of a species or higher taxonomic group

border cells: specialized ovarian follicle cells that control the polarity of the Drosophila oocyte

branchial arches: (=pharyngeal arches) segmental structures between the pharyngeal pouches each comprising a cartilaginous arch, blood vessel and cranial nerve

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cancer: uncontrolled growth of the body’s own cells; often displaying local invasive behaviour and metastasis

carcinoma: a cancer derived from an epithelial tissue

caudal: relating to the tail, often equivalent to posterior

cellular blastoderm: stage of insect development at which the syncytial blastoderm becomes divided into cells

chim(a)era: =genetic mosaic, especially a mammalian embryo in which cells of one genotype have been injected into an embryo of a different genotype.

chorion: (1) extraembryonic membrane of amniotes, similar to the amnion but forming an outer layer. (2) an extracellular layer surrounding an insect or fish egg

chromatin: DNA combined with chromosomal proteins in a manner which helps regulate its expression and behavior

clade: a taxon which consists of all the descendants of a common ancestor

cleavage: a type of cell division that occurs without growth, such that daughters are smaller than the mother; typically found in the early stages of development

clonal analysis: information obtained about developmental mechanisms by studying the position and differentiation of the progeny of a single labelled cell

clone: (1) population of cells derived from a single progenitor cell, either in vivo or in vitro. (2) DNA molecule prepared by molecular cloning

cloning: (1) assembling a DNA sequence with the use of restriction enzymes and ligases followed by insertion into a cloning vector and growth to a useful quantity. (2) growth of a colony of cells from a single cell. (3) formation of a whole organism from one cell

coelom: body cavity lined with mesoderm

commitment: of a cell or tissue region indicates that it is programmed to follow a particular developmental pathway or fate

compartment: a region of an embryo within which a clone of cells may move around but whose boundaries it does not cross

competence: ability to respond to an inductive signal

conditional knockout: a knockout mouse in which the ablation of the gene occurs under a particular set of circumstances controlled by the experimenter

constitutive: describes a gene or gene product that is active continuously

convergent extension: morphogenetic movement in which a cell sheet elongates and narrows because of active movements of the constituent cells to alter the overall packing arrangement

coronal section: =frontal section

cortex: outer region of a cell, especially an egg or oocyte, comprising a few microns thickness beneath the plasma membrane

cortical granules: secretory vesicles containing vitelline membrane components, released from the egg at fertilization

cortical rotation: rotation of the zygote cortex relative to the internal cytoplasm

Cre-lox: system used to bring about DNA recombination or excision events at particular sites under controlled circumstances

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dauer larva: resistant dormant state of C.elegans

deep cells: cells in the interior of an early embryo, especially in fish embryos

definitive endoderm: the embryonic endoderm of an amniote embryo, as opposed to extraembryonic endoderm

dermamyotome: part of the somite forming the dermatome and myotome

dermatome: part of the somite forming the dorsal dermis

dermis: the connective tissue layer of the skin

determinant: (=cytoplasmic determinant) substance localized to part of an egg or blastomere that causes the cells that inherit it to acquire a particular developmental commitment

Deuterostomia: group of animal phyla characterised by radial cleavage, regulative development and formation of the anus from the blastopore

developmental constraint: aspect of the developmental mechanism that prevents the viability of some types of mutant

diencephalon: the posterior part of the vertebrate forebrain

differentiation: the acquisition during development of the properties of mature functional cells The term usually refers to the last step of development rather than to earlier events of commitment.

diploblasts: animal taxa with only two germ layers: the cnidarians and ctenophores

dispersed transcription: mode of transcription initiation in which there are multiple weak start sites that are distributed over a region of about 50 to 100 nucleotides. Dispersed promoters are typically found in housekeeping genes that maintain a steady level of transcription (by J.T. Kadonaga, 2012)

distal: further from the body

distal tip cell: cell acting as a signaling center for the C.elegans germ line

dominant: type of mutation that produces a phenotype in the presence of the wild type allele

dominant negative: type of mutation that antagonises the effect of the wild type allele

dorsal: the upper surface (or back) of an animal

dorsal lip: dorsal part of the ring shaped blastopore of an amphibian embryo. The tissue just above the dorsal lip is Spemann’s organizer

dysgenesis: defective or abnormal development of an organ (source:

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ecdysis: the act of molting or shedding an outer cuticular layer (source: Merriam-Webster)

ectoderm: the outer of the three embryonic germ layers

ectopic: not in its usual position; as in ectopic structures or ectopic gene expression

egg: strictly the female gamete after completion of the second meiotic division (=ovum). Also a fertilized egg (=zygote). Also often loosely used for secondary oocytes and early embryos

embryoid bodies: structures resembling embryos which are formed by ES cells or teratocarcinoma cells when they are removed from a growth-promoting medium

embryonic induction: the 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

endoderm: the inner of the three germ layers

endosperm: the storage tissue in the seeds of most angiosperms (flowering plants), derived from the fusion of one male gamete with two female polar nuclei (source:

endothelial cells: the cells lining blood vessels and forming capillaries

enhancer: regulatory region of DNA that controls the expression of a gene; often independently of its precise position in relation to the gene

enhancer trap: a transgenic line containing a reporter gene whose activity is regulated by an endogenous enhancer

enveloping layer: outer layer of an early fish embryo, composed of flattened cells

epiblast: upper layer of a mammalian or avian blastoderm, or fish germ ring

epiboly: active spreading and increase in area of a cell sheet

epidermis: the epithelial part of the skin

epigenetic: may be used for anything to do with development, but nowadays more usually refers to mechanisms of gene control based on DNA methylation or chromatin structure

epistasis: in general an effect exerted on the phenotype of one gene by the activity of another. In particular used to describe the suppression of one mutant phenotype by a mutation in a different gene

epithelium: a tissue type in which cells are arranged as a single or multilayered sheet lying on a basement membrane

equivalence group: set of cells with the same competence

extracellular matrix: material filling the space between cells, and also such structures as basement membranes, vitelline membranes etc

extraembryonic membranes: structures derived from the zygote which are not part of the embryo itself but serve to support or nourish it

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facultative gene: a gene transcribed only when needed or when a cell receives a signal from its surroundings (source:

fate map: diagram of an embryo or organ rudiment showing where each part will move and what it will become in the course of normal development

feeder layer: tissue culture cells treated to prevent division, which provide an environment enabling the growth of other cells that cannot be grown in standard media

floor plate: midventral part of the vertebrate embryo spinal cord

floxed: a DNA sequence including loxP sites, which is a substrate for Cre-mediated recombination

FLP system: method for inducing recombination at desired genomic sites, particularly used in Drosophila

focused transcription: mode of transcription initiation in which there is a single predominant transcription start site or a cluster of start sites within several nucleotides. Focused promoters are generally associated with regulated genes (by J.T. Kadonaga, 2012)

follicle cells: somatic epithelial cells of the ovary

forebrain: region of the vertebrate embryonic brain that will become the telencephalon and diencephalon

forward genetics: analysis of a biological phenomenon starting from a mutant phenotype

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gain of function: of a mutation, confers additional or altered activity on the gene product

gametes: haploid reproductive cells; sperm or eggs

gametogenesis: the development of gametes from germ cells

gametophyte: the gamete-producing, usually haploid generation in the life cycle of a plant (source:

gap genes: genes whose expression defines a body region in the Drosophila embryo

gastrula: stage of development in which gastrulation takes place

gastrulation: phase of morphogenetic movements in early development that brings about the formation of three germ layers

gene trap: type of insertional mutation in which a reporter gene is introduced into the locus of an endogenous gene. The function of the endogenous gene is usually destroyed, but its regulatory elements drive the reporter in the normal expression pattern

genetic mosaic: an organism composed of two or more types of cell that are genetically different

genotype: relating to a single organism or to a genetic line, the genotype means the particular alleles present at specific genetic loci

germ cells: cells belonging to the germ line

germ layers: the ectoderm, mesoderm and endoderm

germ line: the cell lineage that will form the gametes, comprising the primordial germ cells, spermatogonia and oogonia, together with other clonal progeny such as nurse cells. The germ line is often formed by the action of cytoplasmic determinants

GFP: =green fluorescent protein

gonad: the somatic structure containing the germ cells

gradient: a continuous change in some property with position. Often used to refer to concentration gradient of a morphogen

green fluorescent protein: (=GFP); a fluorescent protein often used as a reporter

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haploid: having a single set of chromosomes, as after meiosis

hemimetabolous: (of insects) relating to incomplete metamorphosis (source:

Hensen’s node: a condensation of cells at the anterior end of the primitive streak of an avian

hermaphrodite: individual organism producing both male and female gametes

hindbrain: region of the brain of a vertebrate embryo that will become the cerebellum and medulla oblongata

holoblastic: type of cleavage where the whole zygote becomes subdivided into blastomeres

holometabolous: (of insects) undergoing complete metamorphosis (source:

homeobox: DNA sequence encoding a homeodomain, which is a 60 amino acid DNA binding domain defining an important class of transcription factors

homeotic gene (=selector gene): a gene whose expression distinguishes two body parts. If mutated then one body part will be converted into the other

homolog: (1) structures or genes which resemble each other because they are homologous, i.e. descended from a common ancestor. (2) the maternally and paternally derived chromosomes that pair with each other at meiosis

homologous recombination: recombination of a transgene into a locus in the genome which has exactly the same sequence

homology: related by descent from a common ancestor

housekeeping genes: genes involved in basic functions needed for the sustenance of the cell. Housekeeping genes are constitutively expressed (source:

Hox genes: subset of homeobox genes that controls anteroposterior specification in animals

hypomorph: mutant allele showing partial loss of function

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imaginal discs: structures in the larva of Drosophila and other holometabolous insects that form the adult cuticle at the time of metamorphosis

imprinting: the expression of a gene from only one of the parental genomes

in situ hybridization: detection of a specific RNA, or occasionally DNA, in a biological specimen by hybridizing to a specific probe and then visualizing the probe with a suitable detection method

inducing factor: a signal substance responsible for an embryonic induction

induction: (1) =embryonic induction. (2) can refer to the regulated activation of transcription of a gene

inner cell mass: the cells within a mammalian blastocyst that form the entire embryo and most of the extraembryonic membranes

insertional mutagenesis: creation of mutations by means of a DNA element, such as a transposon or retrovirus, that will integrate at random into the genome and disrupt endogenous genes in the process

instar: period in the life cycle of an organism in between two moults

intermediate mesoderm: region of mesoderm in a vertebrate embryo lying between the somites and the somatopleure

invagination: internalization of a cell sheet by movement led by a free edge

involution: internalization of a cell sheet by movement led by a free edge

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knock-in: introduction of a transgene to a specific locus in the genome by homologous recombination

knock-out: null mutation made by targeted mutagenesis

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lateral inhibition: regional specification mechanism that involves isolated cells or cell clusters differentiating in a particular direction, and emitting an inducing factor that inhibits the surrounding cells from doing the same

lateral plate: region of mesoderm of a vertebrate embryo lying lateral to the somites

left-right asymmetry: differences between left and right sides of the body in terms of gene expression, morphogenetic movements or differentiation

locus: position in the genome occupied by a particular gene

longitudinal section: a section made parallel to the long axis of the organism, often but not necessarily in the sagittal plane

loss of function: usually describes a mutation which causes the protein product of the gene to be inactive or of lower activity than the wild type

lumen: the cavity within an organ, often lined with an epithelium

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macromeres: large blastomeres

marginal zone: (1) in an amphibian embryo the ring around the blastula that invaginates during gastrulation. (2) In a chick embryo the junction of the area pellucida and area opaca.(3) In the embryonic vertebrate central nervous system the cell-poor region near the pial surface

maternal effect: situation where the phenotype of the embryo corresponds to the genotype of the mother rather than its own genotype. This arises from defects in the assembly of the oocyte.

medial: relating to the midline of the organism

mediolateral: the direction from, or line joining, the midline to the lateral extremities of an animal

meiosis: the final division leading to the formation of gametes, in which the chromosome complement is halved

meristem: the undifferentiated plant tissue from which new cells are formed, as that at the tip of a stem or root (source:

meroblastic: type of cleavage where only part of the zygote cleaves and the remainder, usually a yolk mass, does not

mesenchyme: tissue type in which cells lie scattered within an extracellular matrix

mesoderm: the middle of the three germ layers

mesonephros: the middle segments of the developing kidney

micromeres: small blastomeres

micropyle: a minute opening in the ovule of a seed plant through which the pollen tube usually enters (source:

midblastula transition (MBT): set of coordinated changes in the late blastula including the onset of zygotic genome transcription

midbrain: region of the embryonic vertebrate brain that will become the optic tecta, or equivalent, and some other structures

molecular cloning: insertion of a nucleic acid sequence into a cloning vector, usually a bacterial plasmid, such that it can be amplified to a useful quantity

morphogen: type of inducing factor to which competent cells can make at least two different responses at different threshold concentrations. This means that the responding cells will form a series of differently committed territories in response to a concentration gradient of the factor

morphogenesis: the aspects of development involving movement of cells or of cell sheets, with the associated formation of structures

morpholino: a chemical grouping that has given its name to a type of oligonucleotide analog which can hybridize with nucleic acids while being resistant to nuclease enzymes

morula: multicellular pre-blastula embryonic stage

mosaic: (1) a type of embryo in which each part continues to develop in accordance with the fate map after separation from the rest of the embryo. (2) =genetic mosaic

mutagen: substance inducing genetic mutations

mutagenesis screens: the process for isolating a large number of mutants affecting some property of the organism. This may be the overall anatomy of the embryo, or some specific organ or cell type

mutant: organism carrying a mutation

mutation: change in the genomic DNA

myotome: the pert of the somite forming striated muscle

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neural crest: migratory cells from the dorsal neural tube of a vertebrate embryo that form a variety of cell types

neural plate: flat sheet of neuroepithelium that will roll up to form the neural tube of a vertebrate embryo

neural tube: the primordium for the vertebrate central nervous system

neuroepithelium: the tissue comprising the neural tube

neurulation: the morphogenetic movements of vertebrate embryos that create the neural plate and neural tube

neutral evolution: evolution arising from the gradual accumulation of mutations which are neither beneficial nor deleterious

Nieuwkoop center: region of the early amphibian embryo that induces Spemann’s organizer

node: condensation of cells at the anterior end of the primitive streak of a mouse embryo, similar to Hensen’s node in the chick

notochord: cartilage-like rod which comprises the dorsal-most part of the mesoderm of a vertebrate or invertebrate chordate embryo

nucleosome: any of the repeating subunits of chromatin, consisting of a DNA chain coiled around a core of histones (source:

null: relating to a mutation that shows a complete loss of function

nurse cells: sister cells of the insect oocyte; 15 nurse cells and one oocyte are formed from female oogonia by four successive mitotic divisions

nymph: The immature form of an insect that does not pass through a pupal stage during metamorphosis (source:

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oocyte: the female germ cell after completion of its mitotic divisions is a primary oocyte, and after completion of the first meiotic division is a secondary oocyte

oogenesis: the development of oocytes

oogonia: the mitotic germ cells that become oocytes

oral: of or relating to the surface of an animal, such as a jellyfish, on which the mouth is situated (source:

organizer: (=signalling center); group of cells emitting an inducing factor

organogenesis: the process, or stage, of formation of individual organs in vertebrate development

otic placodes: epidermal structures forming the ear vesicles

ovule: The female gamete and its protective and nutritive tissue, which develops into the dispersal unit or seed after fertilization in seed plants (source:

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P-granules: (=polar granules)

pair-rule genes: genes controlling the segmentation pattern of Drosophila that are expressed a one stripe for each prospective pair of segments

paracrine signaling: a form of cell signaling in which the target cell is near the signal-releasing cell (source:

parasegments: the initially formed segmental repeating structures in insect embryos

paraxial mesoderm: plate of mesoderm that forms the somites of a vertebrate embryo

parthenogenesis: asexual development from oocytes, without a genetic contribution from the sperm

permissive: (1) of inductions, a signal required for the continuation of a particular developmental pathway, but which does not control alternative developmental fates. (2) of temperatures, the temperature at which a ts mutant phenotype is not displayed

pharyngeal arches: see branchial arches

phenocopy: the same phenotype as that produced by a mutation, but produced by some other means

phenotype: the collected characteristics of an organism, particularly in relation to other members of the same species. It is mostly determined by the individual’s genotype

photosynthesis: The process in green plants and certain other organisms by which carbohydrates are synthesized from carbon dioxide and a source of hydrogen (usually water), using light as an energy source (source:

phylogenetic tree: classification scheme which depicts the taxa in terms of their actual evolutionary relationships

phylotypic stage: developmental stage at which members of an animal group most closely resemble each other

pistil: the female, ovule-bearing organ of a flower, including the stigma, style, and ovary (source:

placode: a thickening of the epidermis which is the rudiment for a structure, usually a sense organ

planar cell polarity: polarity displayed by neighboring cells within a cell sheet

planula: the flat, free-swimming, ciliated larva of a cnidarian (source:

pluripotent: able to form more than one type of differentiated cell

polar body: chromosome set arising from a meiotic division of the oocyte and expelled from the oocyte as a small vesicle

polar granules (=P granules): associated with germ plasm of C.elegans

polarity: (1) of cells, a difference between one end and the other, e.g. apico-basal polarity. (2) of an embryo or part of an embryo, a difference in commitment between the regions along a particular axis, e.g. animal-vegetal polarity

pollen: a fine powdery substance produced by the anthers of seed-bearing plants, consisting of numerous fine grains containing the male gametes (source:

polyp: a cnidarian in its sedentary stage. Polyps have hollow, tube-shaped bodies with a central mouth on top surrounded by tentacles (source:

polyphenism: ability of a single genome to produce alternative phenotypes in response to environmental clues (source:

positional cloning: cloning of a gene starting from a mutation, which is mapped to very high resolution to find the gene within which it lies

posterior: the tail end of an animal

primitive ectoderm: =epiblast of mammalian embryo

primitive endoderm: first formed layer of endoderm in mammalian embryo, contributes to extraembryonic membranes

primitive streak: the region of convergence and invagination of cells in a gastrulating amniote embryo

primordial germ cells: germ line cells at a stage before they become oogonia or spermatogonia

primordium: an organ or a part in its most rudimentary form or stage of development (source:

programmed cell death: (=apoptosis) death of cells by a process involving the activation of caspase enzymes, which avoids the release of toxic products into the surroundings. Often occurs in embryonic development

progress zone: the undetermined mesenchymal cells at the distal tip of a vertebrate limb bud

promoter: the region of a gene 5’ to the coding sequence where the RNA polymerase binds. It may also contain regulatory sequences binding transcription factors that control gene expression

pronephros: anterior segments of the developing kidney

proximal: nearer to the body

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radial cleavage: symmetrical type of cleavage

recessive: a mutation that produces no phenotype if it is present alongside the wild type

redundancy: partial or complete overlap of function of two or more genes

regeneration: regrowth of a missing body part at an adult or late developmental stage

regulated genes: (=facultative genes) genes transcribed only when needed or when a cell receives a signal from its surroundings (source:

reporter gene: usually a transgene that produces a product that is very easy to detect and hence “reports” on the nature and activity of its regulatory environment

retinal pigment epithelium: pigmented layer of the eye derived from the optic cup

reverse genetics: investigations on a known gene, especially by creation of loss of function mutants

rhombomeres: segmental structures in the hindbrain of vertebrate embryos

RNAi: (=RNA interference) method of inactivating a specific mRNA by introducing a short complementary double stranded RNA sequence

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sagittal: the medial plane of an organism, separating left and right halves of the body

sarcoma: cancer derived from connective tissue (in broad sense)

scaffold: three dimensional extracellular matrix used in tissue engineering; often made of synthetic polymers rather than natural extracellular components

sclerotome: the part of the somite forming the vertebrae

segment polarity genes: genes in Drosophila that are expressed in one stripe per prospective segment

segmentation: subdivision of the body, or of an appendage, into repeating structural units, usually involving all germ layers. In older literature can also be a synonym for cleavage

signaling center: (=organizer); group of cells emitting an inducing factor

somatic cells: all cells of an organism that are not the germ line; may be collectively referred to as the soma

somatic mesoderm: the layer of lateral mesoderm lying exterior to the coelom

somites: segmented mesodermal structures in vertebrate embryos that give rise to vertebrae and striated muscles

specification: type of developmental commitment of cells or a tissue explant that is manifested on culture in isolation, but is not irreversible

Spemann’s organizer: the dorsal blastopore lip region of an amphibian embryo that emits BMP inhibitors and thereby dorsalizes the surrounding tissue

spiral cleavage: asymmetrical type of cleavage in which quartets of micromeres are cut off in alternating directions

splanchnic mesoderm: layer of lateral mesoderm lying internal to the coelom

squamous: describes a thin flat type of cell, usually found in a squamous epithelium

stamen: the male reproductive organ of a flower, consisting of a stalk (filament) bearing an anther in which pollen is produced (source:

stem cell: a cell that is undifferentiated; survives a long time; divides to produce more copies of itself; and also divides to produce progeny destined to differentiate

stigma: the receptive apex of the pistil of a flower, on which pollen is deposited at pollination (source:

style: the usually slender part of a pistil, situated between the ovary and the stigma (source:

superficial cleavage: cleavage of nuclei without formation of cells

syncytium: a mass of cytoplasm containing many nuclei

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tailbud: (1) region at the posterior of a vertebrate embryo that generates part or all of the tail. (2) the phylotypic stage of vertebrate embryos

telencephalon: the anterior part of the vertebrate forebrain

temperature sensitive: (=ts) describes mutations that have an effect at one, usually high, temperature but not at another, usually lower, one

teratocarcinoma: pluripotent tumour derived from germ cells

teratogenic: leading to teratological effect

teratology: the study of developmental abnormalities arising from exposure of the embryo to noxious stimuli such as toxic substances, radiation or infection

threshold response: sharp and discontinuous change in cell state occurring at a particular concentration of an inducing factor

tissue culture: growth of cells in vitro, usually as a cell monolayer

tissue engineering: the growth of tissues and organs in vitro, sometimes on a three-dimensional scaffold, either for transplantation or for temporary replacement of the function of a damaged organ

tissue stem cell: (=adult stem cell) stem cell found in a renewal tissue; normally committed to forming just cell types found in that tissue

totipotent: indicates that a cell population is capable of differentiating into any cell type in the organism

transcription factor: protein controlling the transcription of specific genes

transdifferentiation: strictly the change of one differentiated cell type to another, without passing through undifferentiated intermediate states. But often used as synonym for metaplasia

transgene: cloned gene introduced into an organism

transplantation: the movement of tissue from one part of an organism to another, or from one individual to another. It can refer either to microscopic grafts carried out on embryos, or to organ transplants in adults

transposable element: (=transposon) a piece of DNA that may occasionally move from one part of the genome to another

trophoblast: the outermost layer of cells of the blastocyst that attaches the fertilized ovum to the uterine wall and serves as a nutritive pathway for the embryo (source:

ts: =temperature sensitive

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vegetal hemisphere: the lower hemisphere of an egg or oocyte

ventral: the lower surface of an animal

ventral furrow: invagination of prospective mesoderm into an insect embryo

ventricular zone: the cells next to the ventricles of the vertebrate central nervous system

vitelline membrane: extracellular membrane around a zygote

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wild type: the normally occurring allele at a genetic locus

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X-inactivation: shutting down of the activity of one of the two X chromosomes in female mammals

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yolk: granules of food reserve deposited in the oocyte and used for embryonic nutrition. Yolk granules contain a few principal proteins together with lipids

yolk sac: in the avian embryo the extraembryonic membrane that adheres to the yolk mass, consisting of extraembryonic mesoderm and endoderm; in the rodent embryo the middle of three membranes eventually surrounding the fetus, derived from the endoderm and mesoderm

yolk syncytial layer: syncytial region at the junction of the yolk cell and the cellular part of a fish embryo

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zona pellucida: transparent extracellular layer surrounding a mammalian early embryo

zone of polarizing activity: (=ZPA) signaling center in the posterior region of the vertebrate limb bud, that controls the anteroposterior pattern of regional specification

zygote: fertilized egg, strictly after the stage of fusion of male and female pronuclei

zygotic: relating to the genome of the embryo as opposed to that of the mother

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