A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z



agar-A polysaccharide complex extracted from seaweed (Rhodophyceae) and used as an inert support for the growth of cells, particularly bacteria and some cancer cell lines.

Ambystoma mexicanum-Mexican axolotl (amphibian). A salamander that shows neoteny. The adult may retain the larval form, but can reproduce. The neotenous, aquatic axolotl will metamorphose into the terrestrial form if injected with thyroid or pituitary gland extract.

animal cap-Pigmented animal hemisphere of the amphibian blastula.

animal pole-In most animal oocytes the nucleus is not centrally placed and its position can be used to define two poles. That nearest to the nucleus is the animal pole, and the other is the vegetal pole, with the animal-vegetal axis between the poles passing through the nucleus. During meiosis of the oocyte the polar bodies are expelled at animal pole. In many eggs there is also a graded distribution of substances along this axis, with pigment granules often concentrated in the animal half and yolk, where present, largely in the vegetal half.

anterior-posterior axis - Body axis extending from the anterior to the posterior pole of a bilaterally symmetric embryo (or animal).

anterior-posterior polarity


apical constriction

archenteron- Cavity formed by the endoderm during gastrulation; will later become the gut lumen.

axial mesoderm -the mesodermal tissue that gives rise to the notochord and somites.

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blastocoel- Fluid-filled cavity that forms in the embryo after the morula stage.

blastocoel roof

blastomeres - One of the cells produced as the result of cell division, cleavage, in the fertilised egg.

blastoporal groove-The groove formed as result of the formation of bottle cells. The groove presumably results from the invagination produced by apical construction, as well as the basal expansion of the basal ends of the bottle cells, which forces nearby tissue to roll.

blastoporal pigment line-The first visible sign on the surface of the amiphibian embryo that gastrulation is underway. The blatoporal pigment line forms as a result of the apical constriction of bottle cells, thereby concentrating the pigment granules near the apex of each of the bottle cells, and marks the initial phase of formation of the blastopore lip. In Xenopus, the dorsal bottle cells constrict first, creating a crescent of pigment resembling a frown.

blastula-Stage of embryonic development of animals near the end of cleavage but before gastrulation. In animals where cleavage (cell division) involves the whole egg, the blastula usually consists of a hollow ball of cells.

bottle cells - Epithelial cells that temporarily become bottle-shaped, owing to the contraction of their apical margins and the expansion of their basal margins; found at the site of initiation of gastrulation in amphibian embryos.

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calcium wave- Chain reaction of intracellular Ca2 + release and uptake that accompanies the cortical reaction.

cell cycle- Period between the formation of a cell by the division of its parent cell and the formation of two new cells by cell division.

cell rearrangement

centrosomal material

closure of the blastopore

convergence and extension- Convergence of an epithelial sheet toward a central site, followed by its extension along a single axis through forceful intercalation of the cells of the epithelium.

cortex- 1. Gel-like cytoplasmic layer just below the egg plasma membrane. 2. Thickened coelomic epithelium of developing gonads.

cortical reaction- Wave of exocytosis that occurs as the cortical granules fuse with the egg plasma membrane and release their contents after sperm-egg fusion.

cytokinesis- Division of the cytoplasm during mitosis.

cytoplasmic determinant

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deep cells -Generally, non-epithelial cells in a vertebrate embryo. In amphibians, deep cells underlie the superficial epithelial cells of the animal cap and marginal zone.

deep cytoplasm


diacylglycerol (DAG) -

diploid genome - Diploid cells have its chromosomes in homologous pairs, and thus having 2 copies of each autosomal genetic locus. The diploid number (2n) equals twice the haploid number and is the characteristic number for most cells other than gametes.

dorsal involuting marginal zone

dorsal lip of the blastopore- Site of initiation of gastrulation in the amphibian embryo. The dorsal lip, which forms at the site of the gray crescent, forms the dorsal margin of the blastopore.

dorsal non-involuting marginal zone

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ectoderm- Germ layer that gives rise to the epidermis and nervous tissue.

egg activation-The process whereby the egg becomes metabolically active, initiates protein and DNA synthesis, and undergoes structural changes to its cortex and/or cytoplasm. Egg activation normally follows fertilization, and is triggered by a wave of calcium release that passes transiently across the egg. Activation can also be induced by mechanical and chemical treatments of unfertilized eggs (parthenogenetic activation).

endoderm- Germ layer that gives rise to the respiratory organs, gut, and the gut accessory glands.

endoplasmic reticulum - (ER) Membrane system that ramifies through the cytoplasm. The membranes of the ER are separated by 50-200 nm and the cisternal space thus enclosed constitutes a separate compartment. The Golgi region is composed of flattened sacs of membrane that together with ER and lysosomes constitute the GERL system.

epidermis-Outer epithelial layer of a plant or animal. May be a single layer that produces an extracellular material (as for example the cuticle of arthropods), or a complex stratified squamous epithelium, as in the case of many vertebrate species.

epithelial cells

extracellular matrix - (ecm; ECM) Any material produced by cells and secreted into the surrounding medium, but usually applied to the non-cellular portion of animal tissues. The ecm of connective tissue is particularly extensive and the properties of the ecm determine the properties of the tissue. In broad terms there are three major components: fibrous elements (particularly collagen, elastin, or reticulin), link proteins (eg. fibronectin, laminin), and space-filling molecules (usually glycosaminoglycans). The matrix may be mineralised to resist compression (as in bone) or dominated by tension-resisting fibres (as in tendon). The basal lamina of epithelial cells is another commonly encountered ecm. Although ecm is produced by cells, it has recently become clear that the ecm can influence the behaviour of cells quite markedly, an important factor to consider when growing cells in vitro: removing cells from their normal environment can have far-reaching effects.

external development

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fibronectin - Glycoprotein of high molecular weight (2 chains each of 250 kD linked by disulphide bonds) that occurs in insoluble fibrillar form in extracellular matrix of animal tissues, and soluble in plasma, the latter previously known as cold-insoluble globulin. The various slightly different forms of fibronectin appear to be generated by tissue-specific differential splicing of fibronectin mRNA, transcribed from a single gene. Fibronectins have multiple domains that confer the ability to interact with many extracellular substances such as collagen, fibrin and heparin, and also with specific membrane receptors on responsive cells. Notable is the RGD domain recognised by integrins, and two repeats of the EGF-like domain. Interaction of a cell's fibronectin receptors (members of the [integrin] family) with fibronectin adsorbed to a surface results in adhesion and spreading of the cell.


fluorescent dextran-A chemically modified form of dextran, a high molecular weight polysaccharide, which carries one of several different fluorescent moities ("tags") that fluoresce upon excitation with the appropriate wavelength of near ultraviolet or visible light. When the molecular weight of the dextran is sufficiently high, these compounds cannot pass through gap junctions. They are therefore useful in lineage tracing, since only cells directly descended from a cell injected with a dextran will fluoresce.

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G1- Phase in the cell cycle between the completion of cell division and the initiation of DNA synthesis.

G2- Phase in the cell cycle between the completion of DNA synthesis and the next cell division.

Gap Phases-The phases of the cell cycle known as G1 and G2, during which relatively less obvious cellular activity is visible.

gastrula-Stage of embryonic developments in animals when gastrulation occurs; follows the blastula stage.

gastrulation - Process by which cells of the blastoderm are translocated to new positions in the embryo, producing the three primary germ layers.


germ layer-The main divisions of tissue types in multicellular organisms. Diploblastic organisms (eg. coelenterates) have two layers, ectoderm and endoderm; triploblastic organisms (all higher animal groups) have mesoderm between these two layers. Germ layers become distinguishable during late blastula/early gastrula stages of embryogenesis, and each gives rise to a characteristic set of tissues, the ectoderm to external epithelia and to the nervous system for example, although some tissues contain elements derived from two layers.

germ plasm - Region of the egg containing the determinants of the germ cell line.

gray crescent - Region of intermediate pigmentation in the marginal zone of the amphibian egg caused by a shift in the pigmented egg cortex toward the site of sperm entry; marks the future site of the dorsal lip of the blastopore.

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induction- Alteration of cell fate as a result of interactions with neighboring cells.


inositol triphosphate (IP3) - (IP3, InsP3) Inositol 1,4,5 trisphosphate is important as a second messenger. It is released from phosphatidyl inositol bisphosphate by the action of a specific phospholipase C enzyme (PLC_) and binds to and activates a calcium channel in the endoplasmic reticulum.

integrin - Superfamily of cell surface proteins that are involved in binding to extracellular matrix components in some cases. Most are heterodimeric with a _ subunit of 95 kD that is conserved through the superfamily, and a more variable _ subunit of 150-170 kD. The first examples described were fibronectin and vitronectin receptors of fibroblasts, which bind to an RGD (Arg-Gly-Asp) sequence in the ligand protein, though the "context" of the RGD seems important and there is also a divalent cation-dependence. Subsequently the platelet IIb/IIIa surface glycoprotein (fibronectin and fibrinogen receptor) and the LFA-1 class of leucocyte surface protein were recognised as integrins, together with the VLA surface protein. The requirement for the RGD sequence in the ligand does not seem to be invariable.

intercalation - 1. Expansion process whereby cells from different layers lose contact with their neighbors and rearrange into a single layer, which consequently spreads laterally, owing to an increase in surface area. 2. Generation of missing positional values during regeneration when cells of disparate positional values are brought together after amputation.

involuting marginal zone-- Vegetal portion of the marginal zone of the Xenopus embryo that turns inside the embryo during involution.

involution- Process by which an expanding epithelium turns over on itself and continues to spread in the opposite direction along its basal margin.

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Keller sandwiches-One of several types of explants of tissue lying immediately animal to the blastopore lip of an early Xenopus gastrula, and including involuting marginal zone, non-involuting marginal zone, and animal cap cells, named for Ray Keller, who devised the technique. In the original Keller sandwich, two such strips of tissue, containing both superficial and deep cells, are placed together such that their deep cells are in contact and their animal and vegetal ends are aligned. Later, culture techniques were devised allowing observation of single strips of tissue (i.e., a single explant containing superficial and deep cells; the 'open-face" sandwich), and the removal of the majority of the deep cells in such a sandwich ("open-face, shaved" explant).

Ray Keller

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leading edge mesoderm -The mesoderm at the extreme vegetal edge of the marginal zone which forms a free edge after the marginal zone is turned inside the embryo. The dorsal leading edge cells form head mesoderm; ventral leading edge cells form heart tissue. Leading edge cells possess lamellipodia at their free margin.

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M- Period of cell division.

marginal zone - Region of intermediate pigmentation between the pigmented animal hemisphere and the unpigmented vegetal hemisphere of the amphibian egg.

maternal mRNA-Messenger RNA found in oocytes and early embryos that is derived from the maternal genome during oogenesis.

mediolateral interdigitation -An intercalation movement in which cells move between one another towards the midline of the forming anterior-posterior (A-P) axis, resulting in the elongation of the entire array of rearranging cells along the A-P axis.

meiosis-A specialised form of nuclear division in which there two successive nuclear divisions (meiosis I and II) without any chromosome replication between them. Each division can be divided into 4 phases similar to those of mitosis (pro-, meta-, ana- and telophase). Meiosis reduces the starting number of 4n chromosomes in the parent cell to n in each of the 4 daughter cells. Each cell receives only one of each homologous chromosome pair, with the maternal and paternal chromosomes being distributed randomly between the cells. This is vital for the segregation of genes. During the prophase of meiosis I (classically divided into stages: Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis), homologous chromosomes pair to form bivalents, thus allowing crossing-over, the physical exchange of chromatid segments. This results in the recombination of genes. Meiosis occurs during the formation of gametes in animals, which are thus haploid and fertilization gives a diploid egg.

mesoderm-Middle of the three [germ layers]; gives rise to the musculo-skeletal, blood vascular, and urinogenital systems, to connective tissue (including that of dermis) and contributes to some glands.

metazoans - Animals whose bodies consist of many cells, as distinct from Protozoa, which are unicellular; all animals commonly recognized as animals. Spnoges (Parazoa, q.v.) though also multicellular, differ so much from other multicellular animals that they are not ussually included in the metazoa.

microfilaments- Contractile cytoskeletal actin filaments of 6-nm diameter.

microtubules- Components of the cytoskeleton composed of hollow cylindrical rods, 25 nm in diameter, formed of 13 rows of solid tubulin protofilaments that run parallel to the microtubule long axis.

mitochondrial - Microscopic bodies occuring in cytoplasm of every cell in varying numbers except in bacteria and blue-green algae (actually the mitochondria are derived from common ancestors as the bacteria and blue green algae). Contain DNA, ribosomes, and many enzyme systems; comprise power plant of cell, producing energy (in form of ATP) for many cell functions.

mitosis -The usual process of nuclear division in the somatic cells of eukaryotes. Mitosis is classically divided into four stages. The chromosomes are actually replicated prior to mitosis during the S phase of the cell cycle. During the first stage, prophase, the chromosomes condense and become visible as double strands (each strand being termed a chromatid) and the [nuclear envelope] breaks down. At the same time the mitotic spindle forms by the polymerisation of [microtubules] and the chromosomes are attached to spindle fibres at their kinetochores. In metaphase the chromosomes align in a central plane perpendicular to the long axis of the spindle. This is termed the metaphase plate. During anaphase the paired chromatids are apparently pulled to opposite poles of the spindle by means of the spindle fibre microtubules attached to the kinetochore, though the actual mechanism for this movement is still controversial. This separation of chromatids is completed during telophase, when they can be regarded as chromosomes proper. The chromosomes now lengthen and become diffuse and new nuclear envelopes form round the two sets of chromosomes. This is usually followed by cell division or cytokinesis in which the cytoplasm is also divided to give two daughter cells. Mitosis ensures that each daughter cell has a diploid set of chromosomes that is identical to that of the parent cell.

morphogenetic movements -



mRNA - (messenger RNA) RNA species that contains the information to specify the amino acid sequence of proteins and that is translated on the ribosome. In eukaryotes mRNA is normally formed by splicing a large primary transcript ( hnRNA). In eukaryotes the mRNA acquires a GTP cap and usually a poly-A tail.

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National Aeronautics and Space Administration (NASA)

neoteny-The persistence in the reproductively-mature adult of characters usually associated with the immature organism.

neural crest cell

neural fold-bilaterally symmetric infoldings of the neural plate that then seal dorsally to form the neural tube.

neural induction-In vertebrates the formation of the nervous system from the [ectoderm] of the early embryo as a result of a signal from the underlying [mesoderm] of the archenteron roof; also known as primary neural induction. The mechanism of neural induction is not yet clear

neural plate-A region of embryonic ectodermal cells, called neuroectoderm, that lie directly above the notochord. During neurulation, they change shape, so as to produce an infolding of the neural plate (the neural fold) that then seals to form the neural tube.

neural tube-The progenitor of the central nervous system. See neural plate, neurulation.

neurulation-The embryonic formation of the neural tube by closure of the neural plate, directed by the underlying notochord.

non-involuting marginal zone- Animal portion of the marginal zone of the Xenopus embryo that spreads in front of the animal cap but does not involute during gastrulation.

notochord-An axial mesodermal tissue found in embryonic stages of all chordates and protochordates, often regressing as maturity is approached. Typically a rod-shaped mass of vacuolated cells. It lies immediately below the nerve cord and may provide mechanical strength to the embryo.

notoplate- Ventral portion of the neural plate in contact with the notochord.

nucleic acid-Linear polymers of nucleotides, linked by 3',5' phosphodiester linkages. In DNA, deoxyribonucleic acid, the sugar group is deoxyribose, and the bases of the nucleotides adenine, guanine, thymine and cytosine. RNA, ribonucleic acid, has ribose as the sugar, and uracil replaces thymine. DNA functions as a stable repository of genetic information in the form of base sequence. RNA has a similar function in some viruses but more usually serves as an informational intermediate (mRNA), a transporter of amino acids (tRNA), in a structural capacity or, in some newly discovered instances, as an enzyme.

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oogenesis-The process of egg formation.

organogenesis - The process of formation of specific organs in a plant or animal involving morphogenesis and differentiation.

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planar inductive signals

planar signals

primary body axes

primary embryonic induction

primary mesenchyme cells

protein synthesis

protists- Group of animals differing from the rest (Metazoa and Parazoa) in consiting of one cell only, i.e. one continuous mass of cytoplasm, but resembling them and plants, and differing from bacteria, in having at least one well-defined nucleus, of eucaryotic type.


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radial intercalation

Reductive division- -Cell division in which the volume of the two daughter cells does not increase. The result is a progressive increase in cell number, without a corresponding increase in the size of the tissue. Reductive divisions are characteristic of cleavage in early embryos.


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S phase - cell cycle phase during which the quantity of DNA doubles, with replication of the chromosomes.

signal transduction - The cascade of processes by which an extracellular signal (typically a hormone or neurotransmitter) interacts with a receptor at the cell surface, causing a change in the level of a second messenger (for example calcium or cyclic AMP) and ultimately effects a change in the cell's functioning (for example, triggering glucose uptake, or initiating cell division). Can also be applied to sensory signal transduction, eg. of light at photoreceptors.

somites- Blocks of tissue in the trunk derived from the originally unsegmented paraxial mesoderm.

Species-specific sperm-egg binding

Hans Spemann (18??-19??)-German experimental embryologist who was instrumental in developing seminal concepts of induction. Spemann received the Nobel Prize in Medicine in 193? for his work. Hilde Mangold, a graduate student in Spemann's laboratory, and Spemann published the results of Mangold's transplantation of the dorsal lip of the blatopore to an ectopic location (the so-called "organizer" experiment) in 1924. Spemann's earlier work included experiments investigating the differnetiation of embryos ligated with hair loops, as well as experiments examining the induction of the eye.

sperm entry point (SEP)

spiral cleavage-Pattern of early cleavage found in molluscs and annelids (both mosaic eggs). The animal pole blastomeres are rotated with respect to those of the vegetal pole. In some molluscs, the handedness of the spiral twist is maternally inherited.


substratum - The solid surface over which a cell moves, or upon which a cell grows: should be used in this sense in preference to substrate, to avoid confusion.

superficial cells-epithelial cells on the surface of an amphibian embryo.

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Taxol-Drug isolated from yew (Taxus brevifolis) that stabilises microtubules: analogous in this respect to [phalloidin] that stabilises microfilaments.

trypan blue - Biological stain used to determine cell viability. Trypan blue is unable to cross intact plasma membranes, and so only labels dead cells.

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ultraviolet light

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vegetal plate -

vegetal pole-The surface of the egg opposite to the animal pole. Often the cytoplasm in this region is incorporated into future endoderm cells.



vital dyes-a substance that imparts a color or fluorescence to living cells (hence the word "vital") without causing pertubation of normal cellular functions. Vital dyes are used to mark specific groups of cells that are to be followed during subsequent phases of development. Examples include Nile blue, Neutral red, and the fluorescent marker rhodamine isothiocyanate.

Walter Vogt (18??-19??) -Embryologist most famous for his use of vital dyes to construct fate maps of amphibian embryos. Vogt placed small vital dye marks on the surface of amphibian embryos at various stages of decelopment to studyt the movements and fates of various regions of the embryo. His fate mapping studies at the gastrula stage as an important prelude to more modern investigations of amphibian gastrulation.

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Xenopus-The genus of African clawed toads, X. laevis is widely used in developmental biology and was formerly used in pregnancy diagnosis. Ovulates easily under influence of luteinising hormone.

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yolk platelets

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