A

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).

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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B

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

blastomere - One of the cells produced as the result of cell division,
cleavage, in the fertilized egg.

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.
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C

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.

cortex - Gel-like cytoplasmic layer just below the egg plasma membrane.

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.
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D

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.

diacylglycerol (DAG) - this, along with IP₃ is a signaling molecule produced from the precursor
molecule PIP₂ by the enzyme Phospholipase C. DAG helps to activate Protein Kinase C and thereby activates
protein phosphorylation cascades.

diploid genome - diploid cells have their 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 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.
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E

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
space thus enclosed constitutes a separate compartment. Source of intracellular Ca⁺⁺.

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 epithelium, as in the case of many vertebrate species.

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). Although ecm is produced by cells, it has recently become clear that the
ecm can influence the behavior of cells quite markedly.
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F

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 recognized 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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G

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
side of sperm entry; marks the future site of the dorsal lip of the
blastopore.
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I

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

inositol triphosphate (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.

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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M

mitosis (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.

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. Sponges (Parazoa) though also multicellular, differ so much
from other multicellular animals that they are not usually 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.

mitochondria - 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.

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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N

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

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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O

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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P

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.
 

R

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

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.

Spemann, Hans - German experimental embryologist who was
instrumental in developing seminal concepts of induction. Spemann received
the Nobel Prize in Medicine in the 1930's 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. Mangold died in a tragic
accident prior to the Nobel award to Spemann. Spemann's earlier
work included experiments investigating the differentiation of embryos
ligated with hair loops, as well as experiments examining the induction of
the eye.

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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T

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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V

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 dye - a substance that imparts a color or fluorescence to living cells
(hence the word "vital") without causing perturbation 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.

Vogt, Walter - 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 development
to study 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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X

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 luteinizing hormone.
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