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hemisphere containing the nucleus (in the case of an egg) or greater
number of cells (in the case of an embryo); opposite the yolky vegetal pole.
Anteroposterior axis The direction in a bilaterally symmetric organism or
embryo de¬ned by an arrow pointing from the head to the tail.
274 GLOSSARY


Anterior Referring to the region of an embryo containing the head;
opposite to posterior.
Antibody A protein produced by the immune system of vertebrate
organisms that combines speci¬cally with another, often foreign,
molecule (antigen). Labeled antibodies are commonly used as tools to
detect the presence and distribution of proteins in cells and embryos.
Antigen A molecule that can evoke the production of antibodies in an
immune-competent organism.
Apical surface The portion of a polarized epithelial cell that contacts
neither adjacent cells nor the basal lamina.
Archenteron The cavity enclosed by the invaginating endoderm during
gastrulation in organisms such as amphibians and sea urchins that later
becomes the gut lumen.
Association constant The ratio for a bimolecular reaction, at chemical
equilibrium, of the concentration of the reversibly formed complex of two
molecules and the product of the concentrations of the non-complexed
molecules; also called the af¬nity constant.
Autocatalysis The promotion of the production of a molecular species by
the same molecular species. In a strict sense, autocatalysis only applies to
the situation in which the molecule acts as a catalyst of the reaction that
produces it, but the term is often used in lieu of the more general
˜˜positive autoregulation.”
Autoregulation In¬‚uence (positive or negative) of a molecular species on
its own production.
Bacterium A category of cell that lacks a nucleus. Bacteria constitute one
of the three major organismal domains. The other two are eukaryotes,
organisms with nucleated cells (e.g., protozoa, slime molds, yeast, animals
and plants), and archaea, which also lack nuclei.
Basal surface The region of an epithelial cell opposite the apical surface
that attaches to the basal lamina.
Basal lamina A sheet-like complex of extracellular matrix macromolecules
produced by, and interposed between, an epithelial cell layer and a second
epithelium or a connective tissue.
Basement membrane A thick extracellular sheet at the interface of
epithelial and connective tissues, comprising a basal lamina and an
additional ¬brous layer produced by the connective tissue.
Basin of attraction A region in the vector ¬eld of a dynamical system in
which all trajectories of the system converge to a stable ¬xed point or
orbit or con¬ned chaotic behavior.
Bifurcation An abrupt change in the number or type of attractors (e.g.,
¬xed-point, limit-cycle) of a dynamical system as a function of a
characteristic parameter (such as a rate constant if the system is composed
of reacting chemical species).
Blastocoel A ¬‚uid-¬lled cavity in a hollow blastula-stage embryo.
Blastoderm The layer of cells, resulting from cleavage in mammalian and
a variety of other species. The blastoderm lies on the surface of the yolk
of avian or reptilian eggs; in long-germ-band insects like Drosophila, it ¬rst
takes the form of a single layer of nuclei beneath the egg cell membrane
(acellular blastoderm) and only later a single layer of cells (cellular
blastoderm).
Blastomere Any of the cells produced by cleavage of a fertilized or
activated egg, that make up a blastula.
GLOSSARY 275


Blastopore An indentation on the surface of a blastula-stage embryo that
represents the site of initiation of gastrulation.
Blastula An early-stage embryo during or at the end of cleavage, before
gastrulation begins.
Bond energy The free energy released upon the formation of a chemical
bond; equivalently, the energy it takes to break the bond.
Brownian motion The non-oriented motion of a single molecule fueled by
its thermal energy and in¬‚uenced by the collisions with surrounding
molecules (such as the motion of a solute particle in a solvent).
Cadherin The collective name of a class of specialized cell adhesion
molecules, forming mostly, but not exclusively, homophilic bonds
(i.e., bonds between pairs of identical molecules).
Carboxyl terminus (C-terminus) One of two ends of a protein: the end
containing a free (uncombined) carboxyl group.
Cartilage A skeletal connective tissue. Cartilage is an elastic solid. Lacking
blood vessels of its own, it is nourished by diffusion from surrounding
tissues. In animals with bony skeletons, much of the skeleton, including
that of the limbs and vertebral column, develops ¬rst as cartilage and is
later replaced by bone.
Caudal Toward the tail. In embryology, usually synonymous with
posterior.
Cdc genes (cell-division-cycle genes) Genes that specify one of a set of
proteins that regulate one or more steps in the eukaryotic cell cycle.
Cdk (cyclin-dependent protein kinase) A protein kinase (product of a cdc
gene) that is active when complexed with a cyclin.
Cell adhesion molecule (CAM) Any member of several classes of integral
membrane proteins that bind to other such molecules on adjacent cells to
mediate cell attachment.
Cell cortex A dense layer of cytoplasm, rich in actin micro¬laments and
actin-binding proteins, that lies directly beneath a cell™s plasma
membrane.
Cell cycle The sequence of molecular and structural events involved in the
replication of DNA and cell division.
Cell division The process by which a cell produces two daughter cells.
Cell fusion The joining of two or more separate cells into a multinucleated
cell with a single plasma membrane.
Cell junction One of several membrane-associated macromolecular
complexes that permit cells to attach to one another or to a substratum.
Cell line A cell population derived from a single progenitor cell that has
accommodated to culture conditions and can grow and replicate
inde¬nitely outside the organism of its origin.
Cell motility The movement of cells by amoeboid locomotion.
Cellular automaton A computer program to study the behavior of a
complex system by representing it in terms of abstract ˜˜cells”
programmed to change state or interact according to speci¬c rules.
Centromere The region of a mitotic chromosome where the sister
chromatids are attached to each other and the kinetochore is assembled.
Centrosome An organelle of animal cells, located near the nucleus, that
organizes the microtubules during mitosis.
Chemotaxis Type of cellular motion driven by a chemical gradient and
powered by the motile apparatus of cells (as opposed to the analogous
process of diffusion, fueled by thermal energy).
276 GLOSSARY


Chordates The group of organisms that contains vertebrates and other
animals with a notochord.
Chromatid One copy of a duplicated chromosome after DNA replication
when it is still joined to the other copy (its sister chromatid) at the
centromere.
Chromatin A complex of DNA, histones, and nonhistone proteins in
eukaryotic cells; present in interphase nuclei and mitotic chromosomes.
Chromosome An individual structure composed of chromatin in a
eukaryotic cell. The chromosomes are long and tangled during interphase,
but become condensed and individually distinguishable during mitosis.
Cilium (plural cilia) A hair-like extension of a cell containing a central
bundle of microtubules and capable of performing repeated beating
movements.
Cleavage A type of cell division in early development in which the embryo
is subdivided with no overall change in mass.
Codon A contiguous group of three nucleotides in a DNA or RNA molecule
that speci¬es a particular amino acid according to the genetic code.
Collagen A family of extracellular proteins, each having signi¬cant
stretches of amino acid sequence of the form Gly-X-Y, where Gly is glycine,
X often proline, and Y often hydroxyproline.
Community effect An inductive interaction in embryonic development in
which the ability of a cell to respond to an inductive signal is enhanced
by, or even dependent upon, other neighboring cells differentiating in the
same way at the same time.
Compartment A population of cells arising during development
exclusively from a small number of founder cells, none of whose progeny
move across boundaries with adjacent compartments.
Competence The capability of a cell or tissue to perform a speci¬c
developmental role, often related to their capacity to respond to a speci¬c
set of signals.
Constitutive Referring to a molecule, cellular structure, or activity that is
not subject to a given type of regulatory mechanism, i.e., that is produced
in all cell types, or at all phases of the cell cycle.
Cortical tension The apparent tension of the plasma membrane, due
primarily to the contractile micro¬laments of the cell cortex.
Convergent extension The rearrangement of a tissue mass characterized
by intercalation of individual cells in one direction resulting in the
elongation of the mass in an orthogonal direction.
Cortical rotation A phenomenon in newly fertilized amphibian eggs in
which the cortical cytoplasm becomes displaced by a characteristic angle
(30—¦ in Xenopus) relative to the cytosolic core.
Covalent bond Strong chemical connection between atoms in a molecule
formed by shared electrons.
Cyclins A class of proteins that regulate the eukaryotic cell cycle by
binding to speci¬c protein kinases and whose concentrations vary in a
periodic fashion.
Cytokinesis The division of the bulk of a cell (cytoplasm and plasma
membrane) into two, as distinct from the division of the nuclear contents,
which is mitosis.
Cytoplasm The interior ¬‚uid compartment of the cell, within the plasma
membrane but outside the organelles.
GLOSSARY 277


Cytosol The ¬‚uid layer of a cell™s cytoplasm, in distinction to the cell
cortex.
Dalton Unit of molecular mass, approximately equal to the mass of a
hydrogen atom, 1.66 — 10’24 g.
Derivative The rate of change of a function in terms of its variable (for
example, the rate of change of concentration in terms of position).
Desmosome A speci¬c type of junctional complex by which adjacent
epithelial cells attach to one another. Desmosomes are connected to
intermediate ¬laments within the attached cells.
Determination The state of a developing cell or tissue in which a
reversible or irreversible decision has been made to follow one of several
possible developmental fates, without an overt change in the cell™s or
tissue™s phenotype.
Diffusion The net drift of molecules in the direction of lower
concentration owing to thermal movement.
Differential adhesion hypothesis (DAH) An interrelated set of assertions
about cell mixtures and tissues in which cells move randomly and readily
past one another. One component of the DAH is that cells take up
consistent relative positions in an aggregate solely on the basis of their
relative adhesive strengths; a second component is that such cell mixtures
and tissues behave like immiscible viscoelastic liquids.
Differential equation A mathematical relationship between a function
and its derivatives.
Differentiation The process by which a cell acquires a specialized type
(e.g., muscle, neuron).
Dimensional analysis An approach based exclusively on the units of
physical parameters used to establish a quantitative relationship between
them.
Diploblast Category of animal generated from an embryo with two germ
layers, ectoderm and endoderm.
Diploid The state of a cell in which it contains two sets of homologous
chromosomes, one from each parent, and thus two copies of each gene.
Dissipative system A system whose energy is not conserved over time.
DNA Deoxyribonucleic acid; the genetic material of eukaryotic cells.
Dominant negative Referring to a gene mutation or a transgene, where
the speci¬ed protein blocks the function of its normal counterpart.
Dorsal mesentery A strip of connective tissue that connects the dorsal
surface of the digestive tube to the inner surface of the dorsal body wall.
Dorsoventral axis The axis of an embryo or body de¬ned by an arrow
pointing from its upper or back (dorsal) surface to its under (ventral)
surface.
Drift velocity The average terminal velocity with which diffusing
molecules move under the in¬‚uence of a constant force.
Dupr© equation An equation that relates the interfacial energy to the
work of adhesion and the work of cohesion.
Dynein A microtubule-associated motor protein that operates in
chromosome movement during mitosis, organelle transport, and the
motion of cilia and ¬‚agella, using energy released from the hydrolysis of
ATP.
Dynamical system A system of interacting components (e.g., chemical
substances) evolving, typically in a complex way, in time.
278 GLOSSARY


Ectoderm The outermost germ layer of an embryo, which gives rise to the
skin and, depending on the species, the nervous system.
Einstein--Smoluchowski equation The equation that relates the diffusion
constant of a molecule to the viscosity of the liquid in which it diffuses.
Elasticity Characteristic physical property of elastic materials, which is
manifest in their ability to recover fully or partially from a deformation
such as elongation or compression.
Electrochemical gradient The driving force that causes an ion to move
across a cell membrane. It is due to the combined effect of a difference in
concentration and in electrical potential across the membrane.
Embryogenesis The formation of an embryo from a zygote, and the
subsequent processes of morphogenesis and differentiation, leading to the
formation of a fully developed organism.
Endocytosis Incorporation of material into a cell by its inclusion in a
membrane-bound vesicle arising from invagination of the plasma
membrane.
Endoderm Innermost layer of a diploblastic (two-layered) or triploblastic
(three-layered) embryo, giving rise to the lining of the digestive tube and,
depending on the species, the accessory organs of the digestive system.
Endoplasmic reticulum (ER) A set of internal membranes in eukaryotic
cells that are sites of lipid synthesis (smooth ER) and the ribosomal
attachment and synthesis of integral membrane and secreted proteins
(rough ER).
Endothelium The characteristic cell layer lining blood vessels and the
heart.
Enzyme A protein that acts as a chemical catalyst.
Epiblast The upper layer of the blastoderm in birds and mammals
(distinct from the hypoblast), which gives rise to all three germ layers
during gastrulation.
Epiboly A morphogenetic movement in some forms of gastrulation in
which a layer of cells moves over and encloses an interior ball of cells.
Epithelioid A type of tissue in which cells make direct contact with their
neighbors via membrane-bound adhesion proteins.
Epithelium A type of epithelioid tissue in which there is a free surface
not attached to other cells.
Equilibrium constant The ratio of the product of the concentrations of
the resulting chemical species to that of the reactants in a reversible
chemical reaction when equilibrium has been reached, i.e., when the rates
of the forward and reverse portions of the reaction are equal.
Exocytosis The process by which most materials are secreted from a
eukaryotic cell; it involves packaging of the material into a
membrane-bound vesicle that then fuses with the plasma membrane.
Extracellular matrix (ECM) A complex network of proteins (such as
collagen and ¬bronectin) and polysaccharides (such as
glycosaminoglycans); the major structural element of connective tissues

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