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cells. Meiosis results in four haploid progeny of the original progenitor
cell, among which are the male or female gametes.
Membrane A sheet-like structure. Cells have external (plasma) and internal
molecular-scale membranes consisting of phospholipid bilayers. Embryos
are surrounded by macroscopic tissue membranes.
Membrane channel One of a variety of protein complexes in phospholipid
membranes that allows the speci¬c transport of small molecules.
Membrane potential The electric potential difference across a membrane
separating two otherwise insulated compartments.
Mesenchymal condensation Rearrangement of mesenchymal cells that
brings them into direct contact with one another, usually transiently, at
locations that serve as primordia for structures such as cartilage
elements, feathers, or hairs.
284 GLOSSARY


Mesenchyme A type of tissue, common to the early embryo and early
stages of organogenesis, in which cells are loosely suspended in a dilute
extracellular matrix.
Mesoderm The middle germ layer in tripoblasts, which gives rise to
muscle, blood, and skeletal and other connective tissues.
Metabolism The totality of the biochemical processes that sustain a living
cell or organism.
Metaphase The stage of mitosis or meiosis at which the chromosomes are
aligned at the cell center prior to separation.
Metazoa The class of multicellular animals.
Micromere Small cell that arises as a result of unequal early cleavages.
Microtubule organizing center Characteristic region within the cells (such
as the centrosome), from which microtubules emanate, typically in a
radial pattern.
Midblastula transition A set of early embryonic changes characterized by
the iniation of transcription from the zygotic genome and pregastrulation
cell movements.
Microfilament A cytoskeletal ¬lament composed of a polymer of the
protein actin.
Microtubule The polymerized form of the protein tubulin; a constituent of
the cytoskeleton that plays central role in vesicular transport and cell
division.
Microvillus (plural microvilli) A small, nonmotile extension of the cell
surface involved in absorption or other surface-dependent functions.
Mitochondrion Membrane-bound organelle of eukaryotic cells, in which
free energy is extracted from cellular fuels and stored, by oxidative
phosphorylation, in the form of ATP.
Mitogen A biological molecule that stimulates the division of one or more
cell types.
Mitosis Stage of the cell division cycle (other than in germ cells), in which
the separation of chromosomes and the eventual separation of the
daughter cells take place.
Mitotic spindle A special cellular structure made of microtubules that
forms in the course of mitosis and meiosis and provides the architectural
basis for the separation of chromosomes.
Mole A quantity of a substance corresponding to its molecular weight in
grams.
Monomer A subunit of a larger structure (e.g., dimer, polymer).
Monte Carlo method A computational approach for studying the behavior
of a complex system by subjecting a model representation of the system
to small random perturbations and simulating its evolution through
states with successively lower energies.
Morphogen A secreted biological molecule that in¬‚uences tissue pattern
or form.
Morphogenesis The shaping of an organism™s body or organs.
Mutant A gene that is altered in its internal sequence relative to a
reference gene.
Navier--Stokes equation The equation of motion of a viscous ¬‚uid, derived
by the application of Newton™s second law.
Nernst equation The equation that relates the ionic concentration
difference to the electrical potential difference across a membrane such
as the cell plasma membrane.
GLOSSARY 285


Neural crest The portion of the neural plate that detaches and disperses
through the embryo, giving rise to the peripheral nervous system,
portions of the cranial skeleton and the adrenal gland, pigment cells of
the skin, and several other cell types.
Neural folds The pair of ridges at the periphery of the neural plate that
eventually fuse to form the neural tube.
Neural plate The thickened region of ectoderm on the dorsal surface of an
embryo where neurulation is initiated.
Neuron A nerve cell; characterized by electrical excitability and extended
processes (axons and dendrites).
Neurulation The set of cell differentiation and morphogenetic events that
lead to the formation of the neural tube, and eventually the spinal cord,
in chordates.
Nieuwkoop center A population of cells that forms opposite the sperm
entry point in the frog embryo and serves as an inducer of the
Spemann--Mangold organizer.
Notochord A stiff rod-shaped tissue in the embryos of chordates that
forms from the central mesodermal layer and acts as an inducer of
neurulation in the overlying ectoderm.
Nuclear envelope A structure that consists of two concentric
phospholipid bilayer membranes surrounding the nucleus which, in
conjunction with the nuclear pores that traverse it, provides a
selective barrier between the interior of the nucleus and the
cytoplasm.
Nuclear pore One of several thousand eight-fold symmetric multiprotein
complexes that traverse the nuclear envelope and serve as selective
conduits for the traf¬cking of molecules between the nucleus and
cytoplasm.
Nucleic acid A polymer constructed of nucleotides, e.g., DNA, RNA.
Nucleotide An organic molecule consisting of a sugar, a phosphate group,
and a heterocyclic nitrogenous base that serves as the subunit of nucleic
acids such as DNA and RNA.
Nucleus The cell organelle that contains the major portion of genetic
material and is the site of the transcription and processing of RNA.
Ontogeny The generation, over the course of development, of an individual
organism, to be contrasted with phylogeny.
Oocyte A cell in the lineage giving rise to the ovum that precedes the ¬rst
(primary oocyte) or second (secondary oocyte) meiotic divisions.
Oogenesis The sequence of biosynthetic and macromolecular assembly
processes that leads to the development of an ovum.
Oogonia Early cells of the female germline that undergo meioisis and
ultimately give rise to ova (eggs).
Open system A material system that exchanges chemicals and energy with
its environment (the opposite of a closed system, in which interactions
exist only between the constituents of the system itself ).
Ordinary differential equation A differential equation that relates a
function of one variable to its ¬rst-order derivatives (in a ¬rst-order
differential equation) or to its higher-order derivatives (in a higher-order
differential equation).
Organizer A set of cells in an embryo and in some fully formed
invertebrate organisms, such as hydra, that in¬‚uences the movement or
differentiation of adjacent tissues.
286 GLOSSARY


Organogenesis A set of cell differentiation, pattern-formation, and
morphogenetic events that lead to the generation of an organ during
embryonic development.
Osmosis The net movement of solvent across a semipermeable membrane
(i.e., one that is permeable to solvent, e.g., water, but not solute), driven by
a difference in solute concentration on either side.
Osmotic pressure The pressure that must be exerted on the side of a
semipermeable membrane containing the higher concentration of solute
to prevent the ¬‚ow of solvent (e.g., water) across the membrane due to
osmosis.
Ovum The egg, or female gamete.
Pair-rule genes A class of nonuniformly expressed genes (e.g., even-skipped,
fushi tarazu) involved in segmentation in insects. The products of pair-rule
genes are generally involved in the regulation of segment polarity genes.
Paracrine A type of local communication between cells that involves
released factors.
Phenotype The outward expression of an organism™s or cell™s
characteristics, to be contrasted with its genotype.
Planar polarity The structural asymmetry of cells in a tissue that
constrains the direction of their elongation, movement, or growth to a
preferred plane or stack of planes.
Partial differential equation A differential equation that relates a function
of more than one variable (e.g., time and distance) to its derivatives.
Percolation theory/transition The mathematical representation of a
(second-order) phase transition in the connectivity properties of systems
composed of randomly or irregularly arranged subunits (e.g., ¬laments,
monomers, metallic islands in an insulating matrix, etc.). Below the
transition the system contains isolated clusters of the subunits, whereas
above the transition, arrived at by increasing the number of subunits, an
interconnected network of the subunits forms that spans the entire
system.
Phagocytosis The engulfment of a particle (e.g., a bacterium or cell
fragment) by a cell.
Phase transition A major alteration in material characteristics (e.g., from
liquid to gas, from one crystal structure to another, from insulator to
conductor), typically triggered by a change in an external physical
parameter (e.g., temperature, pressure, chemical concentration).
Phosphorylation Attachment of a phosphate group to another molecule.
Phylogeny The generation, over the course of evolution, of a given type of
organism, to be contrasted with ontogeny.
Plasma (or cell) membrane The lipid bilayer that constitutes the boundary
between the interior of a cell and its microenvironment.
Pluripotent cell A cell that is capable of giving rise to a variety of
terminally differentiated cell types.
Piconewton (pN) Unit of force equal to 10’12 N.
Polar body The products of meiosis I and meiosis II other than the
secondary oocyte or ovum.
Posterior Referring to the region of an embryo containing the tail;
opposite to anterior.
Polyspermy An abnormal condition whereby an egg is fertilized by more
than one spermatozoon.
GLOSSARY 287


Pressure The force acting on unit area in the normal direction (the
outward normal in the case of a closed surface).
Prokaryote A class of organisms whose DNA is not organized into nuclei.
Promoter Sequence of DNA within a gene that binds transcription factors
and related proteins, thereby mediating control of the gene™s activity.
Prophase The initial stage of mitosis when chromatin condenses into
discrete chromosomes, the nuclear envelope breaks down, and spindle
¬bers form at opposite poles of the cell.
Protein A polymer of amino acid residues linked by peptide bonds. The
amino acid sequences (primary structure) of different proteins are
speci¬ed by corresponding DNA sequences contained in genes.
Protein kinase An enzyme that transfers a phosphate group to a protein.
Protein phosphatase An enzyme that removes a phosphate group from a
protein.
Proteoglycan A macromolecular component of the cell surface or
extracellular matrix, consisting of a (core) protein with one or more
glycosaminoglycan chains attached to it.
Proximodistal axis The axis of an appendage such as the vertebrate limb,
de¬ned by an arrow pointing from the site of its attachment on the body
to its tip.
Protozoa Class of free-living single-celled eukaryotes.
Radial intercalation Movement of cells from different layers between one
another along the embryo™s radius during convergent extension so as to
form a thinned cell mass.
Reaction--diffusion mechanism A speci¬c means of pattern formation in
which chemical components react with one another as well as diffuse at
different rates.
Receptor A protein or protein complex that mediates a biological activity
upon binding to another molecule, its ligand.
Replication The process by which a cell™s DNA is duplicated.
Rostral Toward the nose. More generally, toward the head or anterior
region of the embryo.
Rostrocaudal axis The axis de¬ned by an arrow pointing from the head
(rostral) region to the tail (caudal) region of an animal or embryo.
S phase The phase of the cell cycle during which chromosomal DNA is
synthesized.
Scale-free network Spatial arrangement of objects (e.g., molecules) with no
dominating characteristic distance between the objects (in contrast with,
e.g., a regular crystalline lattice, which has a well-de¬ned lattice constant).
Scaling The characteristic power-law dependence of a physical quantity
(e.g., mass, conductivity, length of shore line, etc.) on the size of the units
(i.e., the scale) of a quantity with which it varies (e.g., concentration,
temperature, length, etc.).
Segmentation The transformation of a uniform tissue mass into tandemly
arranged tissue blocks.
Segment polarity gene One of several genes in the Drosophila body
segmentation process (e.g., engrailed, wingless) whose expression
distinguishes the anterior and posterior portions of the developing
segment.
Separatrix A geometric structure (a line in two dimensions, a plane in
three dimensions) dividing distinct basins of attractions in a vector ¬eld.
288 GLOSSARY


Signal transduction The transmission of biochemical or mechanical
changes within a cell in response to an external stimulus, typically
leading to changed cell behavior.
Sister chromatids The two joined chromatids of a single replicated
chromosome.
Somatic cell A cell of the body or its organs, in contrast to a cell of the
germ line.
Somatopleure The outer of the two sheets into which the paraxial tissue
(that lying on either side of the notochord) of the three-layered vertebrate
embryo splits subsequent to gastrulation. The somatopleure contains the
somatic mesoderm and the ectoderm and eventually forms the body wall
(cf. splanchnopleure).
Somite A block of tissue, present in a series of pairs, arrayed along the
central axis of vertebrate embryos. The somites give rise to the vertebral
bones, associated connective tissues, intervertebral discs, and body wall
and limb muscles.
Sorting The propensity of distinct living tissues composed of adhering and
motile cells to separate and form boundaries of ˜˜immiscibility” across
which no signi¬cant motion of cells takes place.
Spanning cluster An interconnected cluster of subunits that forms at a
percolation transition and extends over the entire system.
Spemann--Mangold organizer A signaling center located at the anterior or
rostral blastopore lip of the frog embryo that initiates gastrulation
movements.
Spermatogonia Early cells of male germ line that undergo meiosis and
eventually give rise to spermatozoa.
Spermatozoon (pl. spermatozoa) The male gamete.
Splanchnopleure The inner of the two sheets into which the paraxial
tissue (that lying on either side of the notochord) of the three-layered
vertebrate embryo splits subsequent to gastrulation. The splanchnopleure
contains the splanchnic mesoderm and the endoderm and eventually
forms the digestive tube (cf. somatopleure).
Stable fixed point A ¬xed point of a dynamical system that attracts
trajectories in the vector ¬eld.
State space Coordinate system spanned by axes measuring the values of
the functional variables (e.g., time-dependent concentrations)

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