Biology 376 - Developmental Biology

Glossary, Fall 2017

 [update]31 Aug 17
This Biology 376 Glossary was begun in Fall 1995 by the Biology 212 class at Lafayette College, and continued to be developed each semester by the instructor and students each year. Note that starting with the 9th edition of 'Developmental Biology' by S. Gilbert (now the 11th edition by Gilbert & Barresi), a glossary was finally added. This online glossary was originally created in part because our text lacked one. Therefore, it is no longer regularly updated.

Please note that the definitions are often given to explain a word found in a particular context found in our text and so may be somewhat narrowly or simply defined. Typically the definitions are provided by the student and modified as necessary by the instructor. Some terms found below are no longer found in the current edition of the text (now the 11th), but they remain in the glossary nonetheless.

To limit the scope of the glossary, gene and protein names are not generally included (although there are a few).

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Within definitions, words in bold are defined under their own heading within the glossary.

A | B | C | D | E | F |G | H | I | J | K | L |M | N | O | P | Q | R |S | T | U | V | W | XYZ


A23187 - mobile-carrier calcium ionophore (allows Ca++ ions to cross cell membranes) originally isolated as an antibiotic from Streptomyces chartreusensis. Also known as Calcimycin, Antibiotic A23187.

ablation - experimental removal or killing of some part of an organism; for example, in experimental embryology, used to determine what effect absence of the structure will have on development of the remaining embryo.

Acetabularia - genus of unicellular marine green algae. These organisms comprise an enormous single cell consisting of a cap, stalk and rhizoid and can be up to 10 cm in size. Experiments involving nuclear transplants among different Acetabularia species helped demonstrate the essential role of the nucleus in determining development of the organism.

acetylcholinesterase (AChE) - enzyme that catalyzes hydrolysis of the neurotransmitter acetylcholine into acetate and choline. Expression of AChE can serve as a marker of skeletal muscle differentiation.

acrosomal vesicle - membrane-bound organelle in the sperm head derived from the golgi apparatus; the vesicle containing enzymes that digest proteins and complex sugars in the outer coverings of an egg. Fusion of the acrosomal vesicle with the plasma membrane of the sperm (in the "acrosome reaction") exposes receptors that bind to the egg surface and is necessary for fertilization.

actin - protein which when polymerized forms microfilaments, part of the cytoskeleton and necessary for cytokinesis and other cellular functions. Also well known as a major component of the contractile apparatus of muscle cells (along with myosin).

agglutination - the state of joining or clumping together by adhesion.

aggregate (noun) - collection of units or particles (e.g., cells) forming a body or mass. (verb) - to form such a body or mass.

albumen - The 'white' of a bird's egg which provides both protein and water for the growing embryo.

allantois - extra-embryonic membrane emerging as a sac from the hindgut's ventral wall; formed from the splanchnopleure (combination of endoderm and splanchnic mesoderm). Found in amniotes, it is one of the four extraembryonic membranes (chorion, amnion, allantois and yolk sac) that are adaptations of the terrestrial egg. It collects waste materials from the embryo, and as a part of the chorio-allantoic membrane can be a site of gas exchange.

allometric growth or allometry - phenomenon whereby parts of the same organism grow at different rates. Contrast with isometric growth.

amniocentesis - prenatal diagnostic procedure in which amniotic fluid is withdrawn from amniotic sac in order to obtain fluid and fetal cells which are analyzed for metabolic and/or genetic disorders, and to test the maturity of the fetus' lungs.

amnion - the innermost membranous sac enclosing the embryo of an amniote; it becomes filled with amniotic fluid. One of the four amniote extraembryonic membranes; derived from the somatopleure (combination of ectoderm and somatic mesoderm)

amniote - higher vertebrate capable of terrestrial reproduction, and having an amnion during its development. Includes reptiles, birds and mammals, which share a common ancestor.

amniotic membrane - see amnion.

amplexus - [Latin, an embracing] form of sexual reproduction seen in frogs wherein the male grasps the female from behind and externally fertilizes the eggs as they are deposited.

ampulla - upper region of the mammalian oviduct, near the ovary. Fertilization typically takes place in this region.

analogous structures - structures having similar function or superficial appearance, but not necessarily sharing a common evolutionary origin (contrast with homologous structures).

anencephaly - [Greek, no brain] condition of forebrain degeneration caused by failure of anterior neuropore closure and consequent prolonged contact of the forebrain with amniotic fluid. Although infants with this neural tube defect are sometimes born alive, the condition is invariably fatal within a few hours or days.

angiogenetic clusters - see blood islands

animal hemisphere - half of an egg or embryo that contains less yolk and/or which divides more rapidly in comparison to the vegetal hemisphere. In eggs or embryos with considerable yolk, the animal hemisphere will be the upper half when the embryo is allowed to settle by gravity - yolky cytoplasm being more dense than yolk-free cytoplasm.

apoptosis - also known as programmed cell death (PCD); an active cellular process ("cell suicide") consisting of a stereotyped set of events including nuclear condensation, chromosome degradation, release of specific cellular proteases and signaling for phagocytosis. Apoptosis/PCD is a normal part of development in most or all tissues, aiding in regulating cell number and morphogenesis.

archenteron - cavity formed during gastrulation by invagination of the vegetal plate cells (in sea urchin) or involution of cells at the blastoporal lip (in amphibians); it will become the interior of the primitive gut. May also refer to the structure forming this cavity.

archeocyte - somatic cell of sponge that can differentiate into all three other cell types of the organism.

aster - [Latin, star] radiating formation of microtubules at each pole of a spindle apparatus, formed during mitosis.

autonomous specification - determination of cell fate by material acquired during mitosis, independent of interactions with neighboring cells (for example, by 'cytoplasmic segregation' of factors into one daughter cell but not the other.)

autoradiography - photographic process used to detect location of radioactive material.

axoneme - motor section of flagellum constructed of microtubules emanating from the centriole at the base of the flagellum.

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Bilateria - group containing most animals, defined by having bilateral symmetry; includes protostomes and deuterostomes. Note that some Bilateria have secondarily evolved radial symmetry in the adult form, although larvae may still retain bilaterial symmetry (e.g. echinoderms).

bindin - acrosomal protein (of sea urchin sperm) mediating species-specific recognition and binding between the sperm and the egg once the sperm has penetrated the egg jelly.

blastocoel - fluid-filled cavity found in the interior of a blastula or blastocyst.

blastocyst - cleavage stage mammalian embryo; a hollow ball of cells made of outer trophoblast cells and an inner cell mass.

blastoderm - cell layer formed during cleavage of telolecithal and centrolecithal eggs.

blastomere - any embryonic cell formed during cleavage.

blastopore - site of gastrulation initiation and later the opening of the archenteron at the vegetal region of certain embryos (e.g., echinoderm and amphibian); in deuterostome embryos it is the future anus of the organism.

blastula - a cleavage stage embryo, typically a hollow ball of cells surrounding a cavity called the blastocoel; this term is used for (among others) echinoderm and amphibian embryos.

blood islands - also known as angiogenetic clusters; masses of splanchnic mesodermal cells found in the yolk sac of amniotes. The first blood-forming tissue of the embryo, responsible for red blood cells and vitelline blood vessels.

bottle cells - epithelial cells found at the initial site of gastrulation, lining the initial archenteron, that temporarily become bottle-shaped; they maintain contact with the outer surface of the embryo, but the majority of the cell is inside the embryo. Also known as flask cells.

branchial arches - see pharyngeal arches.

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cAMP - cyclic 3',5' - adenosine monophosphate, an important intracellular second messenger molecule formed from ATP by the enzyme adenylate cyclase. Also used by the slime mold Dictyostelium for extracellular cell-to-cell communication to promote chemotaxis during aggregation and to promote spore cell differentiation during migration and culmination.

capacitation - change in mammalian sperm that occurs after exposure to female genital tract making the sperm competent to undergo the acrosome reaction; this change is necessary for penetration of the cumulus matrix and for fertilization. Numerous molecular changes in the sperm are associated with capacitation, but the extent to which each event causes sperm capacitation is uncertain.

central nervous system (CNS) - in the vertebrates, the brain and spinal cord; derived from the neural tube. Contrast with the peripheral nervous system (PNS), derived from the neural crest.

centriole - microtubule-based structure that divides prior to mitosis; with the pericentriolar material constitutes the centrosome it is associated with the poles of the spindle apparatus during karyokinesis.

centromere - see kinetochore.

centrosome - microtubule organizing center that contains the centriole; it divides and organizes the poles of the mitotic spindle apparatus during mitosis.

chelator - substance that binds particular ions, removing them from solution; e.g., EDTA is a chelator of divalent cations such as Mg++.

chemotaxis - directed movement of a cell or organism toward (or away from) a chemical source.

chordate - organism having a notochord at some stage of development - a rigid cartilaginous rod in the back extending from anterior to posterior; this group includes the vertebrates.

chorion - one of the four extraembryonic membranes of amniotes; it forms from the somatopleure (ectoderm and somatic mesoderm). In birds and reptiles, the membrane adheres to the shell and is highly vascularized to serve in gas exchange. In mammals, it forms the fetal contribution to the placenta, made by trophoblastic tissue and extraembryonic mesoderm, containing blood vessels that allow exchange of materials with maternal circulation.

chorionic somatomammotropin - aka placental lactogen, a hormone that promotes maternal breast development during pregnancy.

chromosomal puff - expanded region of a polytene chromosome indicative of active messenger RNA synthesis.

cloning vector - intentionally designed artificial DNA construct used by molecular biologists to amplify selected pieces of DNA inserted into the construct; examples include plasmid, phage, phagemid, cosmid, fosmid, yeast artificial chromosome (YAC) and bacterial artificial chromosome (BAC). Cloning vectors minimally contain an origin of replication, selectable marker gene (e.g., ampicillin resistance gene), and multiple cloning site containing unique restriction enzyme sites; other useful features may also be present.

compaction - event in early cleavage-stage mammalian embryo during which blastomeres become tightly joined, forming gap junctions enabling the exchange of ions and small molecules to pass from one cell to the next.

congenital (adj.) - existing at or before birth; acquired at or during birth.

conjugation - form of sexual reproduction (exchange of genetic material) used by some unicellular organisms.

contractile ring - the mechanical agent of cytokinesis, made of actin microfilaments. It is located in the cortical cytoplasm and provides the necessary force to split a single cell into two cells.

cortical granules/vesicles - membrane-bound structures in the egg, derived from the Golgi apparatus, and found just beneath the plasma membrane. The cortical reaction is the release of their contents (proteases, mucopolysaccharides, and peroxidases) by fusion with the plasma membrane; the changes caused by the cortical reaction help to prevent polyspermy.

convergent extension - cell movement resulting in tissue elongation via intercalation of adjacent cells in an epithelial sheet to form a narrower, longer strip of tissue.

ctenophore - radially symmetric diploblastic animal also known as comb jelly. Although superficially similar to cnidarians (aka coelenterates) such as jellyfish, their phylogenetic affinities are uncertain. (That is, it is unclear how they are related to other animals.)

cyclic AMP - see cAMP

cyclin - protein active in regulating the cell cycle, typically synthesized and degraded during the cell cycle to regulate the activity of a cyclin-dependent kinase. Cyclin B is the large protein subunit of mitosis promoting factor (MPF); cyclin B is synthesized and degraded during the cell cycle to regulate MPF activity. Cyclin B plus cyclin dependent kinase 1 (cdk1, aka cdc2, aka p34 kinase) together form an active MPF protein. There are many other cyclins that regulate the cell cycle.

cytokinesis - division of the cytoplasm and plasma membrane of a single cell into two cells.

cytoplasmic localization - phenomenon wherein factors (morphogenetic determinants) are found in a specific region of the oocyte and/or are later segregated to specific blastomeres (cytoplasmic segregation). Associated with autonomous specification as a mechanism of cell specification.

cytoplasmic segregation - restriction of factor(s) into one daughter cell but not the other to specify cell fate, a mechanism associated with autonomous specification.

cytotrophoblast - inner cellular layer of the trophectoderm (trophoblast), between the syncitiotrophoblast, and chorionic villus capillaries; part of the mammalian placenta. In contrast to the syncitiotrophoblast, made of individual mononucleate cells.

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Dalton - measure of molecular weight or mass. One hydrogen atom has mass of 1 Da. Proteins and other macromolecule molecular weights are usually measured in kDa or kD (kilodaltons) - 1000 Da.

dauerblastula - permanent "blastula" of ciliated epidermal cells formed by experimental isolation of animal pole cells of the sea urchin embryo. ["dauer" is from the German dauern - "to endure"]

delamination - splitting of one cellular sheet or layer into two parallel layers.

deoxynucleotides - components of DNA, containing the phosphate, sugar and organic base; when in the triphosphate form, they are the precursors required by DNA polymerase for DNA synthesis (i.e., ATP, CTP, GTP, TTP).

deuterostomes - broad classification of triploblastic animals including echinoderms and chordates that tend to share certain embryological traits; among these the formation of the "mouth second" (hence the name) during gastrulation, after the future anus, which is comes from the blastopore, the site of gastrulation initiation. (Contrast with protostomes)

dideoxynucleotides - chain-terminating precursors of DNA synthesis that block further polymerization when added to the end of the DNA strand by DNA polymerase. These nucleotides lack a 3'-OH (hydroxyl) group necessary for continued 5'-to-3' DNA synthesis. Dideoxynucleotides are used in molecular biology for Sanger-type DNA sequencing, and in medicine as anti-retroviral drugs for the treatment of HIV infection (e.g., ddI, ddC, and AZT).

differential RNA synthesis - concept that states that cells are different from each another because they turn on (transcribe mRNAs from) different sets of genes; thus each cell makes different proteins appropriate to its particular function.

differentiation - process whereby cells acquire their mature morphological and biochemical characteristics. Differentiation is often considered a 'final step' of development in which cells take on their mature function.

differentiation-inducing factor (DIF) - a low molecular weight lipid that induces posterior cells of a Dictyostelium slug to differentiate as stalk cells as opposed to spore cells.

discoidal cleavage - incomplete division of the blastodisc, a region of yolk-free, active cytoplasm; characteristic of birds, fishes and reptiles.

DNA ligase - protein that joins (ligates) DNA strands; used by cells for DNA repair, by molecular biologists for gene cloning.

dorsolateral hinge points - during neurulation of bird embryos, regions involved in bending of the lateral neural plate; wedge-shaped neural cells are anchored to the surface ectoderm to form additional furrows near the connection of the neural plate to the remainder of the ectoderm.

dynein - a microtubule-dependent motor protein; cytoplasmic dyneins work in organelle transport and mitosis, whereas the closely related ciliary dyneins are attatched to outer doublet microtubules in a cilium or flagellum, providing sliding/bending force used in (for example) sperm propulsion.

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ectoderm - (1) the outer cellular membrane of a diploblastic animal. (2) a: the outermost of the three primary germ layers of a triploblastic embryo. b: a tissue (as neural tissue) derived from this germ layer. In the vertebrates, ectoderm splits into three major divisions: neural ectoderm (neural tube), neural crest, and epidermal ectoderm.

ectopic (adj.) - misplaced, not in the normal location.

ectopic pregnancy - implantation of the embryo in the oviduct or other site outside the uterus, typically caused by failure of zona pellucida to keep blastocyst from adhering to oviduct walls; it is a dangerous condition that can cause life-threatening hemorrhage in the mother of the developing embryo.

endoderm - One of the three primary germ layers formed in the embryo, moved into interior by cell movements during gastrulation. In vertebrates, this innermost layer of cells goes on to form the linings of the gut (esophagus, stomach, intestines, rectum, colon), pharyngeal pouch derivatives (tonsils,thyroid, thymus, parathyroid glands), lungs, liver, gall bladder, pancreas. In amniotes, extraembryonic endoderm participates in the formation of the allantois and yolk sac.

endostyle - a ciliated, mucus-secreting groove in the ventral surface of the pharnyx of non-vertebrate chordates (e.g, tunicates and lancelets); it aids in transporting food to the esophagus. Recent molecular evidence supports the traditional view that the endostyle is homologous to the vertebrate thyroid gland.

enhancer - DNA sequence in a gene that influences the gene's expression by increasing or decreasing its rate of RNA synthesis (via effects on binding and transcription of the basal transcription apparatus including RNA Polymerase); can be located upstream (5'), downstream (3') or within the protein coding sequence of the gene.

embryology - study of embryogenesis, the development of animals and plants from fertilization to birth/hatching.

epiboly - literally, "over the ball," usually the growth of epidermal ectoderm to cover the surface of the embryo during gastrulation.

epigenesis - theory holding that development is a gradual process of increasing complexity. (This contrasts with preformationism, which holds that the organism is already present in the gamete(s), merely growing and unfolding during development.) For example, organs are formed de novo in the embryo rather than increasing in size from pre-existing structures.

epithelial (adj.) - belonging to a sheet of tightly joined, polarized cells.

erythrocyte - red blood cell.

erythropoiesis - red blood cell (erythrocyte) development, including pluripotential stem cell division, restriction of potential and differentiation into a mature red blood cell containing hemoglobin.

exogenous (adj.) - arising from a source outside the organism or cell.

exon - segment of DNA sequence in a gene that will be transcribed in the nucleus, spliced to other exons, and transported to the cytoplasm as a part of the mature mRNA; see also intron.

extra-embryonic membranes - tissues arising during development of amniote embryos that do not become a part of the 'embryo proper' that hatches or is born, considered to be adaptations essential to a terrestrial egg. Formed from dual layers of extra-embryonic ectoderm plus mesoderm (outer membranes), or extra-embryonic mesoderm plus endoderm (inner membranes). See chorion, amnion, allantois and yolk sac.

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fate map - diagram that takes the larval or adult structure of an organism and "maps" it onto the region of the embryo from which it arises

fertilization cone - a prominence extending from the surface of some eggs at the moment of, or in some cases allegedly shortly before contact with a sperm.

fission - asexual reproduction in which the parent organism divides into two or more parts, each developing into genetically identical individuals.

flask cells - see bottle cells

founder cells - daughter cells created by asymmetric cell divisions resembling a stem cell divsion pattern during early cleavage of C. elegans (for example) that will give rise to specific sets of differentiated cells.

fruiting body - in Dictyostelium, the final structure derived from a grex/slug/pseudoplasmodium, an aggregation of thousands of myxamoebae forming a single structure composed of spore cells and a stalk.

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gastrulation - stage in animal development following cleavage characterized by extensive cell movement and rearrangement to form a "three-layered" embryo of ectoderm, mesoderm and endoderm.

gene cloning - isolation and amplification of selected pieces of DNA by recombinant DNA techniques.

genome - complete haploid complement of DNA (including all genes) from the chromosomes of the nucleus of an organism.

genomic DNA - nuclear DNA of the chromosomes.

genomic equivalence - concept that each cell in the body has the same genetic material and therefore all the information necessary to create a complete organism. Animal cloning from a somatic nucleus 'proves' this idea.

gill arches - see pharyngeal arches.

golgi apparatus - stack of intracellular membrane compartments (organelle) that function to modify and package secreted and integral membrane proteins; during spermatogenesis, the golgi apparatus causes the formation of an acrosomal vesicle in the sperm head.

gonidia - reproductive cells of colonial photosynthetic eukaryotes such as Volvox.

granular/granule cell - most abundant neuron type in the cerebellar cortex of the vertebrate brain.

gray crescent - region of the egg seen in some amphibian zygotes formed by the 30 degree shift of cortical cytoplasm toward the site of sperm entry; it marks the future site of gastrulation initiation (dorsal lip of the blastopore).

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head mesenchyme - mesoderm that will give rise, along with cranial neural crest cells, to connective tissue, bone and musculature in the head.

Hensen's node - regional thickening of cells at the top (anterior) of the primitive groove through which gastrulating cells migrate anteriorally to form tissues in the future head and neck. The functional equivalent of the dorsal lip of the blastopore ('organizer') in amphibians, the region is found in birds, reptiles and mammals (strictly, the mammalian equivalent is called simply the 'node'). Also known as the primitive knot.

heterogamy - state of having gametes of different sizes produced by different mating types or sexes.

hinge point - during neurulation, region of initial bending of the neural plate to fold into the neural tube; wedge-shaped hinge point cells are anchored to underlying notochord (medial hinge point) or surface ectoderm (dorsolateral hinge point).

histocompatibility antigens - cell surface glycoproteins that differ from individual to individual; their recognition as foreign by a host organism is responsible for rejection of grafted foreign tissue.

histones - basic proteins that bind and "package" eukaryotic DNA; major protein constituent of chromosomes.

holoblastic cleavage - complete cleavage - major pattern of embryonic cell divison in which cytokinesis completely separates cells during division; it is typically seen in smaller eggs containing moderate (mesolecithal) to sparse (isolecithal) yolk. Examples of eggs that divide holoblastically include those of amphibians, mammals, non-vertebrate chordates, echinoderms, most molluscs, annelids, flatworms, nematodes.

homologous recombination - process whereby stretch of DNA in a chromosome is replaced by a homologous (highly similar) DNA molecule for the purpose of altering the gene's function.

hyalin - protein released by cortical granules forming a coating around the sea urchin egg; the hyaline layer provides support for the blastomeres during cleavage.

hyaluronidase - enzyme that degrades hyaluronic acid (a glycosaminoglycan extracellular matrix constituent).

hydrogenate - add hydrogen atoms to a molecule.

hydrolyze - break a bond in a molecule by adding water.

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in situ hybridization - detection of the location of nucleic acid sequences in a cell or organism.

inducer gene - gene encoding the repressor protein of the lac operon; when lactose binds the repressor protein, the lac operon is induced.

induction - alteration in cell fate resulting from external signal (inducer), a form of cell-cell signaling.

ingression - migration of individual cells from surface layers into the interior of the embryo.

inner cell mass - in a mammalian blastocyst, cells that will generate the 'embryo proper' plus some extraembryonic membranes, in contrast to the outer trophoblast.

intercalate - to insert or interpose

intron - 'intervening sequence,' a stretch of nucleic acid sequence spliced out from the primary RNA transcript before the RNA is transported from the nucleus to the cytoplasm as a mature mRNA; can refer either to the RNA sequence or the DNA sequence that from which the RNA is transcribed. See also exon.

invagination - the infolding of a sheet of cells, much like the indenting of a hollow rubber ball when poked.

involution - a type of cell movement during gastrulation which involves the inturning or inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.

ionophore - lipid soluble substance that forms a channel or acts as a carrier in a lipid bilayer membrane to allow specific ions can move across the membrane. A23187 / Calcimycin is an example of a calcium ionophore.

isogamous - having haploid gametes that are similar in size, structure and motility.

isometric growth - growth that occurs at the same rate for all parts of an organism so that its shape is consistent throughout development. Contrast with allometric growth.

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karyokinesis - the mitotic division of the nucleus.

kDa - see Dalton.

kinetochore - structure that attaches a eukaryotic chromosome to the spindle apparatus for separation during mitosis; adjacent to the centromere.


larva - immature (non-reproductive) post-embryonic form of many animals, which hatches from an egg and may look significantly different than the adult (reproductive) form.

lanugo - Thin and closely spaced hairs which are the first hairs in the human embryo. This type of hair is usually shed before birth and is replaced by the short and silky vellus.

luciferinase - enzyme used by fireflies to produce light.

lymphocyte - type of white blood cell (immune system cell).

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macromere - large blastomere; in the sea urchin embryo, the four relatively large cells that result from the fourth cleavage of the vegetal tier are macromeres. Contrast with micromere and mesomere.

marginal zone - region near the equator of the amphibian blastula, where the animal and vegetal hemispheres meet; gastrulation begins among these cells.

melanocyte - pigment cell. In the vertebrates, melanocytes are derived from neural crest, migrate dorsolaterally as melanoblasts to reside in the basal layer of the epidermis. Mature melanocytes contribute pigment (melanin) in the form of membrane-bound melanosomes to developing keratinocytes (epidermal skin cells) and hair or feather shafts.

medial hinge point cells - The midline neural plate cells in birds derived from the anterior midline of Hensen's node and epiblast region that are involved in the intial bending of the neural plate during neurulation. These cells become wedge-shaped, and are anchored to the notochord.

meroblastic cleavage - incomplete cleavage, characteristic of zygotes with large accumulations of yolk.

merogones - egg fragments (in sea urchins) that can divide and develop, even if they have only a haploid nucleus.

mesenchyme - mesodermal cells in a developing embryo with the ability to move freely and individually.

mesoderm - primary embryonic germ layer of triploblastic animals found between the outer ectoderm and the inner endoderm, which (in chordates) gives rise to notochord, bone, cartilage, muscle, other connective tissues, somatic gonad, urogenital tracts, kidneys, heart and circulatory system, blood, and portions of extraembryonic membranes (in amniotes).

mesomere - blastomere exhibiting a size intermediate between a macromere and a micromere.

metaplasia [Greek, 'change in form'] - the transformation of one differentiated cell type into another; the phenomenon is evidence for genomic equivalence.

micromere - small blastomere; in the sea urchin embryo, the four small cells that result from the fourth cleavage of the vegetal tier of cells (half the cells resulting from that tier's cleavage) are micromeres. Contrast with macromere and mesomere.

mitotic spindle apparatus - microtubule-based structure present during mitosis to which chromosomes attach and are separated toward opposite poles of the dividing cell.

morphogenesis - creation of form or structure during development.

MPF - maturation/mitosis promoting factor - heterodimeric protein that initiates prophase of mitosis and alters the activity of diverse proteins that function in mitosis by phosphorylation; active MPF contains both cyclin B (large) and cyclin-dependent kinase 1 (small) subunits. Activity of MPF is regulated both by the synthesis and degradation of cyclin B subunit, and phosphorylation.

morula - [Latin, mulberry] early cleavage stage embryo (blastula) resembling a mulberry.

myogenesis - differentiation of skeletal muscle.

myxamoeba - in Dictyostelium discoideum, the solitary haploid cell of the vegetative life cycle that lives on bacteria and reproduces by binary fission until the food supply is exhausted.

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neural crest - in the vertebrates, one major division of the ectoderm; cells that arise from the top of, or near the dorsal site of closure of the neural tube during neurulation. Neural crest cells migrate away from the neural tube to give rise to the periperal nervous system, various endocrine & paraendocrine cells, pigment cells, and in the head and neck (cranial neural crest) some connective tissue, facial cartilage, muscle and bone (traditionally mesodermal derivatives).

neural tube - hollow cylindrical structure of neuroepithelial cells (in chordate embryos) that will give rise to the brain and spinal cord (central nervous system); an ectodermal derivative.

neuroblast - dividing neuronal precursor cell

neurula - vertebrate embryo during neurulation.

neurulation - organogenesis of the nervous system in vertebrate embryos during which dorsal neuroectoderm cells of the neural plate (typically) roll up to form the neural tube which gives rise to the central nervous system.

Nieuwkoop center - vegetal cells of presumptive dorsal endoderm that signal overlying equatorial/marginal cells in the amphibian blastula to form dorsal mesoderm/organizer.

notochord - rigid cartilaginous rod found at the dorsal midline in all chordate embryos (it is their defining feature) derived from dorsal mesoderm (chordamesoderm). In the vertebrates, it is typically a transient embryonic structure.

nucleic acid hybridization - coming together (annealling) of single-stranded nucleic acid sequences by hydrogen bonding of complementary bases to form double-stranded molecules; this process is the basis for molecular biological techniques in which a labeled probe sequence is used to detect another identical or similar sequence (e.g., Southern hybridization, Northern hybridization).

nucleosome - unit of chromatin consisting of a short length of DNA (about 140 bp) wrapped twice around a core of eight histone proteins (two each of H2A, H2B, H3 and H4).

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oligonucleotide - piece of single-stranded nucleic acid, short stretch of nucleotides.

oogamy - a specialized form of heterogamy, which involves the production of large, relatively immotile eggs by one mating type and small, motile sperm by the other.

operator - site on DNA at which repressor protein binds to prevent transcription, e.g. in the lac operon.

organogenesis - creation of specific tissues and bodily organs by cell interaction and rearrangement following gastrulation.

oviparous - producing offspring from externally laid eggs. Contrast with viviparous

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P element - mobile sequence of DNA (transposon/transposable element) found in Drosophila that can 'hop' in and out of fly chromosomes; can be used with transgenic techniques.

parthenogenesis - special reproductive strategy in which unfertilized eggs undergo cell division and embryogenesis to develop into viable adult individuals ("virgin birth"). The embryo develops without a genetic contribution from the sperm, although in some species fertilization is necessary for egg activation.

parasympathetic nervous system - part of the autonomic nervous system that regulates the involuntary activity of glands, smooth muscle, and cardiac muscle. It is derived from the vagal and sacral neural crest cells.

PCR - see polymerase chain reaction.

peripheral nervous system (PNS)- in the vertebrates, nervous system outside of the central nervous system (CNS), the brain and spinal cord). Derived from neural crest cells which migrate away from neural tube during development, including components such as the enteric, sympathetic, and parasympathetic nervous systems, etc.

phage - virus that infects bacteria; altered phage can be used as cloning vectors. (short for bacteriophage - "bacteria eater")

pharyngeal arches - columns of mesenchyme found in the neck of the developing vertebrate embryo derived from cranial neural crest. In lower vertebrates, blood vessels formed here become part of the gills; in higher vertebrates derivatives include portions of the jaw and middle ear. Also known as branchial arches, gill arches, or visceral arches.

phosphodiesterase - enzyme that cleaves a phosphodiester bond, for example, that which cleaves cAMP into AMP.

phosphorylation - addition of a phosphate group (PO4 -3); e.g., to a protein to alter its function.

placenta - embryonic/maternal organ that serves nutritional and respiratory functions of the mammalian fetus; composed of embryonic chorion and maternal uterine endometrium, allowing provision of oxygen and nutrients to the fetus and removal of carbon dioxide and other waste products.

plasmid - circular double-stranded DNA molecule found in bacteria; artificially engineered plasmids are used as cloning vectors.

pluripotent - of a cell or nucleus, having the ability to produce many, but not all of the cell types in an organism (Contrast with totipotent). Note: In mammalian development, cells (e.g., embryonic or other stem cells) capable of giving rise to all cell types of the embryo, but not extraembryonic tissues (derivatives of trophoblast) are called 'pluripotent.' [For example, in a recent issue of Nature on ES cells, 'pluripotent' was defined in one article as "able to give rise to all cell types found in the embryo and adult animal."] This usage is different from that used with other organisms and is a potential source of confusion. In mammalian systems, the word "multipotent" is used for a cell capable of producing some, but not all cell types of the embryo.

poly-A tail - polyadenosine tail - a sequence of repeatingadenosine ribonucleotides added to the 3' end of a newly transcribed pre- mRNA before it exits the nucleus. This modification can be up to 200-300 bases long and functions to increase the stability and translatability of the mRNA.

polyinvagination islands - disconnected clusters of cells of the primary hypoblast that are the first to migrate beneath the epiblast of the avian blastoderm.

polymerase - enzyme that links together like units (monomers) into a polymer. For example, RNA polymerase synthesizes a polynucleotide chain (RNA) using ribonucleotide monomers.

polymerase chain reaction - powerful DNA synthesis and amplification technique allowing the amplification of a specific sequence from among many others; thoeoretically PCR can be used to detect as little as one specific DNA molecule.

polyspermy - entry of more than one sperm into the egg during fertilization.

polytene chromosome - large chromosome resulting from repeated DNA replication and alignment of homologs in the absence of mitosis; found in salivary glands and some other tissues of dipteran insects.

posterior neuropore - posterior open end of the neural tube that must close during the development of the embryo; failure to close results in spina bifida

precocious (adj.) - developing earlier than normal.

preformationism - 17th century theory of inheritance that hypothesized that all the organs of an adult were prefigured in miniature within either the sperm or the ovum.

primitive folds - thickened ridges of the primitive streak; formed by convergent flow of epiblast.

primitive streak - thickening of the epiblast cell layer caused by movement of mesodermal cells into the blastocoel; this structure is characteristic of avian, reptilian and mammalian gastrulation.

proliferate - to grow or multiply by rapidly producing new tissue, parts, cells, buds, or offspring.

promoter - region of gene that binds RNA polymerase and transcription factors to initiate transcription.

pronucleus - haploid, gametic nucleus (from sperm or egg) in a fertilized egg, prior to fusion to form a zygote nucleus

prosencephalon - anteriormost division of the developing vertebrate brain (forebrain) that further subdivides into the telencephalon and the diencephalon.

protamine - protein that binds DNA in sperm, replacing histones and allowing chromosomes to become more highly condensed than possible with histones.

protostomes - broad classification of animals that tend to share certain embryological traits; among these the formation of the "mouth first" (hence the name) during gastrulation, before the future anus. (The site of gastrulation initiation, the blastopore, becomes the mouth.) Major protostome phyla include molluscs, annelids and arthropods.


quiescent (adj.) - of a cell, not dividing, in G0 phase (having exited the cell cycle).

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Radiata - group of simple animals with radial symmetry, including the cnidarians (or coelenterates) and ctenophores. Contrast with Bilateria.

regulation - the ability of cells to change their developmental fates according to their location within the whole organism. Regulation can be manifested experimentally as an ability for a cell or cells to develop normally following removal or addition of other cells or tissue.

repressor protein - DNA regulatory protein that binds the operator (a specific DNA sequence) in the absence of the inducing substance (in the case of the lac operon, lactose).

restriction endonuclease - aka restriction enzyme - enzyme that cuts double-stranded DNA at a specific sequence.

retrotransposon - A transposable DNA element (transposon) which is replicated through an RNA intermediate via reverse transcriptase. Within their characteristic long terminal repeats (LTRs), some retrotransposons encode retroviral-like proteins (e.g., gag, pol) for reverse transcriptase and integrase. These are likely derived from retroviruses; other retroransposons lack viral genes. Retrotransposons are ubiquitous elements in eukaryotic genomes. Examples include yeast Ty elements, Drosophila copia elements, and human LINE1 elements (which comprise 17% of the human genome).

retroviral vector - artificial DNA construct derived from a retrovirus, used to insert sequences into an organism's chromosomes.

rhombencephalon - the posteriormost of 3 primary vesicles (bulges) formed from the anterior neural tube. The hindbrain further subdivides into a posterior myelencephalon which will give rise to the medulla oblongata, and an anterior metencephalon, which will give rise to the cerebellum and pons. Also known as hindbrain)

ribosome - protein synthesis 'machine,' made of ribosomal RNAs and proteins.

RNA polymerase - enzyme that synthesizes RNA from ribonucleotide precursors, using a DNA template.

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senescence - 1) aging; 2) in cell biology, a process related to, but distinct from, programmed cell death wherein the cell permanently exits the cell cycle and can no longer divide [aka cellular senescence].

somatopleure - amniote embryonic tissue layer consisting of somatic (lateral plate) mesoderm and ectoderm.

somite - block of dorsal mesodermal cells adjacent to the notochord during vertebrate organogenesis. These transient structures define the segmental pattern of the embryo, and subsequently give rise to vertebrae and ribs, dermis of the back, and skeletal muscles of the back, body wall and limbs.

spindle fiber - microtubule found in the cell during mitosis, upon which chromsomes are moved.

splanchnopleure - amniote embryonic tissue layer consisting of splanchnic (lateral plate) mesoderm and endoderm.

stereoblastula - embryo produced by spiral cleavage, characterized by the absence of a blastocoel; formed by embryos of annelid worms, turbellarian flatworms, nemertean worms, and all molluscs except cephalopods.

stomodeum - anterior part of the embryonic alimentary canal formed as an invagination of the ectoderm; the future mouth.

structural gene - gene or part of gene encoding protein sequence.

synapse - a highly specialized junction between two neurons, or between a neuron and an effector cell (e.g., muscle or gland cell), at which electrical and/or chemical signals are passed from one cell to another.

syncitiotrophoblast - highly invasive, multinucleated derivative of the cytotrophoblast near the inner cell mass in the implanting mammalian embryo.

syncitium - multinucleate cell; may arise either from nuclear division (karyokinesis) without cell division (cytokinesis), e.g., in Drosophila syncitial blastoderm, or from fusion of mononucleate cells (e.g. vertebrate skeletal muscles). [plural: syncitia; adjective form: syncitial]

syndrome - a collection or group of signs and symptoms that occur together and characterize a particular disease or abnormality.

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telolecithal (adj.) - containing a large amount of yolk concentrated at one pole of the egg.

teratogen - [Greek, monster-former] external agent that causes abnormalities (birth defects) during development, including any of various chemicals, or treatments/conditions such as hyperthermia, radiation, or viral infection.

thalidomide - drug which is a powerful human teratogen. First synthesized in Germany in 1954 as a new antihistamine, the drug was found to be a safe and effective sedative. It was widely used beginning in 1958 to relieve symptoms of morning sickness and nausea in pregnancy (although never approved in the US). By the 1960's, the teratogenic effects of the drug had become apparent; the most common birth defects included severe limb abnormalities and malformed internal organs. More recently, the drug was approved for treatement of multiple myeloma and Hansen's disease (leprosy), under strict prescribing conditions.

totipotent (adj.) - of a cell or nucleus, possessing the ability to produce all the differentiated cell types of the organism. (Contrast with pluripotent)

transcription - process of RNA synthesis from DNA template.

transcription factor - DNA binding protein that turns genes on or off (regulates the level of transcription) by binding to enhancer elements in DNA and interacting with RNA polymerase at the promoter.

transcytosis - process of transport of substance across an epithelium by uptake into and release from coated vesicles

transgenic (adj.) - of an organism, having DNA artificially inserted into the genome.

transient (adj.) - short-lived or temporary

transfection - incorporation of DNA into a cell by incubation of the two together under appropriate conditions.

translation - process whereby mRNA code is used by the ribosome to synthesize a polypeptide chain (protein) from amino acid precursors.

transposable element - see transposon

transposon - mobile segment of DNA capable of "hopping" into or out of chromosomal DNA; a complete transposon encodes an enzyme called a transposase that can mediate the excision and insertion of the transposon DNA. (See also retrotransposon)

triploblastic (adj.) - having three embryonic germ layers (ectoderm, mesoderm, and endoderm); characterizes all animals except cnidarians, ctenophores and sponges which are considered diploblasts, lacking true mesoderm.

trophoblast - in a mammalian blastocyst, the outer superficial layer of cells surrounding the inner cell mass. This specialized tissue forms a trophic interface between the embryo and mother during cleavage, and subsequently forms the embryonic portion of the placenta upon implantation into the endometrial epithelium of the uterine wall. A portion of cytotrophoblast cells (near the inner cell mass) form the highly invasive syncytiotrophoblast which penetrates into the endometrial stroma.

tubulin - protein which when polymerized forms microtubules, part of the cytoskeleton, and necessary for (among other processes) karyokinesis.

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vegetal hemisphere - typically, the yolky half of an egg or embryo, opposite the animal hemisphere. In many embryos, cells in the vegetal hemisphere divide more slowly than those in the animal hemisphere.

visceral (adj.) - pertaining to the internal organs of the body

visceral arches - see pharyngeal arches.

vitelline envelope/membrane- membrane outside the plasma membrane forming a fibrous mat over the sea urchin egg; becomes the fertilization membrane via the cortical reaction.

viviparous (adj.) - giving birth to live young.

Volvocaceans - group of colonial eukaryotes (photosynthetic green algae), including Volvox.

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yolk platelets - membrane-bound discs containing high concentrations of yolk found in eggs.

yolk plug - a patch of vegetal cells (endoderm) that remains exposed in the blastopore after the formation of the ventral lip during gastrulation.

yolk sac - the first of four extraembryonic membranes of amniotes to form during embryogenesis. Like the allantois, it arises from the splanchnopleure (endoderm and splanchnic mesoderm) to surround the mass of yolk in reptile and bird eggs. It is connected to the midgut by the yolk stalk. The yolk sac also forms in mammals, despite the absence of yolk.

zona pellucida - a thick extracellular matrix surrounding the mammalian ovum (egg) which binds sperm and initiates the acrosome reaction of the sperm.

zona reaction - in mammals, modification of the zona pellucida that blocks polyspermy; enzymes released by cortical granules digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm.

zygote - diploid cell created by the union of two haploid gametes; a fertilized egg.

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