Developmental Biology - All Vocabulary

Polyspermy

Entrance of more than one sperm into a single egg, resulting in abnormal development.

Genomic equivalence

Idea that all of the cells in a multicellular organism contain the same genetic information.

Morula

In amphibians, a cleavage-stage embryo formed of a solid cluster of blastomeres without a well-developed blastocoel.

Homeotic selector genes

In flies a set of genes whose products specify the characteristic structures of each segment.

Cavitation

In mammalian embryos, creation of a blastocoel by secretion of fluid by the outermost cells of the morula into the interior region.

Blastocyst

In mammalian embryos, embryonic stage characterized by the inner cell mass being positioned on one side of a ring of trophoblast cells.

Node

In mammals, a thickening at the anterior end of the primitive streak that serves as the functional equivalent of the dorsal lip of the amphibian blastopore.

Rotational cleavage

In mammals, cleavage pattern in which second cleavages are perpendicular, meridional in one blastomere and equatorial in the other.

Primary mesenchyme

In sea urchins, micromere-derived cells that are the first to ingress into the blastocoel and later differentiate into skeletal spicules.

Analogy

In two or more species, superficially similar structures or characteristics that arise from similar function rather than being derived from a common ancestral structure.

Archenteron

Initial invagination into a gastrulating embryo that forms the primitive gut.

Endoderm

Innermost cell layer of a developing embryo that contributes to the inner lining of the digestive tube, pharynx, and respiratory tube.

Syncytial blastoderm

Intermediate product of superficial cleavage consisting of thousands of nuclei sharing a common cytoplasm and arranged in a single layer at the periphery of an insect egg.

Primary embryonic induction

Key induction in vertebrate embryos in which the progeny of dorsal lip cells induce the dorsal axis and the neural tube.

Germinal neuroepithelium

Layer of rapidly dividing neural stem cells that compose the original neural tube.

Blastoderm

Layers of dividing cells during discoidal cleavage.

Morphogenetic determinants

Materials in the zygote cytoplasm that are segregated during cleavage and ultimately direct cell differentiation in later development.

Duplication and divergence

Mechanism of evolutionary change whereby modules are copied, followed by differentiation and specialization among the redundant structures.

Cortical granules

Membrane- bound packets of proteolytic enzymes, mucopolysaccharides, adhesive glycoproteins, and hyalin protein in eggs that are secreted from the cell upon fertilization.

Superficial cleavage

Meroblastic cleavage limited to a surface layer of cytoplasm at the periphery of the egg.

Intermediate mesoderm

Mesoderm between the somitic dorsal mesoderm and lateral plate mesoderm that forms the urinary system and genital ducts.

Somitic dorsal mesoderm

Mesoderm derived from paraxial dorsal mesoderm that is organized into somites and will contribute to the axial skeleton, dorsal dermis, and skeletal muscles of the back and limbs.

Lateral plate mesoderm

Mesoderm that lies lateral to the intermediate mesoderm that contributes to heart, blood vessels, blood cells, linings of the coelom, and mesodermal tissues of the limbs except muscles.

Mesoderm

Middle cell layer of a developing embryo that contributes to the notochord, skeletal muscles, dermis of the skin, heart, kidney, blood and blood vessels, bone, etc.

Differential adhesion hypothesis

Model of cell interactions which predicts that cells rearrange themselves into the most thermodynamically stable pattern, where the interfacial free energy is minimized.

Axoneme

Motor apparatus in the sperm tail consisting of two central microtubules surrounded by an array of nine doublet microtubules (9 + 2 arrangement).

Collective migration

Movement of cells as part of a group.

Myoblast

Muscle precursor cell derived from the myotome.

Dorsolateral pathway

Neural crest migration route between the dermis and epidermis of the skin.

Ventral pathway

Neural crest migration route from the roof of the neural tube down through somites.

Glial cells (glia)

Non-neuronal cells of the central and peripheral nervous systems that provide structural and functional support to neurons.

Blastopore

Opening of the archenteron through which cells invaginate during gastrulation.

Somitomeres

Organized whorls of cells within paraxial mesoderm that serve as precursors to somites

Ectoderm

Outermost cell layer of a developing embryo that contributes to the epidermis, central nervous system, and neural crest derivatives.

Trophoblast

Outermost cells of a mammalian blastocyst that contribute to the chorion of the placenta.

Mosaic development

Pattern of embryogenesis associated with autonomous specification in which the embryo consists of a group of self-differentiating parts.

Compaction

Phenomenon in mammalian embryos in which blastomeres become tightly packed to form a compact morula.

RNA interference

Phenomenon in which expression of a single gene is silenced by introduction of a double-stranded RNA sequence corresponding to part of the mRNA.

Morphs (Ecomorphs)

Phenotypic variants that result from environmental differences.

Teratogen

Physical and chemical agent in the environment that causes developmental disruptions.

Developmental constraints

Physical, morphogenetic, and phyletic restraints that limit the number of body plans that can be created by evolution.

Neural plate

Portion of the dorsal ectoderm destined to become the neural tube and consisting of a flat sheet of columnar cells.

Conditional specification

Position-dependent specification of cell fate by interaction between a cell and its neighbors.

Homology

Presence in two different groups of organisms of a structure, behavior, pathway, or gene that is shared by descent from a common ancestor.

Genetic assimilation

Process by which an initially environmentally induced phenotype through selection becomes fixed in a population so that the phenotype develops even in the absence of environmental induction.

Epigenetic inheritance systems

Processes by which phenotypic variation not associated with DNA sequence variation is inherited.

Cellular blastoderm

Product of superficial cleavage consisting of thousands of cells arranged in a single layer at the insect egg periphery.

Regeneration

Reactivation of development in postembryonic life to restore missing or damaged tissues.

Epigenetic

Refers to heritable changes in gene expression that are not due to changes in DNA sequences.

Morphallaxis

Regeneration from existing cells through transdifferentiation without proliferation.

Epimorphosis

Regeneration through dedifferentiation followed by redifferentiation.

Epiphyseal plate

Region near the ends of long bones that remain cartilaginous during development to allow for continued elongation of the bone.

Stem cell-mediated regeneration

Regrowth of missing structures from stem cells.

Orthologs

Related genes in different species that are descended from the same gene in a common ancestor.

Paralogs

Related genes that are derived from gene duplication within a single genome.

Homologous genes

Related genes that exist within a species or in different species that are descended from the same gene in a common ancestor.

Parasegment

Section of gene expression along the AP axis of a Drosophila embryo before segmentation that develops into the posterior portion of one body segment and the anterior portion of the next segment.

Promoter

Sequences in the near-upstream region of a gene to which RNA polymerase binds to bring about transcription.

Gene regulatory network

Set of connections among genes that specifies cell identity.

Alternative RNA splicing

Situation in which a pre-mRNA can be processed in more than one way to produce different mRNA molecules encoding different polypeptides.

Permissive interaction

Situation in which a responding tissue only needs to a generally suitable environment to differentiate in some predetermined way.

Instructive interaction

Situation in which a specific signal from an inducing tissue is needed to bring about changes in gene expression in the responding cell.

Dissociation

Situation in which changes in development in one module do not affect development of other modules.

Correlated progression

Situation in which evolutionary changes in one module of an embryo affect development in another module.

Pleiotropy

Situation in which expression of an allele of a single gene affects more than one aspect of the phenotype.

Genetic heterogeneity

Situation in which mutations in different genes can produce the same phenotype.

Reciprocal induction

Situation in which one tissue receives and responds to a signal from another tissue in part by sending a signal back, creating a response in that tissue.

Phenotypic heterogeneity

Situation in which the same mutation produces different effects in different individuals.

Dendrites

Small branching processes that extend from a neuron that pick up action potentials from other cells.

Stem cell niche

Specialized compartments within organs in which stem cells reside and receive from surrounding cells signals related to division and differentiation.

Acrosomal vesicle

Specialized secretory vesicle in the sperm that carries enzymes used to digest the outer coverings of eggs.

Autonomous specification

Specification of cell fates by the cell's own internal cytoplasmic components without reference to neighboring cells.

Syncytial specification

Specification of cell fates prior to cellularization of a multinucleate cleavage-stage egg.

Satellite cells

Stem and progenitor cells found alongside muscle fibers.

Adult stem cells

Stem cells that are found in tissues after embryogenesis is complete.

Competence

The ability of a tissue to respond to an inductive signal.

Phenotypic plasticity

The ability of individuals to express different phenotypes under different environmental conditions.

Nucleosome

The basic unit of chromatin organization, consisting of a short DNA segment wrapped around a core of histone proteins.

Blastocoel

The central cavity within a blastula.

Convergent extension

The elongation of a cell layer in one dimension and shortening in another dimension.

Slow block

The fertilization envelope, a mechanical block to polyspermy that becomes active about 1 minute after fertilization.

Heterometry

Evolutionary change in the amount of a structure or level of gene expression.

Allometry

Evolutionary mechanism in which the relative growth rates of different modules changes with respect to each other.

Heterochrony

Evolutionary mechanism in which the relative timing of development in different modules of a developing system change with respect to each other.

Heterotypy

Evolutionary production of a new kind of gene or structure.

Molecular parsimony

Existence and use throughout the animal kingdom of a relatively small set of regulatory genes that are used to build animal body plans.

Heterotopy

Expression of an existing structure or function in a different part of the body.

Differential gene expression

Expression of different subsets of the genome in different cell types in multicellular organisms.

Vitelline envelope

Extracellular matrix of an egg that controls binding of sperm to egg and access of sperm to the egg plasma membrane.

Muscle fiber

Extremely large, elongated cell containing many nuclei and contractile proteins actin and myosin.

Osteogenesis

Formation of bone.

Chondrogenesis

Formation of cartilage.

Primary neurulation

Formation of the neural tube by infolding of neural ectoderm and pinching off from surface ectoderm.

Secondary neurulation

Formation of the neural tube by sinking of a solid cord of neural ectoderm into the embryo followed by cavitation.

Cortical granule reaction

Fusion of cortical granules in the egg with the plasma membrane, resulting in exocytosis of granule contents into the space between the membrane and the vitelline envelope.

Acrosomal reaction

Fusion of the acrosomal vesicle with the sperm cell membrane and extension of the acrosomal process.

Germ cells

Gametes and gamete precursors.

Morphogenesis

Generation of form and structure within an organism during development.

Maternal effect genes

Genes of the maternal genome that are transcribed and accumulated during oocyte development and affect development of the embryo derived from the egg.

Apoptosis

Genetically programmed cell death.

Determination

A (relatively) irreversible level of commitment to a particular developmental fate characterized by the ability of a cell or group of cells to differentiate to a particular fate even when placed into a different region of the embryo.

Neural crest

A band of cells along the border where the neural tube pinches off from the surface ectoderm that migrate to various parts of the embryo and produce diverse structures.

Fast block

A barrier to fertilization by additional sperm mediated by depolarization of the egg cell membrane potential and lasting about one minute after fertilization.

Signal transduction pathway

A biochemical pathway that detects the presence of a signal molecule at the cell surface and brings about a response in the cell to the presence of the signal.

Totipotent cell

A cell such as a zygote from which all cells of the embryo and, in mammals, the trophoblast are derived.

Stem cell

A cell which divides to form one daughter cell that remains a stem cell and one daughter cell that can differentiate to a specific cell type.

Enhancer

A cis-acting DNA regulatory element that can activate transcription even at a great distance upstream or downstream from the promoter.

Silencer

A cis-acting DNA regulatory element that represses transcription even at a great distance upstream or downstream from the promoter.

DNA microarray

A collection on a slide of a very large number of different DNA fragments, each of which acts as a probe to monitor the presence and expression of a large number of genes or transcripts simultaneously.

Chromatin

A complex of DNA and proteins that makes up chromosomes.

Resting membrane potential

A constant electric potential across a membrane in its normal state.

Syncytium

A cytoplasm that contains many nuclei.

Oocyte

A developing or immature egg cell.

Sperm activation

A dramatic increase in sperm metabolism and motility in response to an attractant from a mature egg.

Ovum

A fully differentiated female germ cell.

Reporter gene

A gene whose product is easy to detect and measure that is used to assay the function of a promoter element or enhancer.

Mesenchyme

A group of cells that are loosely connected or not connected and that function as independent units.

Epithelium

A group of cells that are tightly connected in a sheet or tube.

Regeneration blastema

A group of undifferentiated cells at the site of a healed wound from which regeneration occurs.

Axon

A large process that extends from a neuron and carries action potentials away from the cell body.

Primitive streak

A long, thickened region in the blastoderm through which cells ingress during gastrulation in birds and mammals.

Contact inhibition of locomotion

A mechanism of dispersion whereby cell movements are inhibited at points of contact and redirected away from points of contact.

Deep homology

A molecular homology found in two groups of organisms which diverged from a common ancestor that existed in the distant past.

Juxtacrine factor

A molecule on the surface of a cell that induces a response in another cell.

Autocrine factor

A molecule produced by a cell that stimulates its own production in the same cell.

Endocrine factor

A molecule produced in one tissue that is secreted into the blood and delivered to a responding tissue in another part of the body; a hormone.

Homeotic mutations

A mutation that causes one structure to be replaced by another one.

Energid

A nucleus and its associated cytoplasmic domain in a syncytial blastoderm.

Exon

A portion of the transcribed region of a gene that is included in the mature mRNA molecule that exits the nucleus.

Intron

A portion of the transcribed region of a gene that is removed from the transcript before it exits the nucleus as a mature mRNA.

Neuroblast

A precursor to a neuron.

Transcription factor

A protein that binds to a promoter region or enhancer/silencer to activate or repress transcription of a particular gene.

Specification

A reversible level of commitment to a particular developmental fate characterized by the ability of a cell or group of cells to differentiate to a particular fate in a developmentally neutral environment.

Pair-rule genes

A set of genes whose products are expressed in stripes that subdivide broad gap gene regions into parasegments.

Segment polarity genes

A set of genes whose products control orientation and cell fates within each segment.

Segmentation genes

A set of genes whose products divide the Drosophila embryo into a repeating series of parasegments along the AP axis.

Gap genes

A set of genes whose products divide the embryo into broad regions and help to specify cell fates within those regions.

Syndrome

A set of symptoms that together define a particular condition or disease.

Paracrine factor

A signal produced by an inducing cell or tissue that diffuses over a short distance and brings out changes in a responding cell or tissue.

Imaginal disc

A small group of cells in an insect embryo or larva that are elaborated during metamorphosis to form adult structures such as eyes, legs, wings, antennae, and certain reproductive structures.

Morphogen

A soluble molecule that is secreted from a source and whose concentration specifies one or more cell types along a concentration gradient.

Northern blot

A stable membrane filter to which RNA from a particular tissue or developmental stage is bound.

Pluripotent stem cell

A stem cell that can generate all of the structures of an embryo, but not the placenta.

Committed stem cell

A stem cell that can give rise to a restricted range of related cell types.

Dynein

ATPase associated with axoneme microtubules that converts chemical energy of ATP into mechanical energy of microtubules sliding past each other, providing the propulsive force of flagella.

Differential cell affinity

Ability of different cells to selectively recognize other cells and substrates, adhering to some and migrating over others.

Epigenesis

Acquisition of a complex, differentiated state from a simple, undifferentiated state.

Somatic cells

All cells of the body that are not germ cells.

Fertilization envelope

An elevated vitelline envelope that forms in response to fertilization and serves as a mechanical block to fertilization by additional sperm.

Blastula

An embryo consisting of a (usually) hollow sphere of blastomeres resulting from cleavage.

Head mesenchyme

Anterior loose mesoderm that contributes to the connective tissues and muscles of the face (along with neural crest cell derivatives).

Neuron

Any of a variety of nerve cells.

Mutagen

Any physical or chemical treatment that increases the rate of mutations.

Pronuclei

Haploid nuclei from sperm and egg in the egg after fertilization and prior to fusion to form a diploid nucleus.

Cloning

Asexual reproduction in which the offspring are genetically identical to the parent.

Epialleles

Heritable variants in chromatin structure and gene expression states that are not based on variation in DNA sequences.

Osteocyte

Bone cell.

Endochondral ossification

Bone formation in which mesenchymal cells differentiate first into cartilage, which is later replaced by bone.

Osteoblast

Cell capable of secreting bone matrix.

Embryonic stem cells

Cells of the inner cell mass of a mammalian embryo from which all of the cells of the body are derived.

Osteoclast

Cells that degrade bone tissue.

Chondrocytes

Cells that secrete extracellular matrix of cartilage.

Radial glia

Cells that span the apicobasal axis of the CNS wall, serving as neural stem cells and providing a scaffold for migration of neural progenitors.

Blastomeres

Cellular products of cleavage.

Induction

Change in cellular behavior in a tissue in response to a signal produced by another tissue.

Egg activation

Conversion of the egg from a metabolically inactive state prior to fertilization to a highly active state required for early embryogenesis following fertilization.

Commitment

Covert changes within a cell that restrict its developmental fate.

Predator-induced defense

Defensive phenotype induced in a prey by chemicals released from a predator.

RT-PCR

Detection of a particular rare RNA in a complex sample by amplification using a combination of reverse transcriptase and polymerase chain reaction.

In situ hybridization

Detection of a specific RNA of interest in place within an embryo, whole organism, organ, or tissue.

Embryogenesis

Development of an organism from zygote to either birth or hatching.

Modularity

Development through a series of discrete and interacting parts.

Malformation

Developmental anomaly caused by a genetic defect.

Disruption

Developmental anomaly caused by an environmental agent.

Metamorphosis

Developmental process by which larval tissues are destroyed and replaced by cells destined to form adult tissues and organs.

Differentiation

Developmental processes by which cells become different from each other within a multicellular organism.

Induced pluripotent stem cells

Differentiated cells that are artificially converted to pluripotency by insertion and expression of particular genes.

Myogenesis

Differentiation of skeletal muscle fibers.

Intramembranous ossification

Direct conversion of mesenchymal tissue into bone.

Compensatory regeneration

Division of differentiated cells to recover the structure and function of a damaged organ.

Chordamesoderm

Dorsal axial mesoderm that differentiates as the notochord.

Organizer

Dorsal lip of the blastopore and its derivatives, which have the ability to coordinate development of the body plan during gastrulation.

Nieuwkoop center

Dorsal-vegetal cells that contain dorsalizing determinants and that induce overlying dorsal mesoderm cells to form the organizer.

Dermatome

Dorsolateral layer of the dermamyotome that decondenses to form mesenchymal precursors of the dorsal dermis.

Dermamyotome

Double-layered epithelium of the somite after decondensation of the sclerotome.

Blastodisc

During discoidal cleavage, region of cleaving cytoplasm at the animal pole of the egg.

Cleavage

During embryogenesis, a rapid series of mitotic divisions of the zygote to form smaller cells.

Organogenesis

During embryogenesis, the generation of body organs.

Gastrulation

During embryogenesis, the rearrangement of blastomeres to produce an embryo with 3 primary germ layers.

Neural tube

Ectoderm-derived dorsal tube that forms the rudiment of the central nervous system.

Invagination

The infolding of an epithelium into an interior space.

Cell lineage

The line of descent from zygote to any particular cell type in a multicellular organism.

Soma

The main cell body of a neuron.

Ingression

The migration of individual cells from an epithelium into a cavity.

Reaction norm

The presence of a continuous range of phenotypes across a range of environmental conditions.

Polyphenism

The presence of two or more distinctly different phenotypes elicited by the environment.

Nuclear RNA (nRNA)

The primary transcripts of genes that exist unprocessed in the nucleus.

Inner cell mass

The small group of cells within a mammalian blastocyst that will give rise to the embryo proper and extraembryonic membranes.

Developmental biology

The study of all developmental processes from fertilization through death.

Embryology

The study of development from fertilization to birth or hatching.

Zona pellucida

The thick vitelline envelope surrounding the mammalian egg.

Paraxial mesoderm

Thick bands of mesoderm that run longitudinally along each side of the notochord and neural tube and form the somites.

Somites

Transient blocks of paraxial mesoderm that determine the segmental pattern of vertebrate embryos.

Neoblast

Undifferentiated cell within a regeneration blastema.

Fertilization

Union of haploid gametes to form a diploid zygote.

Sclerotome

Ventral medial cells of the somite that decondense to form mesenchymal precursors to vertebral cartilage.

Myotome

Ventromedial layer of the dermamyotome that decondenses to form mesenchymal precursors of skeletal muscles.