Developmental Biology Terms Starting With B
Developmental Biology Glossary: B
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Blastocoel
/ BLAS-toh-seel / · Greek blastos (germ) + koilos (hollow)
Blastocoel is the fluid-filled central cavity of a blastula-stage embryo, formed as dividing blastomeres arrange into a hollow sphere and fluid accumulates between them.
As cleavage proceeds, blastomeres pump ions into the interior of the embryo, drawing water osmotically to create and expand the blastocoel. Its relative size varies considerably among species: frog blastulas have a large blastocoel occupying much of the interior, while the blastocoel of amniote blastocysts is comparatively smaller. During gastrulation, cells migrate through or around the blastocoel as tissues undergo invagination and rearrangement to form the three primary germ layers.
The blastocoel also contains signaling molecules, including members of the TGF-beta family, that influence the fates of cells lining its walls.
In assisted reproductive technology, embryologists assess human blastocysts partly by grading blastocoel expansion on a scale of 1 to 6, where a grade-4 blastocyst has a cavity filling more than half the embryo volume. Blastocysts with higher expansion grades have significantly higher implantation rates after transfer.
Early embryos are solid balls of cells at every stage. Many embryos form an internal fluid-filled space during the blastula stage, and this cavity is mechanically and chemically important for subsequent development.
In sea urchin (Strongylocentrotus purpuratus) embryos, the blastocoel occupies roughly 40 percent of the blastula volume by the late blastula stage. During gastrulation, primary mesenchyme cells ingress into the blastocoel and later secrete the larval skeleton from within that space.
Blastoderm
/ BLAS-toh-derm / · Greek blastos (germ) + derma (skin)
Blastoderm is the single layer of cells that forms on the surface of a yolk-rich egg after cleavage divisions and gives rise to most of the embryo's tissues through subsequent gastrulation.
In birds and reptiles, the blastoderm forms as a disc of cells sitting on top of the yolk rather than surrounding it, because the large yolk mass prevents complete cleavage of the egg. The upper epiblast layer of the blastoderm gives rise to the embryo proper through gastrulation, while the lower hypoblast contributes to extraembryonic membranes. In a freshly laid chicken (Gallus gallus domesticus) egg, the blastodisc is approximately two to three millimeters in diameter and already contains roughly 60,000 cells.
A primitive streak forms along the midline of the epiblast, and cells ingress through it to generate the mesoderm and definitive endoderm.
In the fruit fly (Drosophila melanogaster), early nuclear divisions occur without cell membrane formation, producing a syncytial blastoderm of roughly 6,000 nuclei sharing a common cytoplasm. Membranes then grow inward to cellularize the blastoderm in about 60 minutes, one of the fastest cellularization events known in animal development.
Embryos always divide to form a hollow ball of cells. In birds and reptiles, the large yolk prevents complete division, so the blastoderm forms as a flat disc of cells sitting on top of the yolk instead of surrounding it.
In zebrafish (Danio rerio) embryos, the blastoderm forms as a cap of cells at the animal pole by the 128-cell stage, approximately 2.25 hours after fertilization. These cells then undergo epiboly, spreading vegetally to cover the yolk cell completely by about 10 hours post-fertilization.
Blastomere
/ BLAS-toh-meer / · Greek blastos (germ) + meros (part)
Blastomere is one of the cells produced by the cleavage divisions of a fertilized egg, characterized by a large nucleus-to-cytoplasm ratio and, in early stages, the potential to contribute to multiple embryonic tissues.
Blastomeres arise during cleavage, when a fertilized egg divides by mitosis without a significant increase in total embryo volume, so each successive division produces smaller cells. In sea urchins (Strongylocentrotus purpuratus), the first few cleavage cycles take as little as 20 minutes each, generating 16 blastomeres within roughly an hour of fertilization. Mammals show indeterminate development; blastomeres separated at the two-cell stage can each form a complete embryo, while in tunicates such as Ciona intestinalis, which show determinate development, even the first two blastomeres have restricted fates tied to inherited cytoplasmic determinants.
The progressive restriction of blastomere potential as cleavage proceeds reflects the accumulation of cell-intrinsic and cell-extrinsic fate-determining signals.
Hans Driesch demonstrated in 1892 that separating the first two blastomeres of a sea urchin embryo produced two complete, normally proportioned larvae rather than two half-embryos. This result overturned the prevailing mosaic theory of development and introduced the concept of developmental regulation.
Every early blastomere has a fixed fate in all animals. In species with regulative development, such as sea urchins and mammals, early blastomeres retain broad developmental potential and can compensate for missing neighbors.
When a mouse blastomere at the two-cell stage is separated from its partner, each cell can still generate a complete blastocyst and, after transfer, a viable embryo. By the eight-cell stage, however, separated mouse blastomeres typically cannot form whole embryos, marking the window during which developmental restrictions emerge.
Blastula
/ BLAS-choo-lah / · Greek blastos (germ) + diminutive -ula
Blastula is an early embryonic stage in animals consisting of a ball or layer of cells surrounding a fluid-filled cavity, formed at the end of cleavage and immediately preceding gastrulation.
The blastula stage begins when cleavage divisions have produced enough cells to organize around a central blastocoel, typically after six to eight hours of division in sea urchins or four to five days in humans. Cells at different positions within the blastula already differ in gene expression, even though they are not yet committed to specific fates. Gastrulation begins when cells in defined regions of the blastula start moving inward, marking the transition from the blastula to the gastrula stage.
In frogs, the blastula consists of roughly 4,000 cells arranged around a large blastocoel, with animal-pole cells smaller and more pigmented than the yolk-rich vegetal-pole cells.
The mammalian equivalent of the blastula, the blastocyst, contains two distinct cell populations: the inner cell mass, which gives rise to the embryo proper, and the trophoblast, which forms the placenta. This distinction arises through a combination of cell position and Hippo signaling pathway activity, with outer cells activating trophoblast genes and inner cells activating pluripotency genes such as Oct4 and Nanog.
A blastula already contains recognizable organs or tissue types. The blastula is a ball or layer of relatively undifferentiated cells surrounding a cavity, and organ formation does not begin until after gastrulation creates the three germ layers.
A frog (Xenopus laevis) blastula develops surface cilia that beat to rotate the embryo within its jelly coat, a behavior that may facilitate oxygen exchange. The blastula stage in Xenopus lasts approximately six hours, from the 128-cell stage at around 6 hours post-fertilization to the onset of gastrulation at roughly 12 hours post-fertilization.
Body Plan
/ BOD-ee plan / · Old English bodig + plan
Body Plan is the fundamental spatial organization of an animal's major structural features, including its axes of symmetry, number of germ layers, and arrangement of body regions, shared by all members of a major animal group.
Body plans are established during early embryogenesis through the coordinated activity of Hox genes and other developmental regulators that translate positional information into tissue identity. These organizational features, including bilateral or radial symmetry, the presence of a coelom, and the number of body segments, remain consistent within a phylum even as individual species diverge in size, coloration, and organ detail. All insects, for example, share a three-part body plan of head, thorax, and abdomen with three pairs of jointed legs, while all vertebrates share a dorsal hollow nerve cord, a notochord at some stage, and pharyngeal slits.
Hox gene clusters are so conserved that mouse Hox genes can partially substitute for their Drosophila counterparts when introduced experimentally, demonstrating deep evolutionary continuity of body plan mechanisms.
The Cambrian explosion, roughly 541 to 485 million years ago, produced representatives of nearly all major animal body plans within a geologically brief interval. Fossil sites such as the Burgess Shale in British Columbia preserve soft-bodied animals from this period, giving paleontologists direct evidence of how diverse body plans appeared and were modified over time.
A body plan describes every anatomical detail of a species. Body plans describe only broad structural organization; individual species vary enormously in anatomy while sharing the same fundamental arrangement.
All arthropods share a body plan defined by a segmented exoskeleton, jointed appendages, and an open circulatory system. Within this shared plan, a horseshoe crab (Limulus polyphemus) has ten walking legs while a honeybee (Apis mellifera) has six, illustrating how species diverge in detail while retaining the same fundamental organization.
Body Segmentation
/ BOD-ee seg-men-TAY-shun / · Old English bodig; Latin segmentum, cutting
Body Segmentation is the developmental organization of an animal's body into a series of repeated structural units along the anterior-posterior axis, each of which can be independently modified to carry out different functions.
Segmentation arises through the periodic activation of developmental gene networks during embryogenesis. In vertebrates, somites form as paired blocks of paraxial mesoderm that bud off from the presomitic mesoderm at regular intervals, with a new somite pair added approximately every 90 minutes in zebrafish (Danio rerio) and every six hours in humans. Each somite gives rise to vertebrae, ribs, skeletal muscle, and dermis of the back, establishing the segmental organization visible in the adult spine.
In insects, gap genes and pair-rule genes subdivide the embryo into segment-sized domains before the segments themselves become morphologically distinct, so segmentation is molecularly established before it is anatomically visible.
The segmentation clock, a molecular oscillator that drives periodic somite formation in vertebrates, involves cycling expression of Notch, Wnt, and FGF pathway genes. In humans, mutations in genes encoding components of this clock, such as HES7 and LFNG, cause spondylocostal dysostosis, a skeletal disorder in which vertebrae and ribs are malformed because somite boundaries form incorrectly.
Segmented animals have identical repeated parts throughout life. Hox genes and other developmental regulators modify each segment's identity, so segments that start as similar units become highly specialized structures such as antennae, wings, or reproductive appendages.
In the fruit fly (Drosophila melanogaster), the embryo is divided into 14 parasegments by pair-rule gene expression within the first three hours after fertilization, well before any physical segment boundaries are visible. Each parasegment is defined by a unique combination of Hox gene expression that determines what adult structures will form from it.