Embryology and Development Chapter 4

Comprehensive vocabulary terms and definitions covering developmental patterns, egg types, cleavage styles, and early embryogenesis in sea urchins and snails based on the lecture transcript.

Metazoans

All animals that undergo gastrulation during development.

Diploblastic animals

Animals with two germ layers and radial symmetry that lack a mesoderm, though some develop striated muscles from mesoderm-like primordia.

Protostomes

Triploblastic organisms with bilateral symmetry where the mouth develops before the anus.

Deuterostomes

Triploblastic organisms with bilateral symmetry where the anus develops before the mouth.

Isolecithal

A type of egg with a small amount of yolk evenly distributed throughout the cytoplasm, found in sea urchins, snails, nematodes, and mammals.

Mesolecithal

A type of egg with a moderate amount of yolk concentrated in the vegetal half, found in amphibians.

Telolecithal

A type of egg with a very large amount of yolk occupying the entire egg except for the blastodisc, found in birds, fish, and reptiles.

Centrolecithal

A type of egg where yolk is concentrated at the center with a thin layer of cytoplasm covering it, found in insects.

Holoblastic cleavage

Complete mitotic division of the egg.

Meroblastic cleavage

Incomplete cleavage occurring in telolecithal eggs where division is observed only in the blastodisc.

Syncytial cleavage

Mitotic cell divisions that occur without cytokinesis.

Superficial cleavage

Mitotic cell divisions with cytokinesis occurring only at the surface of the egg.

Invagination

The infolding of a sheet of cells into the embryo.

Involution

The inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.

Ingression

The migration of individual cells into the embryo.

Delamination

The splitting or migration of one cell sheet into two sheets.

Epiboly

The expansion of a sheet of cells over other cells.

Arbacia punctulata

The Atlantic purple sea urchin, typically characterized by isolecithal eggs and radial holoblastic cleavage.

Mesenchyme

A loosely organized tissue; in sea urchins, it is divided into skeletogenic (primary) and non-skeletogenic (secondary) types.

Vegetal plate

The flattened and thickened region at the vegetal pole of a sea urchin blastula that eventually invaginates.

Skeletogenic mesenchyme cells

Descendants of micromeres that ingress into the blastocoel and fuse to form syncytial cables and calcium carbonate spicules.

Archenteron

The primitive gut formed by the invagination of the vegetal plate in a gastrulating embryo.

Convergent extension

A rearrangement process where the wall of the archenteron narrows and extends by reducing its circumference from 20-30 cells to approximately 8 cells.

Filopodia

Actin-rich protrusions extended by non-skeletogenic mesenchyme cells to pull the archenteron toward the oral ectoderm.

Pluteus larva

The swimming sea urchin larva where the ectoderm forms the shell, the mesoderm forms the skeleton, and the endoderm forms the digestive system.

Autonomous specification

A type of specification where the fated descendants of a cell, such as sea urchin vegetal micromeres, will form their specific structures even if moved to another location.

Conditional specification

A type of specification where cells, such as sea urchin animal cap cells, require external signals to determine their fate and can be re-specified.

${\beta}$-catenin

A transcription factor that accumulates in the nuclei of vegetal cells to specify the vegetal half of the sea urchin embryo; its absence leads to animalization.

Wnt signaling pathway

A signaling pathway involving secreted proteins that activates ${\beta}$-catenin by destabilizing the APC-containing destruction complex via Frizzled and Disheveled.

GSK-3

Glycogen synthase kinase 3, a ubiquitous protein and component of the APC-containing complex that targets ${\beta}$-catenin for degradation in the absence of Wnt.

Animalization

A default developmental pathway in sea urchins where the embryo becomes an all-ectoderm ball in the absence of ${\beta}$-catenin activation.

Imaginal rudiment

A small cluster of cells on the left side of the pluteus larva from which the adult sea urchin develops during metamorphosis.

Stereoblastula

A snail blastula that lacks a blastocoel.

Dextral coiling

Normal snail shell coiling to the right, determined by the D allele.

Sinistral coiling

Reversed snail shell coiling to the left, resulting from the recessive null d allele.

Polar lobe

A bulb of cytoplasm extruded by snail zygotes before cleavage that contains maternal RNAs and endomesodermal determinants.