Medical Embryology - Cleavage, Blastulation, and Gastrulation

Comprehensive vocabulary flashcards covering egg ootypes, main and specific patterns of cleavage (holoblastic vs meroblastic), stages of early development like blastulation and gastrulation, the three primary germ layers, and types of cell movements during gastrulation.

lekithos

A Greek term meaning yolk.

isolecithal

Eggs with sparse, evenly distributed yolk (iso = equal, uniform).

mesolecithal

Eggs with a moderate vegetal yolk disposition (meso = middle).

telolecithal

Eggs with dense yolk throughout most of the cell (telo = end).

centrolecithal

Eggs with yolk located in the center (centro = center), typical of most insects.

blastos

A Greek term meaning germ or sprout.

Holoblastic cleavage

Complete cleavage where the entire egg is divided into cells.

Meroblastic cleavage

Incomplete cleavage where only a portion of the egg is divided.

Radial cleavage

A specific pattern of isolecithal holoblastic cleavage found in echinoderms and amphioxus.

Spiral cleavage

A specific pattern of holoblastic cleavage found in annelids, molluscs, and flatworms.

Bilateral cleavage

A pattern of cleavage found in tunicates (holoblastic) and cephalopod molluscs (meroblastic).

Rotational cleavage

A specific pattern of holoblastic cleavage found in mammals and nematodes.

Displaced radial cleavage

A specific pattern of holoblastic cleavage found in amphibians like Xenopus laevis.

Discoidal cleavage

Meroblastic cleavage occurring at the animal pole of telolecithal eggs in fish, reptiles, and birds.

Superficial cleavage

Meroblastic cleavage occurring in centrolecithal eggs, typical of most insects.

Blastula

A hollow sphere of cells (blastomeres) formed during an early stage of embryonic development after cleavage.

Blastocoel

The fluid-filled cavity inside the blastula.

Gastrulation

The phase where the single-layered blastula is reorganized into a trilaminar (three-layered) gastrula structure.

Ectoderm

The outer germ layer that gives rise to the epidermis, the central nervous system, and neural crest cells.

Mesoderm

The middle germ layer that forms the notochord, bone tissue, kidney tubules, red blood cells, and muscles.

Endoderm

The internal germ layer that forms the digestive tube, respiratory tube, pharynx, thyroid, and lung cells.

Invagination

The infolding of a region of cell sheets into the embryo, such as the sea urchin endoderm.

Involution

The inward movement of an expanding outer layer over the basal surface of an outer layer, such as amphibian mesoderm.

Ingression

The migration of individual cells from the surface layer into the interior of the embryo, seen in sea urchin mesoderm.

Delamination

The splitting of one cellular sheet into two or more parallel sheets, as seen in mammalian hypoblast formation.

Epiboly

The movement of epithelial sheets (usually ectodermal cells) that spread as a unit to enclose deeper layers.

IMZ

Involuting Marginal Zone; the region in Xenopus where cells move inward during gastrulation.

Archenteron

The primitive gut cavity formed during gastrulation.

Yolk plug

A mass of yolk-rich endodermal cells that remains exposed in the blastopore during late gastrulation in Xenopus laevis.