Gastrulation
It is the cell movement that gives rise to: ectoderm, mesoderm and endoderm.

The cells that will form the endodermal and mesodermal organs are brought inside the embryo.
The ectodermal cells that will form the skin and nervous system are spread over the outside surface.

The topology of gastrulation varies depending on whether the embryo starts as a spherical blastula((sea urchins and frogs) or sheets of cells (bird and mammals). The basic outcome of is the same with:

Endoderm on the inside
Mesoderm in the middle
and ectoderm on the outside.

Many of the genes and mechanisms are conserved.

Initiation

In xenopus it is initiated at the future dorsal side of the embryo, just below the equator in the region of the gray cresent. This is the marginal zone where animal and vegetal poles meet, the vegetal cells have very little yolk.

Steps:

The prospective endodermal cells invaginate to form a slit like blastopore a the dorsal equator.
At the dorasl lip of the blastopore, the sheet of deep marginal zone cells involutes.
At the same time the animal cells spread as a unit to enclose the deeper layers of the embryo

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

The first cells to involute will form the most anterior endodermal and mesodermal cells, the last will form the posterior structures. The cells are patterned as they involute.

	Created by Beckie Thorne about 9 years ago

University Histology/developmental Note on Blastocyst formation and gastrulation, created by Beckie Thorne on 16/05/2015.