Gastrulation Prepared by: W.R. JACINTO

Rearrangement of blastomeres to establish multilayered body plan by morphogenetic movements. Cells forming endodermal and mesodermal organs are brought inside Cells to form the ectoderm are spread over outside surface Cells are brought close enough together for inductive interactions

Sea urchin Determination of blastomeres Studies have shown that by the 60-cell stage, most of the embryonic cell fates are specified, but that the cells are not irreversibly committed.

Invagination of the vegetal plate to one-half way into the blastocoel

Ingression of primary mesenchyme Ingression from vegetal plate at the late blastula Cells develop filopodia to test substrates then ingress Cell disassociate from the monolayer derived from the micromere

Invagination Cells move to fill gaps created by ingression Cells move inward – forms archenteron Blastophore – opening of the archenteron

Archenteron formation - main feature First formed by invagination and elongation into the blastocoel Second, presence of secondary mesenchyme at the tip of the archenteron extending filopodia toward the opposite wall of the blastula. Third, after contact, secondary mesenchyme transform into Pigment cells, blastocoelar cells, coelomic pouches and circumesophageal musculatiure.

Bilaminar wall ruptures to form the oral opening; blastophore becomes the anus. Skeleton rods arise- leads to formation of pluteus larva.

Amphibian

Initiated at the future dorsal side, in the region of the gray crescent – marginal region where the animal meets the vegetal Bottle cells’ pull inside caused invagination Animal pole cells undergo epiboly Goosecoid and nogin genes - activated

Chordamesoderm – cells passing over dorsal lip Cells undercut the ventrally – result to formed yolk plug

Birds Already in progress at the time of laying Forms primitive streak – landmark in birds, reptiles and mammal

P. streak extends from posterior to anterior Migrating cells move from dorsal to ventral Primitive groove, primitive knot or Hensen’s node, primitive ridges Head process – where notochord is

Regression of primitive streak Migrating cells becomes the (a)pharyngeal endoderm of the foregut (b) head mesenchyme and pre-chordal plate (c) notochord

Goosecoid gene in Hensen’s node and Koller’s sickle As avian gastrulation draws to a close, the ectoderm has surrounded the yolk, the endoderm has replaced the hypoblast, the mesoderm has positioned itself between these regions.

Mammals

Cell migrating to the node become the notochord Notochord cells are integrated in the endoderm in mouse