Gastrulation leads to the formation of the three germ layers - ectoderm, endoderm and mesoderm. The separation of the three germ layers is achieved by means of the invagination of the presumptive mesoderm, the anterior and posterior midgut primordia and the hindgut anlagen. After invagination, these anlagen remain organized as a tube and share a common lumen. One important consequence of gastrulation is that large areas of the blastoderm become topologically distinct. The withdrawal of 1250 cells from the surface of the blastoderm, brought about by ventral furrow formation and amnioproctodeal invagination, leads to the separation of mesodermal and endodermal anlagen from the ectodermal anlage, and from this stage onwards the cellular behaviour of the three germ layers is conspicuously different.
Invagination of the ventral furrow is initiated by apical constrictions of cells located in the middle of the mesodermal anlage (ventral plate). As the cells also flatten, the blastoderm nuclei become displaced basally, probably being pushed by the cytoplasm as consequence of a slight shortening of the cells.Invagination of the cells forming the endodermal anterior midgut occurs slightly later than that of the ventral furrow, and the invaginating cells undergo shape changes similar to those described above for the ventral furrow. The invagination of the amnioproctodeum starts at the beginnig of germ band elongation with a dorsal displacement of a plate of cells at the posterior pole of the embryo. The original tubular architecture of the mesoderm primordium in the ventral furrow is lost at the onset of mitotic activity, when the cells dissociate to divide. They loose their epithelial character and form an irregular mesenchym. This disaggregated appearance of mesodermal cells persists during the fast phase of germ band elongation (first and second mesodermal mitoses). Extensive mixing of mesodermal cells does not seem to occur during early germ band elongation neither in the anteroposterior nor in the lateromedial dimension. On the contrary, it appears that the mesodermal cells retain the same topological relationships with the overlying ectodermal cells that their anlage had in the blastoderm.
Gastrulation, cross section (1.2 MB)..........additional information