THE ORIGIN OF GASTRULATION

Gastrulation is a crucial developmental event occurring in all Metazoa . A simple form of gastrulation can be observed in some diploblastic M etazoa. The egg of these animals gives rise to a blastula, which is a hollow, single-layer sphere of epithelial cells. The blastula transfor ms into gastrula by a progressive change in cell shape known as in inv agination. A particular region of the epithelium forms a depression in the blastula. The depression later deepens into a secondary cavity wh ich communicates with the exterior by a single opening, called the bla stopore. This results in a concentric arrangement of the embryonic cel ls into two layers surrounding a digestive cavity. The non-invaginatin g cells form the ectodermal layer, whereas the invaginating cells form the endodermal layer. Invagination is thought to be the basic process by which a blastula-like ancestral organism evolved into a two-layer, gastrula-like animal. Conceivably, this transformation can be ascribe d to a cytoskeleton-driven mechanism causing the presumptive endoderma l cells to modify their shape so as to invaginate. This evolutionary s cheme implies that endoderm played a more important role than ectoderm in embryogenesis of primitive Metazoa. A similar trend can be discern ed in early development of present-day animals: the imvaginating regio n functions as a major morphogenetic territory in the embryo because i t contains several gene products that directly or indirectly trigger g astrulation.