L'origine de la gastrulation

Abstract

Gastrulation is a crucial developmental event occurring in all Metazoa. A simple form of gastrulation can be observed in some diploblastic Metazoa. The egg of these animals gives rise to a blastula, which is a hollow, single-layer sphere of epithelial cells. The blastula transforms into gastrula by a progressive change in cell shape known as invagination. A particular region of the epithelium forms a depression in the blastula. The depression later deepens into a secondary cavity which communicates with the exterior by a single opening called the blastopore. This results in a concentric arrangement of the embryonic cells into two layers surrounding a digestive cavity. The non-invaginating 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 ascribed to a cytoskeleton-driven mechanism causing the presumptive endodermal cells to modify their shape so as to invaginate. This evolutionary scheme implies that endoderm played a more important role than ectoderm in embryogenesis of primitive Metazoa. A similar trend can be discerned in early development of present-day animals: the invaginating region functions as a major morphogenetic territory in the embryo because it contains several gene products that directly or indirectly trigger gastrulation. [References: 65]