The evolution of amniote gastrulation: the blastopore ‐primitive streak transition

In the animal kingdom, gastrulation, the process by which the primary germ layers are formed involves a dramatic transformation in the topology of the cells that give rise to all of the tissues of the adult. Initially formed as a mono‐layer, this tissue, the epiblast, becomes subdivided through the internalization of cells, thereby forming a two (bi‐laminar) or three (tri‐laminar) layered embryo. This morphogenetic process coordinates the development of the fundamental body plan and the three‐body axes (antero‐posterior, dorso‐ventral, and left‐right) and begins a fundamental segregation of cells toward divergent developmental fates. In humans and other mammals, as well as in avians, gastrulating cells internalize along a structure, called the primitive streak, which builds from the periphery toward the center of the embryo. How these morphogenetic movements are orchestrated and evolved has been a question for developmental biologists for many years. Is the primitive streak a feature shared by the whole amniote clade? Insights from reptiles suggest that the primitive streak arose independently in mammals and avians, while the reptilian internalization site is a structure half‐way between an amphibian blastopore and a primitive streak. The molecular machinery driving primitive streak formation has been partially dissected using mainly the avian embryo, revealing a paramount role of the planar cell polarity (PCP) pathway in streak formation. How did the employme...
Source: Wiley Interdisciplinary Reviews: Developmental Biology - Category: Biology Authors: Tags: Advanced Review Source Type: research