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Asterias blastopore2/1/2024 a, Grobben’s observations of two types of developmental fate of the blastopore. doi: 10.1002/wdev.262 For further resources related to this article, please visit the WIREs website. 1: Theories for the evolutionary origin of mouth and anus in the bilaterians. How did the employment of this machinery evolve? The reptilian branch of the amniote clade might provide us with useful tools to investigate the evolution of the amniote internalization site up to the formation of the 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. 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. How these morphogenetic movements are orchestrated and evolved has been a question for developmental biologists for many years. 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. 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. 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. 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.
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