Practice question

Mammalian gastrulation converts bilaminar disc of epiblast and hypoblast into three germ layers. It begins posteriorly with appearance of primitive streak, linear epithelial thickening where epiblast cells undergo epithelial to mesenchymal transition via Wnt and Nodal signaling, downregulating E-cadherin and ingressing to form mesoderm and definitive endoderm. Brachyury T marks ingressing cells. Node at anterior tip acts as organizer secreting antagonists patterning axis. Primitive streak therefore corresponds to blastopore of amphibians, initiating body plan formation, axis specification, gastrulation movements, germ layer segregation for organogenesis and patterning events.