Practice question

Inner cell mass segregates into epiblast and primitive endoderm, also termed hypoblast, characterized by Gata6 and Sox17 expression. Primitive endoderm cells migrate along blastocoel roof to line cavity, later differentiating into parietal endoderm lining trophectoderm and visceral endoderm surrounding epiblast. Parietal and visceral endoderm combine with extraembryonic mesoderm to form yolk sac providing early hematopoiesis, nutrient transport and signals such as Dkk1 and Cerl1 antagonizing Nodal to position primitive streak. Thus primitive endoderm progenitors generate supportive yolk sac membranes essential for patterning, nutrition and early embryonic organization.