Practice question

Mammalian blastocyst consists of outer trophectoderm and cluster of cells at embryonic pole termed inner cell mass. Trophectoderm progenitors express Cdx2 and differentiate into placenta and trophoblast giant cells contacting uterus. Inner cell mass cells retain expression of Oct4, Nanog, Sox2, Klf4 maintaining pluripotency to generate all three germ layers of fetus, plus primitive endoderm giving yolk sac and amnion. Polar body elimination and zona pellucida are not embryonic lineages. Consequently inner cell mass is source of embryonic stem cells and exclusively forms embryo proper, while trophectoderm provides extraembryonic support essential for implantation and nutrient exchange.