Practice question

Totipotent cells, by forming trophectoderm lineage and primitive endoderm plus embryonic lineages, robustly support full fetal development including formation of placenta and yolk sac membranes essential for maternal-fetal nutrient exchange, gas exchange, and immunoprotection during implantation. Pluripotent cells, such as inner cell mass-derived embryonic stem cells cultured in vitro, efficiently form ectoderm, mesoderm, endoderm derivatives like neurons, muscle, gut epithelium, but contribute poorly to extraembryonic trophoblast without genetic manipulation or forced Cdx2 expression. This molecular restriction reflects epigenetic silencing of trophoblast program via Oct4-mediated repression of Cdx2, illustrating progressive narrowing of fate potential as development proceeds forward.