Inner Cell Mass Development

Following fertilization, the mammalian embryo self-organizes into a hollow ball of cells, the blastocyst. The first two cell types that appear within the blastocyst are known as trophoblast and inner cell mass (ICM). Trophoblast will form placenta, while the ICM splits into two additional lineages before implantation—pluripotent epiblast (future fetus), and hypoblast (future yolk sac). Thus the ICM is a transitory compartment that lasts only until all cells have been allocated to one of the three lineages that make up the blastocyst. This article summarizes the cellular and molecular basis of ICM development and its evolutionary context.