(A) At day E3.5, the blastocyst comprises an outer TE destined to populate the placenta and an ICM destined to form the embryo. TE cells not in direct contact with the ICM form the mural TE, whereas those adjacent to the ICM form the polar TE. The polar TE gives rise to the ExE, from which TS cells can be derived in vitro. (B) Following implantation, mural TEs initiate the first wave of TGC differentiation to form 1° TGCs, which contribute directly to the P-TGC population. Cells within the polar TE continue to proliferate and populate the ExE. Some 2° TGCs arise directly from the ExE. Along with 1° TGCs, they compose the P-TGC population that lines the implantation site. Cells within the ExE then differentiate to form the chorionic plate and the EPC. The chorionic plate is responsible for populating the mouse labyrinth with SynTs and a subset of 2° TGCs called S-TGCs. Together, these cells are responsible for the transport functions of the placenta. Cells within the EPC can either differentiate into a population of lineage-committed progenitors known as SpTs, which then differentiate into 2° TGCs, or they can directly differentiate into various 2° TGCs subtypes.