(A) In the mouse, the fertilized egg undergoes three rounds of cleavage, producing an eight-cell embryo that then undergoes compaction. From the eight-cell stage onward, cell divisions produce two populations of cells, those that occupy the inside of the embryo and those that are located on the outside. The blastocoel cavity begins to form inside the embryo beginning at the 32-cell stage and continues to expand as the embryo grows and matures into the late blastocyst stage. Cdx2 becomes upregulated in outside, future TE cells, starting at the 32-cell stage, while Oct4 expression becomes limited to the ICM in the early blastocyst stage. By the late blastocyst stage, while continuing to express Oct4 ubiquitously, the ICM contains a population of Nanog-positive EPI cells and a population of Gata6-positive PE cells. (B) Development is similar in the early human embryo, although compaction occurs at the 16-cell stage and the mutually exclusive expression patterns of CDX2 and OCT4 are not established until the late blastocyst stage. The expression patterns of NANOG and GATA6 in the human preimplantation embryo have not yet been characterized.