Following fertilization in the oviduct, the mammalian embryo undergoes a relatively slow round of cleavages to produce an eight-cell morula.

However, due to the variability and regulative nature of mammalian embryos, experimental evidence for establishing these early fates remains incomplete.

[3][4] These genes suppress Cdx2 and the inside cells maintain pluripotency generate the ICM and eventually the rest of the embryo proper.

Although this dichotomy of genetic interactions is clearly required to divide the blastomeres of the mouse embryo into both the ICM and TE identities, the initiation of these feedback loops remains under debate.

The asymmetric division of epigenetic information during these first two cleavages, and the orientation and order in which they occur, may contribute to a cell's position either inside or outside the morula.

These pluripotent cells, when grown in a carefully coordinated media, can give rise to all three germ layers (ectoderm, endoderm, and mesoderm) of the adult body.

[15] Blastomeres are dissociated from an isolated ICM in an early blastocyst, and their transcriptional code governed by Oct4, Sox2, and Nanog helps maintain an undifferentiated state.

One benefit to the regulative nature in which mammalian embryos develop is the manipulation of blastomeres of the ICM to generate knockout mice.

In mouse, mutations in a gene of interest can be introduced retrovirally into cultured ES cells, and these can be reintroduced into the ICM of an intact embryo.