What Drives the Formation of Trophectoderm During Early Embryonic Development?

In the mouse, the lineages of cells that give rise to trophectoderm and ICM are generally held to arise between the 8-cell and 16-cell stage of development. This model assumes that all blastomeres have essentially equivalent potential in terms of their fate through the first three rounds of cell division. There is, however, accumulating evidence that the blastomeres of 2-cell stage conceptuses may be compositionally different and contribute unequally to trophectoderm and ICM of the blastocyst. Here, we evaluate these competing points of view relating to when commitment to the trophectoderm lineage occurs in mammals, describe some of the genes that drive trophectoderm specification, and discuss the implications of the two hypotheses in relation to outcomes in commonly used reproductive technologies. Much of what is presently known has been derived from studying the mouse, but where information is available from other species, and particularly from cattle, it has been included.