Abstract
During mouse embryogenesis, totipotency is gradually lost, and, at the 16-cell stage, blastomeres begin to bifurcate into trophectoderm (future placenta) and inner cell mass (future fetus). Although this process is well studied, when and how blastomeres acquire the competence for lineage specification remains unclear. Here, we describe that CEBPa becomes up-regulated at the transition from the two- to the four-cell stage by NR5A2 and is also selectively expressed in the trophectoderm at the blastocyst stage. Its knockout decreases the proportion of trophectoderm cells and delays the morula to blastocyst transition. Conversely, CEBPa overexpression in mouse embryonic stem cells, used as a proxy, drives their differentiation into trophectoderm-like cells, enabling the identification of CEBPa-regulated trophectoderm-specific enhancers. A subset of these enhancers, associated with key trophectoderm-related transcription factor genes, is primed or activated in four- and eight-cell embryos. Together, our data suggest that CEBPa plays a role in the installment of trophectoderm competence before the first lineage bifurcation and in trophectoderm specification.