The first choice of the preimplantation embryo: How compaction and polarity build cell identity

The decision of a blastomere to become inner cell mass or trophectoderm relies on the integration of two types of information: position and polarity. Compaction and polarization play key roles in this first lineage decision, since they alter both the intra- and the intercellular organization of the embryo. In-depth studies of early embryogenesis using the mouse model have provided new insights into the molecular regulation of compaction, polarization and lineage specification. However, how these processes first emerge and influence subsequent molecular and cellular events remain open questions in the field. In this review, we summarize the chain of events that lead to the generation of the first two cell lineages, outlining how compaction and polarization can build cell identity. Such processes, despite running in parallel, are subjected to different regulatory pathways. Then, if under specific circumstances one of the regulatory pathways is affected, embryos may achieve compaction but may have severe problems to acquire full developmental potential.