Epiblast and primitive endoderm differentiation: fragile specification ensures stable commitment.

Specification of the epiblast and primitive endoderm is one of the earliest differentiation steps during embryogenesis. In vitro tracking of pluripotency markers in ESCs suggests that epiblast specification may be plastic; however, live imaging of blastocysts, as detailed in a recent paper from Xenopoulos et al (2015), showed that, unlike in ESCs, fate commitment in vivo is largely irreversible.