Enhancer status in the primitive endoderm supports unrestricted lineage plasticity in regulative development

Mammalian blastocyst formation involves the specification of trophectoderm followed by the differentiation of the inner cell mass into either epiblast or primitive endoderm. During this time, the embryo maintains a window of plasticity and can redirect its cellular fate when challenged experimentally. In this context, we found that the primitive endoderm alone was sufficient to regenerate a complete blastocyst and continue normal postimplantation development to term. We identify anin vitropopulation similar to the early primitive endodermin vivo, that exhibits the same embryonic and extra-embryonic potency, forming three dimensional embryoid structures. Commitment in early primitive endoderm is suppressed by JAK/STAT signalling, collaborating with OCT4 to safeguard enhancer status enabling multi-lineage differentiation. Our observations support the notion that transcription factor persistence underlies plasticity in regulative development and highlights the importance of primitive endoderm in perturbed development.