Regulation of mouse blastocyst primitive endoderm differentiation by p38-mitogen-activated-kinases (p38-MAPKs) is inner-cell-mass (ICM) autonomous and unrelated to cavity expansion defects in ICM specification.

During early mouse blastocyst ICM maturation, we previously described that pharmacological inhibition of p38-MAPK (p38-MAPKi) significantly impairs primitive endoderm (PrE) differentiation from an initially uncommitted population of ICM cells but does not affect pluripotent epiblast (EPI) specification. A recent report details a positive role for blastocyst cavity expansion in assisting ICM lineage formation and marker gene expression. As p38-MAPKi also results in smaller cavity volumes, we addressed to what extent p38-MAPKi mediated impaired PrE differentiation is driven by ICM autonomous or cavity expansion mechanisms. We compared ICM differentiation phenotypes associated with either chemically inhibited cavity volume expansion and p38-MAPKi, on the individual cell and ICM lineage population levels. Whilst recapitulating previously observed decreases in expression of both EPI and PrE markers, we discovered cavity expansion phenotypes are manifest in impaired numbers of specified EPI and increased numbers of uncommitted cells; rather than impaired PrE differentiation, as observed after p38-MAPKi. Moreover, using both 2D ES-cell and 3D ICM organoid models, we show PrE differentiation is also significantly impaired by p38-MAPKi in the absence of a blastocyst cavity; a result recapitulated in cultured immuno-surgically isolated early blastocyst ICMs, in which an outer PrE and inner EPI population are ordinarily formed. These data confirm the early blastocyst requirement for p38-MAPK activity to permit PrE differentiation from uncommitted ICM progenitors is primarily ICM autonomous rather than caused by impaired cavity expansion.