Chemically-defined induction of a primitive endoderm and epiblast-like niche supports post-implantation progression from blastoids.

The early mammalian embryo (blastocyst) contains three co-developing types of stem cells. Two supporting extraembryonic types - the trophectoderm and the primitive endoderm - encase and guide the pluripotent epiblast that eventually forms all body tissues. Unlike embryos, stem cell-based models of the embryo can be generated in large numbers and subjected to high-content screens as a basis for basic and biomedical discoveries (Rivron et al., 2018a; Vrij et al., 2016b). Here, we show that aggregates of naive Embryonic stem cells cultured in chemically-defined conditions and exposed to combinatorial screens of signaling molecules, rapidly (48 hours) and efficiently (80%) set apart PrE-like cells. These cells produce a basal lamina, generate progenitors resembling both visceral and parietal endoderm, and polarize co-developing Epiblast cells to form a proamniotic cavity. In blastoids, stem cell-based models of the early blastocyst (Rivron et al., 2018b), this combination of signals increases the ratio and number of Gata6+/Nanog+ cells and promotes the survival, growth and morphogenesis of a post-implantation-like Epiblast in vitro. Modeling early embryonic development in chemically-defined in vitro conditions shows that the primitive endoderm forms via a specific combination of signaling pathways and sufficient to drive the development of the Epiblast.