Modeling post-gastrula development via bidirectional pluripotent stem cells.

The absence of stem cells capable of efficiently generating both trophoblast and epiblast lineages has hindered precise recapitulation of embryonic development. Through high-content chemical screening, we established an (AS and LY) AL medium to generate mouse bidirectional pluripotent stem cells (BPSCs) characterized by concurrent expression of OCT4 and CDX2. Mouse BPSCs demonstrated highly plastic differentiation into trophoblast, epiblast and primitive endoderm (PrE) lineages in vitro within 48 h without exogenous inducing factors and efficiently contributed to embryonic and extraembryonic tissues in vivo. Mechanistically, hyperactivation of the Wnt signaling pathway breaks the early lineage differentiation barrier by initiating a Lef1-dependent bypass. Remarkably, integration of BPSCs with PrE induction system enables high-efficiency generation of E8.5-stage embryo models. These advanced models complete gastrulation and recapitulate definitive developmental milestones including brain morphogenesis, neural tube closure, cardiac contraction, somite patterning, and primordial germ cell specification. Moreover, human cells cultured under AL conditions acquire an OCT4 and CDX2 double-positive state and corresponding gene expression profiles, revealing conserved functionality of this culturing platform across species. These findings highlight BPSCs as a powerful tool for investigating early lineage specification and post-gastrulation embryonic development.