β-Catenin Drives Apical–Basal Polarization to facilitate TE lineage commitment In Vitro and In Vivo

Embryo polarization is critical for the first cell fate segregation. While mechanisms underlying its initiation have been described, the intrinsic signaling pathways that regulate this process remain poorly understood. Here, we show that mouse embryonic stem cells, when aggregated under defined conditions, recapitulate the first lineage segregation to generate trophectoderm (TE)-like cell populations and undergo self-organized morphogenesis into blastocyst-like structures. In the blastoid-forming medium, we identify CHIR99021 is essential for the generation of blastoids from both ESCs and totipotent-like cells. CHIR99021 promotes cell polarization and TE differentiation by activating the WNT/β-catenin pathway and upregulating associated genes. Consistent with this, genetic ablation of β-catenin abolished the cell polarization and disrupted blastoid formation from ESCs, a defect that was restored by β-catenin overexpression. Moreover, β-catenin depletion compromised cell polarization in natural embryos. Collectively, this study establishes the Wnt/β-catenin as a critical regulator initiating polarization in vitro and in mouse early embryo development.