Boundary-guided cell alignment drives mouse epiblast maturation.

Symmetry breaking and pattern formation occur throughout embryonic development. In early mouse development, a mass of non-polarized epiblast cells in the blastocyst forms the egg cylinder, while cells become apico-basally polarized and build a radial configuration. However, it remains unclear what drives the formation of this tissue architecture. Here we demonstrate that the orientational patterning of epiblast cells is dictated by heterogeneous tissue boundaries, which then defines central lumen positioning. We show that epiblast cells progressively orient perpendicular to the visceral endoderm boundary-which is enriched with the basement membrane protein laminin and the cell surface receptor active integrin β1-but parallel to the extraembryonic ectoderm interface. These orientation dynamics are consistent with general boundary-induced alignment effects in polar materials, with a topological defect predicting the position at which the proamniotic cavity nucleates. The knockout of laminin γ1 and integrin β1 confirms the essential role of adhesion at the epiblast and visceral endoderm boundary. The established epiblast pattern, in turn, facilitates ERK activation-a key cell signalling pathway-to ensure proper epiblast maturation. Together, these findings present the mechanistic basis and functional significance of epiblast tissue patterning.