Tension-Induced Nematic Ordering Organizes Epithelial Morphogenesis

Collective force regulation in epithelial sheets is essential for sculpting tissue form, but the mechanisms linking large-scale forces to emergent cellular material properties remain elusive. Here we investigate collective force regulation during the closure of the Hindbrain Neuropore (HNP), a critical gap in the cranial neural tube of mouse embryos. This morphogenetic event is driven by the coordinated action of directional cell migration and a high-tension actomyosin purse-string at the gap rim. We show that these force-generating mechanisms alone are insufficient to explain the reproducible patterns of cell shape and nematic ordering found at the closing boundary. Instead, we present a crucial regulatory mechanism: a mechanical feedback loop where the purse-string tension acts as a localized mechanical cue that generates shear stress, inducing a persistent nematic order within the adjacent cells. This emergent anisotropy fundamentally alters cell shapes and, by suppressing local cell neighbor exchanges, promotes tissue solidification, thereby imposing a mechanical memory on the newly fused tissue. This physical framework is validated by comparing cell patterning found in wild-type mice to that in chick embryos, which naturally lack the purse-string tension.