Hedgehog-driven adaxial cell constriction patterns slow muscle fate and somite boundary remodeling in the presomitic mesoderm.

How morphogen signaling interfaces with cell behaviors and mechanics to coordinate body axis patterning has remained largely unclear. Here, we uncover a mechanochemical program that directs slow muscle fate commitment and somite boundary remodeling in the presomitic mesoderm (PSM). Hedgehog, secreted from the underlying notochord, triggers cytoskeletal remodeling and basal constriction in adaxial cells, progressively reducing notochord contact and allowing adjacent cells to engage the morphogen source. Concurrently, directional rearrangements of dorsal and ventral medial cells shift prospective somite boundaries posteriorly, converting straight borders into V-shaped patterns before segmentation. Vertex modeling and mutant analyses indicate that Hedgehog-dependent constriction of adaxial cells initiates the tissue deformation, while somite segmentation stabilizes this architecture. Using an integrative multi-omics approach with targeted gene validation, we identified acta1a (a major skeletal muscle actin isoform) and arhgef25a (a RhoGEF) as key participants in adaxial cell morphogenesis in the PSM.