Shh induces symmetry breaking in the presomitic mesoderm by inducing tissue shear and orientated cell rearrangements

Future boundaries of skeletal muscle segments are determined in the presomitic mesoderm (PSM). Within the PSM, future somitic cells undergo significant changes in both morphology and position. How such large-scale cellular changes are coordinated and the effect on the future border formation is unknown. We find that cellular rearrangements differ between cell populations within the PSM. In contrast to lateral somitic cells, which display less organized rearrangement, the adaxial cell layer undergoes significant tissue shearing with dorsal and ventral cells sliding posteriorly. This shear is generated by orientated intercalations of dorsally and ventrally located adaxial cells, which induces a chevron-like pattern. We find Shh signaling is required for the tissue shear and morphogenesis of adaxial cells. In particular, we observe Shh-dependent polarized recruitment of non-muscle myosin IIA drives apical constrictions, and thus the intercalations and shear. This reveals a novel role for Shh in regulating cell mechanics in the PSM.