Plexin/Semaphorin antagonism orchestrates collective cell migration and organ sculpting by regulating epithelial-mesenchymal balance.

Cell behavior emerges from the intracellular distribution of properties such as protrusion, contractility, and adhesion. Thus, characteristic emergent rules of collective migration can arise from cell-cell contacts locally tweaking architecture, orchestrating self-regulation during development, wound healing, and cancer progression. The testis-nascent-myotube system allows dissection of contact-dependent migration in vivo at high resolution. Here, we describe a role for the axon guidance factor Plexin A in collective cell migration: maintaining cell-cell interfaces at a precise point on the mesenchymal-to-epithelial continuum. This is crucial for testis myotubes to migrate as a continuous sheet, allowing normal sculpting-morphogenesis. Cells must maintain filopodial N-cadherin-based junctions and remain ECM-tethered near cell-cell contacts to spread while collectively moving. Our data further suggest Semaphorin 1b is a Plexin A antagonist, fine-tuning activation. This reveals a contact-dependent mechanism to maintain sheet integrity during migration, driving organ morphogenesis. This is relevant for mesenchymal organ sculpting in other migratory contexts such as angiogenesis.