Oscillatory cortical forces promote three dimensional mesenchymal cell intercalations to shape the mandibular arch

Multiple organ primordia are composed of a volume of confluent cells. Although mechanisms that shape tissue sheets are increasingly understood, those which shape a volume of cells remain obscure. We show here that the spatial distributions of cell cycle time, tissue elasticity and viscosity are important for growth of the mandibular arch in the mouse embryo, but do not adequately explain tissue shape. Rather, mesenchymal cells intercalate in 3D to narrow and elongate the middle region of the arch. Using a knock-in vinculin tension sensor, we show that cortical force oscillations promote mesenchymal cell intercalations. Wnt5a functions as a spatial cue to orient cortical actomyosin and promotes high amplitude oscillations, in part by regulating cytosolic calcium fluctuations in a Piezo1-dependent manner. Our data support cell neighbour exchange as a conserved mechanism that drives morphogenesis of a volume of tissue and is spatially coordinated by pathways which tune cytoskeletal oscillation.