Force patterning drives cell flows and 3D spatial order in auditory epithelia

During development, coordinated cell behavior drives the morphogenesis of epithelia into intricate structures essential for their physiological functions. How this coordination is achieved in epithelia composed of multiple cell types remains unclear. We study development of the avian auditory epithelium comprising sensory hair cells (HCs) and non-sensory supporting cells (SCs). Initially, HCs and SCs are arranged into mosaics by Notch-Delta signaling. During development, HCs partially extrude from the epithelium, establish a ten-fold gradient in apical surface area across the tissue, and intercalate with SCs to form near-hexagonal order. Using a combination of experiments and a 3D-vertex model, we show that an increase of contractility at the apical junctions between SCs compared to those between HCs and SCs drives spatial order within the epithelial plane and along the apical-basal axis. A faster increase of HC apical area at one end of the epithelium than the other leads to opposite fluxes of HCs and SCs and establishes the observed gradients in apical surface area and density of HCs along the auditory epithelium while maintaining a uniform degree of hexagonal order throughout. Our findings reveal that patterned junctional contractility can spatially coordinate cell behavior across both the plane and depth of a mixed epithelium, providing a general mechanism for generating complex three-dimensional tissue architectures during organogenesis.