Elasticity and plasticity of epithelial gap closure

Epiboly, during which a tissue closes around the surface of the egg, pervades animal development. This epithelial gap closure involves cell intercalations at the edge of the gap, but the interplay between these plastic cell rearrangements and the elasticity of the tissue is not understood. Here, inspired by serosa closure in the beetle Tribolium, we study this interplay in a minimal continuum model of the closure of a circular gap bounded by a contractile actomyosin cable. We reveal how two different closure mechanisms emerge at the tissue scale from the energy barrier E_{b} to and the energy ΔE released by intercalation: If E_{b}≫ΔE, cells intercalate into the gap to close it. For a fluidized tissue in which E_{b}≪ΔE, however, cells deintercalate from the boundary into the bulk of the tissue, consistently with Tribolium serosa closure. We also discover an emergent mechanical role of the inhomogeneities of the actomyosin cable observed in Tribolium. Our work thus explains the mechanical basis for Tribolium serosa closure and processes of epiboly and wound healing more generally.