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. Here, inspired by serosa closure in the beetle Tribolium, we study the interplay between these plastic cell rearrangements and the elasticity of the tissue in a minimal continuum model of the closure of a circular gap bounded by a contractile actomyosin cable. We discover two different closure mechanisms at the tissue scale depending on the energy barrier $E_\text{b}$ to and the energy $\Delta E$ released by intercalation: If $E_\text{b}\gg\Delta E$, cells intercalate into the gap to close it. For a fluidised tissue in which $E_\text{b}\ll\Delta E$, however, cells deintercalate from the boundary into the bulk of the tissue, and we reveal an emergent mechanical role of inhomogeneities of the actomyosin cable. Our work thus explains the mechanical role of tissue fluidisation in Tribolium serosa closure and processes of epiboly and wound healing more generally.