Phase transitions in confluent epithelia

Despite the absence of gaps and interstitial structures, confluent layers of epithelial cells are able to migrate collectively and remove excess cells by extrusion. While in common with foams and other passive confluent fluids, both these phenomena crucially rely on the active remodelling of the cellular network, via topological transformations known as T1 and T2 processes. Combining analytical predictions from active hydrodynamics and renormalization group methods with statistics from extensive numerical simulations of a multiphase field model, we show that both collective migration and cell extrusion can be thought as continuum phase transitions, with the former being in the same universality class of the Kosterlitz-Thouless transition and the latter reminiscent of sublimation in solids.