Non-equilibrium Rigidity Transitions in Embryonic Tissues

The physical state of embryonic tissues emerges from collective interactions among constituent cells. Here, we present a computational framework that includes key features at the cellular level, namely the presence of extracellular spaces, complex cell shapes and tension fluctuations that enables the description of non-equilibrium dynamics and emergent mechanics in embryonic tissues. We capture two limits of previously reported rigidity transitions in equilibrium (jamming transition and density-independent transition) and elucidate a novel non-equilibrium transition governed by junctional tension fluctuations. We find that tissues are maximally rigid at the structural transition between confluent and non-confluent states, with active tension fluctuations controlling stress relaxation and tissue fluidization. Comparing the simulation results to developing tissues during zebrafish body axis elongation, we show that tissues are solid-like and behave like wet foams at short timescales, with adhesion levels controlling the degree of cellular confinement. However, active T1 transitions by tension fluctuations control long timescale stress relaxation and tissue fluidization. Our results highlight an important role of non-equilibrium tension dynamics in developmental processes.