Fluctuations and entropy enable neural crest cell ingression

The second law of thermodynamics explains the dissipative nature of embryonic development as an exchange of energy-dependent order for proportionately greater output of heat and waste. Recent work on granular matter provides a path by which to define the roles of passive, stochastic mechanisms in nonequilibrium systems. Here, we apply such a framework to examine the role of thermodynamic parameters to cell ingression, the movement of cells from one tissue layer to another that has been attributed, in part, to directional cues. Using the murine neural crest as a model system, we provide evidence that a stochastic mechanism, rather than a proposed stiffness gradient, underlies cell ingression. Cortical fluctuations representing effective temperature and cell packing configurations generate an entropic trap that promotes cell ingression. The results imply dissipative mechanisms that transiently disorder tissue underlie some morphogenetic events.