Adhesion-driven rigidity transition decoupled from density-driven jamming triggers epithelial organization in embryonic tissues.

The active regulation of tissue material properties via phase transitions is central in morphogenesis. Transitions occur abruptly at critical points in different control parameters, such as cell density, shape or adhesion. Whether these parameters are interdependent, and perform redundant or distinct functions, is unknown. Here we show that depending on the co-regulation of multiple control parameters, a tissue not only tunes its deformability but also its morphogenetic trajectory. We theoretically define a phase diagram capturing the material states of zebrafish pluripotent tissues undergoing epiboly-a tissue movement occurring during gastrulation-and show that they simultaneously cross critical points in cell density, connectivity and adhesion strength. We then combine optogenetics, biophysical measurements and quantitative morphometrics to independently modulate each parameter in vivo, and identify adhesion as the main determinant of tissue rheology. Further decoupling adhesion from density and inducing adhesion-driven rigidification in unjammed pluripotent tissues is sufficient to switch their morphogenetic program and trigger epithelial organization. This switch in tissue reorganization is achieved via tricellular junction formation, followed by lumenogenesis and the initiation of apical polarity. Our work reveals that the nonlinear dynamics of emergent tissue mechanics are mechanisms of tissue organization and morphogenesis.