Tissue phase transitions in development: more than just mechanics

In many developing systems, tissue morphogenesis relies on dynamic deformability patterns emerging through material phase transitions. Transitions abruptly occur at crucial points in microscopic cell properties, including cell density, shape or adhesion, termed control parameters. Unlike in non-living materials, where phase transitions are a passive response to external inputs tuning the control parameters (e.g. temperature or pressure), transitions in living tissues are actively driven by internal cues, including growth and patterning signals. Consequently, phase transitions may not only regulate tissue deformability, but further integrate tissue mechanical changes with mechanochemical signalling, instructing the formation of complex, self-organised and adaptive spatiotemporal patterns. In this Review, we explore both canonical (mechanics centred) and non-canonical (beyond mechanics) roles of embryonic tissue phase transitions, particularly in relation to growth control and cell fate specification. We propose tissue phase transitions as a strategy by which developing systems spatiotemporally couple processes across molecular, cellular and tissue levels, facilitating developmental robustness and precision.