Dynamic heterogeneity and hidden fluidity in dense epithelial tissues.

Epithelial tissues maintain organ integrity while continuously remodeling during morphogenesis, repair, and disease. At high cell densities, these tissues often appear mechanically arrested in a disordered, solid-like state, raising the question of how they retain the ability to reorganize. Here, we show that, unlike thermal glasses, dense epithelial tissues do not exhibit caging behavior but instead behave as a complex fluid. Cells display subdiffusive creep together with Fickian yet non-Gaussian dynamics and compressed exponential relaxation, hallmarks of stress-driven fluidity. This fluidity arises from the tissue's structural and mechanical organization rather than from cell division or extrusion, which only transiently enhance local dynamics. Fast-moving cells organize into collective, anisotropic clusters whose spatial heterogeneity correlates with local structural entropy and soft vibrational modes. Together, these findings reveal a hidden fluidity in densely packed epithelia that supports mechanical stability while preserving the capacity for remodeling during development, wound healing, and early tumor invasion.