The complex microscopic dynamics of cells in high-density epithelial tissues

Epithelial tissues line the surfaces of vital organs and are often densely packed in a disordered, mechanically arrested state, referred to as jammed or glassy state. While collective migration at low-density regimes has been extensively studied, the microscopic dynamics within such jammed epithelia remains poorly understood. Here, we reveal that contrary to expectations from thermal systems with glassy dynamics, jammed epithelial monolayers do not exhibit cage effects that reflect the temporary spatial trapping of the particles. Instead, cells display sub-diffusive creep and Fickian yet non-Gaussian dynamics, accompanied by compressed exponential relaxation, features that reveal stress-driven fluidity. We show that cell divisions and extrusions transiently do enhance local motion, they are insufficient to fluidize the tissue globally. Fast-moving cells form collective, anisotropic clusters, and these dynamic heterogeneities correlate with local structural entropy and low-frequency vibrational modes. These findings challenge the conventional view that jammed tissues are static and inert structures, uncovering a hidden fluidity that can be expected to play a critical role in morphogenesis, wound healing, and early tumor progression.