Shape-independent fluidity in epithelial cell monolayers.

Tissue fluidity regulates biological processes such as embryonic development, wound healing, and cancer metastasis. In confluent epithelia, where cell packing fraction is effectively fixed, the prevailing paradigm postulates that fluidity is governed by a geometric shape index determined by the balance of cortical tension and intercellular adhesion. Here, we report that reducing cell-cell adhesion triggers an increase in fluidity with no change in cell shape index, cell density, substrate traction, or junctional line tension. The observed decoupling of shape and fluidity reveals that current vertex models, which treat adhesion as contributing solely to interfacial tension, are incomplete. To reconcile these findings, we extend the theoretical framework to account for the dual nature of adhesion-its thermodynamic role in setting interfacial adhesion energy at the cell-cell junctions and its kinetic role in generating viscous drag due to relative motion between adjacent cells. This generalized model quantitatively captures the experimental data, demonstrating that the interplay between adhesive energy and dissipative friction is essential for epithelial fluidity.