Tissue fluidization by cell-shape-controlled active stresses

TL;DR

This study models how active stresses along cell axes influence tissue mechanics, revealing transitions from solid to fluid states, formation of patterns like rosettes, and the emergence of topological defects similar to those in epithelial tissues.

Contribution

It introduces a minimal model linking cell shape-controlled active stresses to tissue rheology and pattern formation, providing new insights into tissue fluidization and defect dynamics.

Findings

Active stress induces transitions from crystalline to fluid tissue states.

Formation of rosette patterns and topological defects observed.

Predicted hotspots for cell rearrangements near defects.

Abstract

Biological cells can actively tune their intracellular architecture according to their overall shape. Here we explore the rheological implication of such coupling in a minimal model of a dense cellular material where each cell exerts an active mechanical stress along its axis of elongation. Increasing the active stress amplitude leads to several transitions. An initially hexagonal crystal motif is first destabilized into a solid with anisotropic cells. Increasing activity further, we find a re-entrant transition to a regime with finite hexatic order and finite shear modulus, in which cells arrange according to a rhombile pattern with periodically arranged rosette structures. The shear modulus vanishes again at a third threshold beyond which spontaneous tissue flows arise. In this last regime, we observe the emergence of cell shape patterns called topological defects, with flow and stress fields around defects agreeing with those observed in epithelial tissue experiments. We further provide a testable prediction of cell-cell rearrangement hotspots near topological defects. Overall, our work connects seemingly distinct features - e.g. rosettes and topological defects - observed across various types of epithelial tissues.