# Mechanical heterogeneity in tissues promotes rigidity and controls cellular invasion

## TL;DR

This study reveals how mechanical heterogeneity in tissues influences their rigidity and cellular invasion, showing that a small fraction of rigid cells can significantly enhance tissue stiffness and affect tumor invasion dynamics.

## Contribution

It introduces a vertex-based model to analyze cell stiffness heterogeneity and uncovers a scaling law for tissue rigidity controlled by the fraction of rigid cells.

## Key findings

- Tissue shear modulus scales with the fraction of rigid cells.
- Rigidity onset occurs at a low fraction of rigid cells (0.21).
- Heterogeneity can create an intermediate solid state that impedes invasion.

## Abstract

We study the influence of cell-level mechanical heterogeneity in epithelial tissues using a vertex-based model. Heterogeneity in single cell stiffness is introduced as a quenched random variable in the preferred shape index($p_0$) for each cell. We uncovered a crossover scaling for the tissue shear modulus, suggesting that tissue collective rigidity is controlled by a single parameter $f_r$, which accounts for the fraction of rigid cells. Interestingly, the rigidity onset occurs at $f_r=0.21$, far below the contact percolation threshold of rigid cells. Due to the separation of rigidity and contact percolations, heterogeneity can enhance tissue rigidity and gives rise to an intermediate solid state. The influence of heterogeneity on tumor invasion dynamics is also investigated. There is an overall impedance of invasion as the tissue becomes more rigid. Invasion can also occur in the intermediate heterogeneous solid state and is characterized by significant spatial-temporal intermittency.