Fluctuations can induce local nematic order and extensile stress in monolayers of motile cells

TL;DR

This paper demonstrates through a mesoscopic model that fluctuations in motile cell monolayers can spontaneously generate local nematic order and extensile stresses, influenced by cell crawling behavior and activity levels.

Contribution

It introduces a model linking fluctuating traction forces and nematic order to tissue stress states, highlighting activity-driven stress types in cell monolayers.

Findings

Local nematic order can spontaneously emerge in tissue dynamics.

Tissue stress can be extensile or contractile depending on activity balance.

Cell crawling along nematic axes induces anisotropic fluctuations.

Abstract

Recent experiments in various cell types have shown that two-dimensional tissues often display local nematic order, with evidence of extensile stresses manifest in the dynamics of topological defects. Using a mesoscopic model where tissue flow is generated by fluctuating traction forces coupled to the nematic order parameter, we show that the resulting tissue dynamics can spontaneously produce local nematic order and an extensile internal stress. A key element of the model is the assumption that in the presence of local nematic alignment, cells preferentially crawl along the nematic axis, resulting in anisotropy of fluctuations. Our work shows that activity can drive either extensile or contractile stresses in tissue, depending on the relative strength of the contractility of the cortical cytoskeleton and tractions by cells on the extracellular matrix.