Experimental identification of force, velocity, and nematic order relationships in active nematic cell monolayers

TL;DR

This study quantitatively links traction force, nematic order, and cell flow in active nematic cell monolayers, revealing force alignment and flow dynamics influenced by nonlinear forces and diffusion.

Contribution

It provides a quantitative analysis of how traction force aligns with nematic order and influences flow, incorporating nonlinear forces and diffusion for the first time.

Findings

Traction force aligns with the gradient of the nematic order tensor.

Flow is described by nonlinear forces and a diffusion term.

Nonlinear forces affect cell density stability and pattern stability.

Abstract

Cell alignment often forms nematic order, which can lead to anomalous collective cell flow due to the so-called active force. Although it is appreciated that cell migration is driven by traction force, a quantitative evaluation of the relationships between the traction force, the nematic patterning, and the cell flow velocity is still elusive. Here we have found that cellular traction force aligns almost perfectly and is proportional in amplitude to the gradient of the nematic order tensor, not only near the topological defects but also globally. Furthermore, the flow in the monolayer was best described by adding nonlinear forces and a diffusion term derived from symmetry considerations. These nonlinear active forces enhance density instability but suppress bending instability, explaining why cell accumulation and dispersion can occur in neural progenitor cell culture while their ordering pattern is stable.