Why Extensile and Contractile Tissues Could be Hard to Tell Apart

TL;DR

This paper investigates the challenge of distinguishing between extensile and contractile active stresses in epithelial tissues, emphasizing the need for simultaneous stress and cell shape measurements for accurate interpretation.

Contribution

The study introduces a continuum model with two-tensor order parameters to clarify the relationship between cell shape and active stress, highlighting measurement limitations.

Findings

Correlating cell shape and flow alone cannot determine active stress type.

Simultaneous stress and cell shape measurements are necessary for accurate analysis.

Tissue-level behavior can resemble either extensile or contractile nematics despite cellular contractility.

Abstract

Active nematic models explain the topological defects and flow patterns observed in epithelial tissues, but the nature of active stress-whether it is extensile or contractile, a key parameter of the theory-is not well established experimentally. Individual cells are contractile, yet tissue-level behavior often resembles extensile nematics. To address this discrepancy, we use a continuum theory with two-tensor order parameters that distinguishes cell shape from active stress. We show that correlating cell shape and flow, whether in coherent flows in channels, near topological defects, or at rigid boundaries, cannot unambiguously determine the type of active stress. Our results demonstrate that simultaneous measurements of stress and cell shape are essential to fully interpret experiments investigating the nature of the physical forces acting within epithelial cell layers.