Energy injection in an epithelial cell monolayer indicated by negative viscosity

TL;DR

This study introduces a method to measure effective viscosity in epithelial monolayers, revealing regions with negative viscosity that indicate energy injection and increased cellular activity, advancing understanding of tissue dynamics.

Contribution

Developed a novel approach to quantify effective viscosity in epithelial tissues, uncovering negative viscosity regions linked to energy injection and cellular activity.

Findings

Negative effective viscosity observed in certain tissue regions.

Regions with negative viscosity showed higher cell speed and vorticity.

Negative viscosity correlates with increased metabolic activity.

Abstract

Epithelial tissues are driven out of thermodynamic equilibrium by internally generated forces, causing complex patterns of motion. Even when both the forces and motion are measurable, it is not yet possible to relate the two, because the sources of energy injection and dissipation are often unclear. Here, we study how energy is transferred by developing a method to measure the effective viscosity from the shear stresses and strain rates within an epithelial cell monolayer. Interestingly, there emerged multicellular regions in which the relationship between shear stress and shear strain rate was negatively proportional, indicating a negative effective viscosity. The negative effective viscosity occurred in regions wherein cell stresses were less efficient at producing tissue deformations compared to regions of positive effective viscosity. Regions of negative effective viscosity consistently exhibited greater cell speed and vorticity, and the cells had elevated metabolic activity, reflecting an increased energy demand in these cells. Our study shows that negative effective viscosity is a useful means of quantifying the flow of energy in living matter.