Are Confluent Cell Layers Extensile or Contractile?

A lot is understood about the ways in which single cells move over a surface, but the motion of confluent layers of epithelial cells, which are coupled through strong intercellular junctions, remains puzzling.

The cells in epithelial layers can be jammed in a glass-like state, they can flock, or they can show active turbulent-like motility with chaotic flows and motile topological defects. Active turbulence characterises active nematics, and we have been trying to understand why cells that are, on average, isotropic can show nematic properties. Moreover, it is surprising that single cells are contractile, whereas the direction of motion of topological defects in many confluent cell layers suggests that the layers are behaving as an extensile material.