Polar Fluctuations Lead to Extensile Nematic Behaviour in Confluent Tissues*

Collective active nematic behaviour has been found to mediate a growing number of important biological processes, such as cell extrusion in epithelial monolayers or the formation of layers in Myxococcus xanthus colonies. Intriguingly, isolated epithelial cells display polar motility and generate contractile nematic stresses when elongated, but exhibit extensile nematic behaviour when part of a tissue. How these cells can exhibit active nematic behaviour at the tissue level is poorly understood and deciphering the mechanisms behind this behaviour is necessary for elucidating fundamental biological processes. Here, we resolve this cellular to tissue level disconnect with a linearized hydrodynamic theory that applies universally in the small fluctuation regime to tissues in both fluid and solid states. We show that polar fluctuations generically generate extensile stresses in confluent tissues, and so can drive extensile collective behaviour in cells that are contractile in isolation. We then validate our theory by demonstrating the appearance of extensile nematic defects in both fluid and solid cell-resolution models with polar active forces. Our results also demonstrate that materials with no inherently nematic active forces can exhibit active nematic collective behaviour, and that the active nematic signatures observed in epithelial tissues can naturally emerge downstream of fundamentally polar processes.