Epithelia are multiscale active liquid crystals

Biological processes such as embryogenesis, wound healing and cancer progression rely on the ability of epithelial cells to coordinate their mechanical activity over length scales that are orders of magnitude larger than the typical cellular size(1). Although this process is regulated by various signalling pathways(2), it has recently become evident that this coordination can be understood using physics tools, of which liquid crystal order is a prominent example(3,4). However, prior studies have given inconsistent results in this respect. Whereas nematic order is often invoked in the interpretation of experimental data(3-5), computational models have instead suggested that hexatic order could serve as a linchpin for collective migration in confluent cell layers(6-8). In this Letter, we resolve this dilemma. Using a combination of in vitro experiments, numerical simulations and analytical work, we demonstrate that both nematic and hexatic order are present in epithelial layers, with the former being dominant at large length scales and the latter at small length scales. In cells on uncoated glass, these different types of liquid crystal order cross over at a length scale of the order of ten cell sizes. Our work provides a framework for deciphering epithelial structure and may lead to a predictive mesoscopic theory of tissues(9).