Epithelia are multiscale active liquid crystals

Abstract

Biological processes such as embryogenesis, wound healing and cancer progression, crucially rely on the ability of epithelial cells to coordinate their mechanical activity over length scales order of magnitudes larger than the typical cellular size. While regulated by signalling pathways, such as YAP (yes-associated protein), MAPK (mitogen-activated protein kinase) and Wnt, this behavior is believed to additionally hinge on a minimal toolkit of physical mechanisms, of which liquid crystal order is the most promising candidat. Yet, experimental and theoretical studies have given so far inconsistent results in this respect: whereas nematic order is often invoked in the interpretation of experimental data, computational models have instead suggested that hexatic order could in fact emerge in the biologically relevant region of parameter space. In this article we resolve this dilemma. Using a combination of in vitro experiments on Madin-Darby canine kidney cells (MDCK), numerical simulations and analytical work, we demonstrate that both nematic and hexatic order is in fact present in epithelial layers, with the former being dominant at large length scales and the latter at small length scales. In MDCK GII cells on uncoated glass, these different types of liquid crystal order crossover at 34 {micro}m, corresponding approximatively to clusters of 21 cells. Our work sheds light on the emergent organization of living matter, provides a new set of tools for analyzing the structure of epithelia and paves the way toward a comprehensive and predictive mesoscopic theory of tissues.