  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 various signalling pathways, it
has recently become evident that this behavior can additionally hinge on a
minimal toolkit of physical mechanisms, of which liquid crystal order is the
most prominent example. 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 serve as a linchpin for collective
migration in confluent cell layers. 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 are, 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 a length scale of the order of ten cell sizes. Our
work sheds light on the emergent organization of living matter, provides a new
framework for deciphering the structure of epithelia, and paves the way toward
a comprehensive and predictive mesoscopic theory of tissues.
