Nematic vs. hexatic order in epithelial tissues: which is the right one?

Epithelial tissues are fundamental in a number of biological processes, such as morphogenesis and cancer development. A fundamental understanding of their dynamics, however, is limited by the current lack of knowledge of the symmetries underlying cells' collective motion. An important progress in this respect, was recently achieved by Saw et. al., who suggested that epithelial tissues could in fact behave as active nematic liquid crystals. In this work, we use a combination of in vitro experiments, numerical simulations and analytical work to identify the emergent order of epithelial tissues. Upon generalising the standard shape tensor to arbitrary ranks, we find that both nematic and hexatic order is in fact present in epithelial layers, with the former being relevant at the large scales and the latter at the short scales. This separation of length scales affects both the topological and dynamical properties of the system. Importantly, neglecting hexatic order leads to a misidentification of topological defects and the appearance of unphysical disclination lines. Finally, we discuss how such an emergent hexanematic order crucially affects the hydrodynamic feedback at different lengthscales.