Hydrodynamics and multiscale order in confluent epithelia

We formulate a hydrodynamic theory of confluent epithelia: i.e. monolayers ofepithelial cells adhering to each other without gaps. Taking advantage ofrecent progresses toward establishing a general hydrodynamic theory of p-aticliquid crystals, we demonstrate that collectively migrating epithelia featureboth nematic (i.e. p=2) and hexatic (i.e. p=6) order, with the former beingdominant at large and the latter at small length scales. Such a remarkablemultiscale liquid crystal order leaves a distinct signature in the system'sstructure factor, which exhibits two different power law scaling regimes,reflecting both the hexagonal geometry of small cells clusters, as well as theuniaxial structure of the global cellular flow. We support these analyticalpredictions with two different cell-resolved models of epithelia -- i.e. theself-propelled Voronoi model and the multiphase field model -- and highlighthow momentum dissipation and noise influence the range of fluctuations at smalllength scales, thereby affecting the degree of cooperativity between cells. Ourconstruction provides a theoretical framework to conceptualize the recentobservation of multiscale order in layers of Madin-Darby canine kidney cellsand pave the way for further theoretical developments.