  We formulate a hydrodynamic theory of confluent epithelia: i.e. monolayers of
epithelial cells adhering to each other without gaps. Taking advantage of
recent progresses toward establishing a general hydrodynamic theory of p-atic
liquid crystals, we demonstrate that collectively migrating epithelia feature
both nematic (i.e. p=2) and hexatic (i.e. p=6) order, with the former being
dominant at large and the latter at small length scales. Such a remarkable
multiscale liquid crystal order leaves a distinct signature in the system's
structure factor, which exhibits two different power law scaling regimes,
reflecting both the hexagonal geometry of small cells clusters, as well as the
uniaxial structure of the global cellular flow. We support these analytical
predictions with two different cell-resolved models of epithelia -- i.e. the
self-propelled Voronoi model and the multiphase field model -- and highlight
how momentum dissipation and noise influence the range of fluctuations at small
length scales, thereby affecting the degree of cooperativity between cells. Our
construction provides a theoretical framework to conceptualize the recent
observation of multiscale order in layers of Madin-Darby canine kidney cells
and pave the way for further theoretical developments.
