  Spatiotemporal patterns in multicellular systems are important to
understanding tissue dynamics, for instance, during embryonic development and
disease. Here, we use a multiphase field model to study numerically the
behavior of a near-confluent monolayer of deformable cells with intercellular
friction. Varying friction and cell motility drives a solid-liquid transition,
and near the transition boundary, we find the emergence of nematic order of
cell deformation driven by shear-aligning cellular flows. Intercellular
friction endows the monolayer with a finite viscosity, which significantly
increases the spatial correlation in the flow and, concomitantly, the extent of
nematic order. We also show that hexatic and nematic order are tightly coupled
and propose a mechanical-geometric model for the colocalization of +1/2 nematic
defects and 5-7 disclination pairs, which are the structural defects in the
hexatic phase. Such topological defects coincide with regions of high cell-cell
overlap, suggesting that they may mediate cellular extrusion from the
monolayer, as found experimentally. Our results delineate a mechanical basis
for the recent observation of nematic and hexatic order in multicellular
collectives in experiments and simulations and pinpoint a generic pathway to
couple topological and physical effects in these systems.
