  Collective cell migration in epithelia relies on cell intercalation: i.e. a
local remodelling of the cellular network that allows neighbouring cells to
swap their positions. While in common with foams and other passive cellular
fluids, intercalation in epithelia crucially depends on active processes, where
the local geometry of the network and the contractile forces generated therein
conspire to produce an ``avalanche'' of remodelling events, which collectively
give rise to a vortical flow at the mesoscopic length scale. In this article we
formulate a continuum theory of the mechanism driving this process, built upon
recent advances towards understanding the hexatic (i.e. $6-$fold ordered)
structure of epithelial layers. Using a combination of active hydrodynamics and
cell-resolved numerical simulations, we demonstrate that cell intercalation
takes place via the unbinding of topological defects, naturally initiated by
fluctuations and whose late-times dynamics is governed by the interplay between
passive attractive forces and active self-propulsion. Our approach sheds light
on the structure of the cellular forces driving collective migration in
epithelia and provides an explanation of the observed extensile activity of in
vitro epithelial layers.
