  Epithelial cells cultured in a monolayer are very motile in isolation but
reach a near-jammed state when mitotic division increases their number above a
critical threshold. We have recently shown that a monolayer can be reawakened
by over-expression of a single protein, RAB5A, a master regulator of
endocytosis. This reawakening of motility was explained in term of a flocking
transition that promotes the emergence of a large-scale collective migratory
pattern. Here we focus on the impact of this reawakening on the structural
properties of the monolayer. We find that the unjammed monolayer is
characterised by a fluidisation at the single cell level and by enhanced
non-equilibrium large-scale number fluctuations at a larger length scale. Also
with the help of numerical simulations, we trace back the origin of these
fluctuations to the self-propelled active nature of the constituents and to the
existence of a local alignment mechanism, leading to the spontaneous breaking
of the orientational symmetry.
