  In vitro epithelial monolayers exhibit a rich repertoire of collective
dynamical behaviours according to microenvironment mechanical features. We
investigate the statistical and dynamical properties of human bronchial
epithelial cell (HBEC) flows in bulk, by analysing their large length-scale and
long time-scale spatiotemporal dynamics. Activity-driven spontaneous collective
flows consist of an ensemble of vortices randomly positioned in space. By
analysing a large population of vortices, we show that their area follows an
exponential law with a constant mean value and their rotational frequency is
size-independent, both being characteristic features of the chaotic dynamics of
active nematic suspensions. Indeed, we find that HBECs self-organise in nematic
domains of several cell lengths. Nematic defects are found at the interface
between domains with a total number that remains constant by the dynamical
balance of nucleation and annihilation events. Near +/-1/2 defects, the mean
velocity field is in faithful agreement with the dynamics of extensile active
nematics.
