  It is increasingly being realized that liquid-crystalline features can play
an important role in the properties and dynamics of cell monolayers. Here, we
present a cell-based model of cell layers, based on the phase-field
formulation, that connects mechanical properties at the single cell level to
large-scale nematic and hydrodynamic properties of the tissue. In particular,
we present a minimal formulation that reproduces the well-known bend and splay
hydrodynamic instabilities of the continuum nemato-hydrodynamic formulation of
active matter, together with an analytical description of the instability
threshold in terms of activity and elasticity of the cells. Furthermore, we
provide a quantitative characterisation and comparison of flows and topological
defects for extensile and contractile stress generation mechanisms, and
demonstrate activity-induced heterogeneity and spontaneous formation of gaps
within a confluent monolayer. Together, these results contribute to bridging
the gap between micro-scale cell dynamics and tissue-scale collective cellular
organisation.
