  There is now growing evidence of the emergence and biological functionality
of liquid crystal features, including nematic order and topological defects, in
cellular tissues. However, how such features that intrinsically rely on
particle elongation, emerge in monolayers of cells with isotropic shapes is an
outstanding question. In this article we present a minimal model of cellular
monolayers based on cell deformation and force transmission at the cell-cell
interface that explains the formation of topological defects and captures the
flow-field and stress patterns around them. By including mechanical properties
at the individual cell level, we further show that the instability that drives
the formation of topological defects and leads to active turbulence, emerges
from a feedback between shape deformation and active driving. The model allows
us to suggest new explanations for experimental observations in tissue
mechanics, and to propose designs for future experiments.
