  Collective cell migration is a highly regulated process involved in wound
healing, cancer metastasis and morphogenesis. Mechanical interactions among
cells provide an important regulatory mechanism to coordinate such collective
motion. Using a Self-Propelled Voronoi (SPV) model that links cell mechanics to
cell shape and cell motility, we formulate a generalized mechanical inference
method to obtain the spatio-temporal distribution of cellular stresses from
measured traction forces in motile tissues and show that such traction-based
stresses match those calculated from instantaneous cell shapes. We additionally
use stress information to characterize the rheological properties of the
tissue. We identify a motility-induced swim stress that adds to the interaction
stress to determine the global contractility or extensibility of epithelia. We
further show that the temporal correlation of the interaction shear stress
determines an effective viscosity of the tissue that diverges at the
liquid-solid transition, suggesting the possibility of extracting rheological
information directly from traction data.
