  Monolayers of anisotropic cells exhibit long-ranged orientational order and
topological defects. During the development of organisms, orientational order
often influences morphogenetic events. However, the linkage between the
mechanics of cell monolayers and topological defects remains largely
unexplored. This holds specifically at the time scales relevant for tissue
morphogenesis. Here, we build on the physics of liquid crystals to determine
material parameters of cell monolayers. In particular, we use a hydrodynamical
description of an active polar fluid to study the steady-state mechanical
patterns at integer topological defects. Our description includes three
distinct sources of activity: traction forces accounting for cell-substrate
interactions as well as anisotropic and isotropic active nematic stresses
accounting for cell-cell interactions. We apply our approach to C2C12 cell
monolayers in small circular confinements, which form isolated aster or spiral
topological defects. By analyzing the velocity and orientational order fields
in spirals as well as the forces and cell number density fields in asters, we
determine mechanical parameters of C2C12 cell monolayers. Our work shows how
topological defects can be used to fully characterize the mechanical properties
of biological active matter.
