  Cell shape changes are vital for many physiological processes such as cell
proliferation, cell migration and morphogenesis. They emerge from an
orchestrated interplay of active cellular force generation and passive cellular
force response - both crucially influenced by the actin cytoskeleton. To model
cellular force response and deformation, cell mechanical models commonly
describe the actin cytoskeleton as a contractile isotropic incompressible
material. However, in particular at slow frequencies, there is no compelling
reason to assume incompressibility as the water content of the cytoskeleton may
change. Here we challenge the assumption of incompressibility by comparing
computer simulations of an isotropic actin cortex with tunable Poisson ratio to
measured cellular force response. Comparing simulation results and experimental
data, we determine the Poisson ratio of the cortex in a frequency-dependent
manner. We find that the Poisson ratio of the cortex decreases with frequency
likely due to actin cortex turnover leading to an over-proportional decrease of
shear stiffness at larger time scales. We thus report a trend of the Poisson
ratio similar to that of glassy materials, where the frequency-dependence of
jamming leads to an analogous effect.
