Do cells sense stress or strain? Measurement of cellular orientation can provide a clue.

We predict theoretically the steady-state orientation of cells subject to dynamical stresses that vary more quickly than the cell relaxation time. We show that the orientation is a strong function of the Poisson's ratio, nu, of the matrix when cell activity is governed by the matrix strain; if cell activity is governed by the matrix stress, the orientation depends only weakly on nu. These results can be used to differentiate systems in which the strain or the stress determine the setpoint for the mechanosensitivity of cells.