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In tissue development, wound healing, and cancer invasion, coordinated cell motion arises from active forces produced by the cells. The relationship between force and motion remains unclear, however, because the forces result from a sum of contributions from activity and the constitutive response of the cell collective. Here, we develop a method to decouple the forces due to activity from those due to constitutive response. As a model of an epithelial tissue, we use a monolayer of epithelial cells in the fluid state, for which the constitutive behavior is that of a viscous fluid. By careful study of the distribution of the ratio between shear stress and strain rate, we show that the order of magnitude of viscosity within the epithelial tissue is 100 Pa h and that increasing (decreasing) the actomyosin cytoskeleton and cell-cell adhesions increase (decrease) the magnitude of tissue viscosity. These results establish tissue viscosity as a meaningful way to describe the mechanical behavior of epithelial tissues, and demonstrate a direct relationship between tissue microstructure and material properties. By providing the first experimental measurement of tissue viscosity, our study is a step toward separating the active and constitutive components of stress, in turn clarifying the relationship between force and motion and providing a new means of identifying how active cell forces evolve in space and time.
McCord et al. (Fri,) studied this question.