Fig. 4. Contractility and stress fiber/focal adhesion formation are needed for micromechanical response of cells to shear flow. (A) Immunofluorescence of actin (green) and vinculin (red) of unsheared (top) and sheared (bottom) Swiss 3T3 fibroblasts. Control fibroblasts exhibit extended stress fibers and large focal adhesions (Inset, bottom). The formation of these cytoskeleton structures was abrogated after cell treatment with inhibitors BDM (middle) or Y-27632 (right). Bar, 20 µm. (B) Comparison of plateau elastic moduli. Only untreated cells exhibited a significant increase in elasticity when subjected to fluid shear stress. This stiffness increase is reduced with BDM treatment, whereas a slight decrease in elasticity is observed with Y-27632 treatment. (C) Relaxation times of the cytoskeleton for each condition, which were normalized by each respective initial (unsheared) value. (D) Intracellular shear viscosity values for each condition, which were normalized by the initial value of the shear viscosity of unsheared cells. For (B) and (D), one-way ANOVA tests yielded P<0.0001; stars denote P values from two-tailed t-tests within conditions (see Materials and Methods). All values represent mean ± s.e.m.