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Fig. 2. Micromechanical response of serum-starved cells to shear flow. (A) Phase-contrast (top) and immunofluorescence (bottom) micrographs of Swiss 3T3 fibroblasts before (left) and after (right) shear flow. Actin (green) and vinculin (red) structures were overlaid with a micrograph of injected nanoparticles, which were colored according to the local value of the cytoplasmic deformability (evaluated at time scale of 0.1 seconds). The nanoparticles were enlarged to aid visualization. Blue denotes the least deformable (stiffest) regions of the cytoplasm; red denotes the most deformable (softest) regions. The inset is a magnified view of focal adhesions at the ends of actin stress fibers. Bar, 20 µm. (B) Shear compliances (i.e. deformability) of the cytoplasm of Swiss 3T3 fibroblasts before (red) and after 40 minutes of shear flow (blue) (n=6). Values represent mean ± s.e.m. (C) Frequency-dependent viscous and elastic moduli of cytoplasm, G'(
) (circles) and G''() (squares). (D) Frequency-dependent phase angle of cytoplasm, 
()=tan-1 (G''/G'), calculated from viscoelastic moduli shown in panel C. From a rheological standpoint, a phase angle of 90° describes the rheological behavior of a liquid; a phase angle of 0° describes the rheological behavior of a stiff solid.
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