Modulation of Rho activity pulse frequency by matrix elasticity. (a) Schematic of the proposed cross talk between cellular contractility signaling and matrix elasticity. Untreated, resting (b–d) or nocodazole-treated (e–g) U2OS cells were grown on matrices of different elasticities and visualized via TIRFM (b–d) or spinning disk microscopy (e–g; see also Video 8). (b and e) Individual images of Myosin IIa localization; kymographs corresponding to white arrows. Arrows indicate activity pulse peaks at the cell border (magenta) or near the cell center (yellow). (c, d, f, and g) Local standard deviation and frequency of myosin activity pulses (c and d; n ≥ 53 cells from three experiments) in unstimulated cells or after nocodazole-induced Rho activation (f and g; n ≥ 23 cells from five experiments). Frame rate is 20/min. Bars, 10 µm. **, P < 0.01; ***, P < 0.001 (c, d, f, and g, unpaired t test).