Defective spreading of Rac1/Cdc42^−/− platelets on fibrinogen upon activation. (A) Washed platelets from wt and Rac1/Cdc42^−/−Mx mice were allowed to adhere on immobilized human fibrinogen (100 µg/mL) upon activation with thrombin (0.01 U/mL, left) or CRP (10 µg/mL, right). (Upper) DIC images were taken at the indicated times (5, 15, and 30 minutes), representative of 4 individual experiments. Bar, 5 µm. (Lower) Statistical analysis of the percentage of spread Rac1/Cdc42^−/−Mx and wt platelets. (Bottom) (1) Roundish, no filopodia, no lamellipodia. (2) Only filopodia. (3) Partial spreading. (4) Full spreading. Arrows indicate altered morphology in Rac1/Cdc42^−/−Mx platelets. (B) Visualization of defective spreading and actin reorganization of Rac1^−/− (middle) and Rac1/Cdc42^−/−Mx (right) platelets by scanning electron microscopy. (Upper) Scanning electron microscopy image of intact platelets. (Lower) Visualization of the cytoskeleton after denudation of the plasma membrane. Bar, 2 µm. (C) Determination of relative F-actin contents in resting, thrombin-activated (1 and 0.01 U/mL), and CRP-activated (10 µg/mL) wt (black) and Rac1/Cdc42^−/−Mx (patterned) platelets (n = 4 per group; ***P < .001).