RhoA^−/− platelets spread normally on fibrinogen but fail to mediate clot retraction. (A-B) Washed platelets of wt and RhoA^−/− mice were allowed to spread on fibrinogen (200 μg/mL) after stimulation with 0.01 U/mL thrombin. (A) Representative differential interference contrast images of 3 individual experiments from the indicated time points (top) and statistical evaluation of the percentage of spread platelets at different spreading stages (bottom). 1, roundish; 2, only filopodia; 3, filopodia and lamellipodia; and 4, full spreading. Scale bar represents 5 μm. (B) Analysis of filamentous actin (red) and tubulin (green) structure in spread (30 minutes) RhoA^−/− and wt platelets by confocal microscopy. Scale bar represents 5 μm. (C) Clot retraction of prp on activation with 5 U/mL thrombin in the presence of 20mM CaCl₂ at the indicated times. Representative images of 2 different experiments are depicted.