SEM analysis of Cdc42^−/− platelets. (A-C) Normal ultrastructure of Cdc42^−/− platelets. (A) Resting platelets immobilized on poly-L-lysine. Scale bar equals 5 μm. (B) Spread Cdc42^−/− and wild-type platelets upon activation with 0.01 U/mL thrombin on human fibrinogen (100 μg/mL). Top panels show SEM of intact platelets. Bottom panels show visualization of the actin cytoskeleton upon denudation of the plasma membrane. Scale bar equals 2 μm. (C) Morphology of Cdc42^−/− and wild-type platelets in suspension at 15 seconds after activation with thrombin (0.1 U/mL; top panels) or ADP (5μM; bottom panels). Scale bar equals 1 μm. (D) Cdc42^−/− platelets exhibit reduced filopodia extension upon adhesion on mouse VWF (20 minutes). Left panels show representative images of 5 experiments. Scale bar equals 2.5 μm. Right panel shows statistical evaluation of filopodia formation according to the number of extensions per platelet (0, 1-3, > 3) in 5 different fields corresponding to a total surface of 9215 μm². The results are mean values ± SD (n = 5 per group). (E) Unaltered agglutination of Cdc42^−/− platelets. Washed Cdc42^−/− and wild-type platelets were stimulated with botrocetin (5 μg/mL) and human VWF (10 μg/mL) in presence of 40 μg/mL integrilin under stirring conditions and agglutination was monitored over 15 minutes on a Fibrintimer 4 channel aggregometer (APACT Laborgeräte und Analysensysteme). Curves are representative of 3 individual experiments.