RhoA^−/− mice display a distinct macrothrombocytopenia but only moderately reduced platelet life span. (A) Analysis of RhoA expression in wild-type (wt) and RhoA^−/− platelets by Western blot. Expression of GPIIIa was used as loading control. Peripheral platelet counts (B) and platelet volume (C) of wt and RhoA^−/− mice measured with a blood cell counter are depicted. Data are presented as mean ± SD of 8 mice per group and are representative of 3 individual measurements. Fl indicates femtoliter. (D) Representative transmission electron microscopy pictures of resting wt and RhoA^−/− platelets. Scale bar represents 2 μm. (E) Determination of the platelet life span in wt and RhoA^−/− mice. Mice were injected with a DyLight 488–conjugated anti-GPIX Ig derivate (0.5 μg/g body weight) to label platelets in vivo. Results are percentage of fluorescently labeled platelets at the indicated days after injection as determined by flow cytometry. Values are mean ± SD of 5 mice per group (**P < .01; ***P < .001).