Morphologic changes associated with platelet apoptosis. Washed human platelets (3.0 × 10⁸/mL) were resuspended in Tyrode buffer in the presence (A-B) or absence (C) of bovine serum albumin (BSA; 5 mg/mL), then incubated with vehicle (dimethyl sulfoxide [DMSO]) or ABT-737 (1μM) for the indicated times. In some experiments, platelets were preincubated with Q-VD-Oph (50μM) or Y27632 (30μM) before treatment with ABT-737. At the indicated time point, platelets were either (A-B) fixed in suspension with paraformaldehyde (2% final) and processed for scanning electron microscopy, or (C) lysed for Western blot analysis using an anti-ROCK1 polyclonal antibody, as described in supplemental Methods. Images are representative of 3 independent experiments. When the experiments described in panel A were performed in the absence of BSA, the ABT-737–induced morphologic change depicted here at 45 minutes was observed after 15-minute treatment, a difference attributed to binding of ABT-737 to BSA.