Platelet apoptosis is characterized by ultrastructural changes distinct from platelet activation. (A) Washed human platelets incubated in HBS retained their normal discoid shape and ultrastructure, whereas treatment with ABT-737 (100nM) for 2 hours resulted in their distension with displacement of granules toward the periphery of the rounded-up cells (black arrowheads). Many of these cells also displayed focal aggregations of cytoskeletal material (black arrow). These changes were all prevented by the incorporation of z-VAD.fmk (50μM) into the medium. Treatment with CRP-XL (2 μg/mL) for 15 minutes resulted in a completely different morphology with the proliferation of numerous filopodia (white arrows) and microparticles. Bars represent 2 μm. (B) Platelets were incubated in HBS with or without ABT-737 (100nM) for 2 hours before analysis by immunohistochemistry and confocal microscopy. Actin was stained using phalloidin/Alexa-488 (green), and tubulin was stained using anti–tubulin/phycoerythrin (red). Bar represents 5 μm.