Microparticle generation from human megakaryocytes. (A) Thin-section electron micrograph showing the surface of an entire human megakaryocyte. Numerous small projections were observed emanating from the surface. Scale bar represents 10 μm. (B) Thin-section electron micrograph showing the megakaryocyte surface at higher magnification. Blebs at various stages are visualized along the surface of this human megakaryocyte. The surface projections are approximately 300 nm in diameter. Scale bar represents 1 μm. (C,D) Microparticles isolated from human megakaryocyte cultures were stained using anti-CD41 antibody and either annexin V or IgG directed at CD42b. This analysis demonstrates that the majority of human megakaryocyte-derived microparticles express PS on their surface and are CD42b⁺. Error bars indicate the standard deviation of 4 independent assays.