Mouse megakaryocytes cocultured with bone marrow endothelial cells produce microparticles. (A) Thin-section electron micrograph showing small projections and vesicles emanating from the surface of a megakaryocyte that has been cocultured with bone marrow–derived endothelial cells. Scale bar represents 2 μm. (B) Mouse megakaryocytes were incubated for 24 hours in the presence or absence of bone marrow endothelial cells. CD41⁺ microparticles within the supernatants were quantified by flow cytometry. Error bars indicate the standard deviation of 3 independent assays.