Mouse megakaryocytes produce microparticles that express phosphatidylserine and multiple platelet markers on their surface. (A) Mouse megakaryocytes were cultured for 3 days and analyzed by flow cytometry. The middle gate (green) indicates particles with FSC and SSC characteristics of mouse platelets. The gate to the left (pink) indicates microparticles. The gate on the right (aqua) indicates megakaryocytes. (B) Microparticles isolated from megakaryocyte cultures were stained using anti-CD41 antibody and annexin V. (C) Microparticles isolated from megakaryocyte cultures were stained with either isotype-matched nonimmune IgG (control) or IgG directed at CD42b, GPVI, or α₂ integrin (immune). Error bars represent the standard deviation of 3 independent assays. (D) Megakaryocyte suspensions were analyzed for megakaryocytes (○) and microparticles (•) on days 3, 4, and 5 of culture. Values relative to the number of megakaryocytes and microparticles, respectively, on day 3 are indicated. Error bars indicate the standard deviation of 4 independent assays.