Megakaryocyte-derived microparticles resemble plasma-derived microparticles with regard to surface markers and expression of full-length filamin A. (A) Microparticles were isolated from mouse platelets incubated with thrombin and collagen, cultured mouse megakaryocytes, or platelet-free plasma. Microparticles were subsequently labeled with nonimmune IgG, anti-CD62P antibody, or anti–LAMP-1 antibody as indicated. Error bars indicate the standard deviation of 4 independent assays. (B) Filamin A was assayed by immunoblot analysis in resting mouse platelets, mouse platelets exposed to thrombin plus collagen, and cultured megakaryocytes. Filamin A was also assayed by immunoblot analysis in microparticles isolated from activated platelets, cultured mouse megakaryocytes, or platelet-free mouse plasma. Full-length filamin A is found in megakaryocyte-derived microparticles and circulating microparticles, but not microparticles from activated platelets.