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We hypothesized that microparticles (MPs) released after ischemia are endogenous signals leading to postischemic vasculogenesis.MPs from mice ischemic hind-limb muscle were detected by electron microscopy 48 hours after unilateral femoral artery ligation as vesicles of 0.1- to 1-μm diameter. After isolation by sequential centrifugation, flow cytometry analyses showed that the annexin V+ MP concentration was 3.5-fold higher in ischemic calves than control muscles (1392±406 versus 394±180 annexin V+ MPs per 1 mg; P<0.001) and came mainly from endothelial cells (71% of MPs are CD144+). MPs isolated from ischemic muscles induced more potent in vitro bone marrow–mononuclear cell (BM-MNC) differentiation into cells with endothelial phenotype than those isolated from control muscles. MPs isolated from atherosclerotic plaques were ineffective, whereas those isolated from apoptotic or interleukin-1β–activated endothelial cells also promoted BM-MNC differentiation. Interestingly, MPs from ischemic muscles produced more reactive oxygen species and expressed significantly higher levels of NADPH oxidase p47 (6-fold; P<0.05) and p67 subunits (16-fold; P<0.001) than controls, whereas gp91 subunit expression was unchanged. BM-MNC differentiation was reduced by 2-fold with MPs isolated from gp91-deficient animals compared with wild-type mice (P<0.05). MP effects on postischemic revascularization were then examined in an ischemic hind-limb model. MPs isolated from ischemic muscles were injected into ischemic legs in parallel with venous injection of BM-MNCs. MPs increased the proangiogenic effect of BM-MNC transplantation, and this effect was blunted by gp91 deficiency. In parallel, BM-MNC proangiogenic potential also was reduced in ABCA1 knockout mice with impaired vesiculation.MPs produced during tissue ischemia stimulate progenitor cell differentiation and subsequently promote postnatal neovascularization.