Platelets contain both pro-and antiangiogenic factors, but their regulatory role in angiogenesis is poorly understood. Although previous studies showed that platelets stimulate angiogenesis in vitro, the role of platelets in angiogenesis in vivo is largely uncharacterized. To address this topic, we used two in vivo approaches, the cornea micropocket assay and the Matrigel model, in four animal models: thrombocytopenic, Lyst bg (platelet storage pool deficiency), glycoprotein (GP) Ib␣͞IL4R transgenic (lacking extracellular GPIb␣, the receptor for von Willebrand factor as well as other adhesive and procoagulant proteins), and Fc␥R ؊/؊ (lacking functional GPVI, the collagen receptor) mice. Adult mice were rendered thrombocytopenic by i.p. administration of an antiplatelet antibody. The number of growing vessels in the thrombocytopenic mice was lower in the cornea assay, and they showed significantly increased appearance of hemorrhage compared with mice treated with control IgG. The thrombocytopenic mice also showed more protein leakage and developed hematomas in the Matrigel model. GPIb␣͞IL4R transgenic mice presented increased hemorrhage in both assays, but it was less severe than in the platelet-depleted mice. Fc␥R ؊/؊ and Lyst bg mice showed no defect in experimental angiogenesis. Intravital microscopy revealed a >3-fold increase in platelet adhesion to angiogenic vessels of Matrigel compared with mature quiescent skin vessels. Our results suggest that the presence of platelets not only stimulates angiogenic vessel growth but also plays a critical role in preventing hemorrhage from the angiogenic vessels. The adhesion function of platelets, as mediated by GPIb␣, significantly contributes to the process. thrombocytopenic mice ͉ blood vessel ͉ ␣-granule ͉ cornea ͉ collagen receptor

The presence of activated platelets and platelet-leukocyte aggregates in the circulation accompanies major surgical procedures and occurs in several chronic diseases. Recent findings that activated platelets contribute to the inflammatory disease atherosclerosis made us address the question whether activated platelets stimulate normal healthy endothelium. Infusion of activated platelets into young mice led to the formation of transient platelet-leukocyte aggregates and resulted in a several-fold systemic increase in leukocyte rolling 2 to 4 hours after infusion. Rolling returned to baseline levels 7 hours after infusion. Infusion of activated P-selectin ؊/؊ platelets did not induce leukocyte rolling, indicating that platelet P-selectin was involved in the endothelial activation. The endothelial activation did not require platelet CD40L. Leukocyte rolling was mediated solely by the interaction of endothelial P-selectin and leukocyte P-selectin glycoprotein ligand 1 (PSGL-1 IntroductionPlatelets are the body's defense mechanism against excessive bleeding caused by endothelial damage. Activated platelets present at the site of injury provide both a prothrombotic surface and a procoagulant surface. Excessive platelet activation occurs in coronary bypass surgery and may result in thrombotic emboli and neurologic complications. 1 Furthermore, many inflammatory diseases including sepsis, 2,3 psoriasis, 4,5 diabetes, 6-8 and cystic fibrosis 9 are associated with circulating activated platelets. These pathologies are also associated with endothelial inflammation.Platelets induce leukocyte adhesion on preactivated endothelial cells in culture. 10,11 In mouse models of atherosclerosis, the role of activated platelets in exacerbating lesion development has been clearly demonstrated. Activated platelets promote monocyte arrest on atherosclerotic lesions and atherosclerotic lesion growth. 12 Studies in our laboratory have also demonstrated the role of platelet P-selectin, in addition to endothelial P-selectin, in atherosclerotic lesion development and maturation. 13 It is important to note that P-selectin on both platelets and endothelium is expressed on the cell surface only on activation of the cells and granule secretion. 14 Early endothelial inflammation is associated with the rapid release of Weibel-Palade bodies and the consequent surface expression of Pselectin and von Willebrand factor (VWF). These molecules mediate rolling of leukocytes and platelets on endothelial cells. 15 This is followed by transcription and expression of molecules such as E-selectin, vascular-cell adhesion molecule 1 (VCAM-1), and other adhesion receptors. 16 These receptors, in turn, mediate slow rolling and adhesion of leukocytes and have been shown to be up-regulated on treatment with activated platelets in the in vitro studies. [17][18][19][20] This sequence of events leads to leukocyte rolling and extravasation at the site of injury or pathologic conditions.Recently, vascular endothelial growth factor (VEGF) secreted from ␣-granules was sh...