Localization of proteins in resting, human platelets using immunoelectron microscopy of ultrathin cryosections. Single immunogold labeling on ultrathin platelet sections was performed with anti-VEGF (A) and antiendostatin (B) antibodies. Double immunogold labeling on platelet sections was performed with the use of anti-VEGF antibody and antiendostatin antibodies. Large gold particles representing anti-VEGF staining (15 nm, arrows) are evident on one population of α-granules and small gold particles (5 nm) representing endostatin staining are abundantly present on a different population of α-granules (arrowheads) (C). Single immunogold labeling on ultrathin platelet sections was performed with antifibrinogen (D) and anti-VWF (E) antibodies. Double immunogold labeling on platelet sections was performed with the use of antifibrinogen antibody, which was revealed with a 15-nm, gold conjugate (arrows) and then with an antibody to VWF, which was revealed with a 5-nm, gold conjugate (arrowheads) (F). Single immunogold labeling on ultrathin platelet sections was performed with anti–P-selectin antibody (G). Gold particles representing P-selectin staining are abundantly present on the α-granules as well as the cell-surface membrane. Bar represents 300 nm. (H) The bar graph shows the quantitation of the percentage of α-granules positive (via immunogold staining) for specific factors. The data represent 3 separate experiments; error bars represent SD. More than 100 granules were scored for each study.