(a) Electron microscopic examination of the dense granule fraction whose electron density has been enhanced by White's cytochemical technique. This shows the high purity of the fraction that is almost exclusively composed of dense bodies. Only few membranes and platelet granules of other types can be identified. (Original magnification × 18,500.) (b, c, and d) Immunolabeling of the dense granule fraction pretreated by the uranaffin technique of Richards and Da Prada10 and embedded in GMA. (b) Labeling of P-selectin (polyclonal anti–P-selectin/GAR 10 nm). P-selectin is present in the membrane of α-granules (A) and in some of the dense granules (D) that are still identifiable, but only discrete labeling is obtained. This might be caused by the interaction of the chemical solutions with the antigenic sites, which could have decreased their reactivity. (Original magnification × 38,500.) (c) Immunolabeling for Gp IIb-IIIa (polyclonal anti–Gp IIb-IIIa/GAR 10 nm) is similar to immunolabeling for P-selectin and is present in the membrane of some dense granules (D) as well as α-granules (A). (Original magnification × 38,500; inset, × 100,500.) (d) Immunolabeling for Gp Ib (polyclonal anti–Gp Ib/GAR 10 nm) is occasionaly found in the membrane of dense granules (D). α-Granules (A) present in this field are not labeled. (Original magnification × 38,500; inset, × 74,500.)