Detection of Gp IIb-IIIa on dense granules by double immunolabeling performed on frozen thin sections of platelets. (a) P-selectin (polyclonal anti–P-selectin/protein A 10 nm; arrowheads)/Gp IIb-IIIa (polyclonal anti–Gp IIb-IIIa/protein A 15 nm; arrows). The presence of P-selectin identifies α-granules (A) and the dense granule membrane (D). Gp IIb-IIIa lines the plasma membrane (pm) and the α-granule membrane (A). Note that it is present in the dense granule membrane (D), where it colocalizes with P-selectin. (Original magnification × 36, 000; inset, × 84,000.) (b) Granulophysin (monoclonal anti-granulophysin/GAM 10 nm; arrowheads)/Gp IIb-IIIa (polyclonal anti–Gp IIb-IIIa/protein A 15 nm; arrows). The anti-granulophysin labels the dense granule membrane, thus allowing identification of these structures (D). Gp IIb-IIIa is also found at their level. Gp IIb-IIIa labels the α-granule membrane (A) and the plasma membrane (pm) as well. (Original magnification × 44,000; inset, × 84,000.)