Detection of Gp Ib in dense granules by double immunolabeling on frozen thin sections of platelets. (a) P-selectin (polyclonal anti–P-selectin/protein A 10 nm; arrowheads)/Gp Ib (polyclonal anti–Gp Ib/protein A 15 nm; arrows). P-selectin is present in the α-granules (A) and dense granules (D). Gp Ib is evident along the plasma membrane (pm), occasional OCS, and α-granules. The dense granule membrane (D) that is marked with P-selectin also contains gold marker for Gp Ib. (Original magnification × 46,500; inset, × 101,000.) (b) Granulophysin (monoclonal anti-granulophysin/GAM 10 nm; arrowheads)/Gp Ib (polyclonal anti–Gp Ib/protein A 15 nm; arrows). Granulophysin labels the dense granule membrane (D) but not the α-granules (A). Gp Ib is found essentially along the plasma membrane (pm). The OCS and some α-granules (A) are occasionaly labeled. It also double-labels the dense granule membrane (D) identified by the presence of granulophysin. (Original magnification × 46,500; inset, × 84,000.)