(a and b) Frozen sections of resting platelets labeled with the dense granule markers granulophysin and P-selectin to identify the morphology of these organelles on cryosections. (a) Labeling of P-selectin (polyclonal anti–P-selectin/protein A 10 nm). P-selectin is present along the α-granules (A) and labels the dense granules (D) that have been partially extracted. The absence of P-selectin permits distinction of the vacuolar structures of the OCS from the dense bodies. (Original magnification × 48,000.) (b) Labeling of granulophysin (monoclonal anti-granulophysin/GAM 10 nm). The gold marker labels vacuolar structures, such as the dense granule limiting membrane (D). Using this technique, the dense granule core appears to have been partially lost, which gives the dense granule a vacuolar appearance. α-Granules (A) and plasma membrane (pm) are negative. (Original magnification × 48,000.)