Immunoelectron microscopy.

Gray platelets immunolabeled for vWF (A) and P-selectin (B). (A) Immunolabeling for the secretory protein vWF is virtually absent from gray platelets; only trace amount of labeling is found in structures resembling empty α-granules (A′); ocs indicates open canalicular system. (B) P-selectin is immunodetected in the membrane of empty vacuolar structures, such as identified as α-granules (A′) deprived of soluble content. Insets show normal platelet α-granules immunolabeled for vWF and P-selectin. (Panels A and B, original magnification × 40 300; insets, × 65 000.)