Ultrastructural immunonanogold localization of SNAP-23 in resting and activated human platelets. (A) Resting platelets demonstrate SNAP-23 staining on the plasma membrane, α-granule membranes, and on membranes of the OCS. SNAP-23 label is observed on the extracellular surface of the plasma membrane (arrows). (B) SFLLRN-stimulated and (C) PMA-stimulated platelets demonstrate more intense staining. In addition, there is abundant SNAP-23 staining on the extracellular surface of the plasma membrane (arrows). Bars, 0.5 μm. See “Materials and methods, Immunofluorescence microscopy” for image acquisition information.