Immunogold labeling of GPVI on ultrathin frozen platelet sections. GPVI was detected using the MoAb F5, specific for an epitope on the extracellular domain. Bound antibody was detected using a second antibody specific for mouse IgG and adsorbed onto 10 nm gold particles. (A) For the control, gold particles were seen both on the surface and associated with internal membranes including those of granules. (B) For the patient's platelets, only an occasional labeling within the SCCS was seen. Bars = 2 μm.