Cryosection of blood platelet from a washed cell suspension incubated with 18- to 20-nm colloidal gold particles coated with Fgn/Au for 5 minutes, then exposed to 1 U/mL of thrombin for 60 seconds. The Fgn/Au particles bind to the cell surface and penetrate into peripheral channels of the OCS. After fixation, freezing, and cryotomy, the frozen thin section was stained with a polyclonal antifibrinogen antibody and protein A bound to 5-nm gold particles. Immunogold beads detecting endogenous fibrinogen are concentrated in intact  granules (G₃) and in  granules (G₁, G₂) in the process of discharging their contents into channels of the OCS. Some of the Fgn/Au particles entering from the outside are mixed with the immunogold beads in the same OCS channels (OCS) communicating with the exterior surface. Original magnification ×60,000. (Reprinted with permission.18)