Use of colloidal gold to examine fibrinogen binding to human platelets.

Fibrinogen labelled colloidal gold was used in a direct labelling procedure with surface activated human platelets. Utilizing this technique, the platelet membrane receptor for fibrinogen was visualized by both scanning and high voltage electron microscopy. Changes in the degree of fibrinogen binding and the whole cell distribution of the fibrinogen receptor are associated with the progression of the morphological transformation induced following platelet activation. While unstimulated platelets do not bind fibrinogen, the amount of fibrinogen bound per platelet increases rapidly during the early stages of shape change characteristic of surface activation. Redistribution of fibrinogen receptors to the central areas of platelets occurs following saturation of receptor sites. The ease of preparation of the label and its easy detection by electron microscopy make it useful for correlative HVEM and SEM studies of the relationship between receptor redistribution and cytoplasmic ultrastructural reorganization.