Electron microscopy of platelet interactions with heme-octapeptide-labeled fibrinogen

Binding of fibrinogen to ADP-activated platelets was visualized by labeling the molecule with heme-octapeptide (microperoxidase) for direct cytochemical staining. Transmission electron microscopy of the platelet aggregates showed most of the fibrinogen distributed widely over the platelet surface in nonbridging rims of 7- to 9-nm thickness. Short peroxidase-positive bridges (less than 25 nm) were found in clusters in regions of close contact between the platelets, but 50-nm bridging corresponding to the length of the molecule was not seen by this method. Thus the fibrinogen appeared to be binding in a predominantly prone rather then upright orientation on the platelets. Abundant 50-nm bridging seen by nonspecific staining appeared unrelated to the length of the fibrinogen molecule because the bridging did not change when the length of the fibrinogen was more than doubled by end-to-end cross-linking with factor XIIIa. It is suggested that the observed binding and bridging of fibrinogen in a prone orientation is promoted by the existence of multiple platelet-binding domains on the molecule.