Localization of ferritin-conjugated anti-fibrin-fibrinogen in platelet aggregates produced in vitro.

A ferritin-conjugated anti-fibrin/fibrinogen was localized by means of light and electron microscopy in artificial in vitro thrombi formed in the presence of the labeled antibody, and in preformed ADP-induced platelet aggregates. The ferritin was distributed throughout the central and peripheral regions of the columns of aggregated platelets in the thrombi. In the preformed ADP aggregates, ferritin was deposited only by infiltration from surrounding plasma and was confined to the periphery of the columns. The even distribution of ferritin in the central zone of the platelet columns of the thrombi indicated a specific reaction had occurred before or during thrombus formation unrelated to infiltration of plasma. In the artificial thrombi, the ferritin-labeled antibody was localized on the surface layer of platelets and on the bridging structures composed of the combined surface layers in the narrow spaces between cohesive platelets. Vesicles and alpha granules within the platelets also were tagged. The absence of obvious fibrin between narrow interspaces and within the platelets indicated that the antibody had reacted with fibrinogen or partly polymerized fibrin at these sites. Many invaginations of the platelet membrane containing dense fibrillar material were interpreted to be alpha granules discharging their contents during the "release reaction" at the time of aggregation. This material, which was tagged by the ferritin-conjugated antibody, merged with the interplatelet bridges to suggest that released fibrinogen from within the platelet contributed to the structural bond and strengthened it. A layer of dense fibrin and altered platelets in the periphery of the columns of aggregated platelets in the artificial thrombi contained the platelets and limited further growth of the aggregates. The fibrin was thought to be derived from infiltrated plasma as well as from released intraplatelet fibrinogen. Platelet fibrinogen thus appeared to take part both in the cohesion of aggregated platelets and in the stabilization of the aggregates formed.