Fibrinogen-containing membrane-associated structures arising at the surfaces of ADP-stimulated blood platelets

Ultrastructural studies of ADP-stimulated gel-filtered human platelets incubated with different concentrations of fibrinogen reveal unusual extracellular structures composed at least partly of the aggregated fibrinogen. Development of these structures depends on the exogenous fibrinogen concentration and duration of the incubation. Fibrinogen-containing extracellular material exists in two different structural forms. As was shown earlier, one of them is represented by dense, amorphous intercellular matrix and is localized mainly in platelet microaggregates (Belitser et al., Thromb. Res., in press). Another one consists of huge sheet-like structures bearing individual or clumped platelets bound either to one or to both of their surfaces. After thrombin treatment, these structures could not be found anymore; sometimes they seem to be substituted by the fibrin-like fibers spatially connected to each other and/or to the platelet membranes. It has been suggested that soluble fibrinogen interacting with its specific membrane receptors undergoes conformational changes promoting intermolecular interactions and resulting in the fibrinogen aggregates formation at the surfaces of the activated platelets. A probable physiological significance of the structures described is discussed. It is supposed that under certain conditions in vivo they may play an important role, in particular as a highly concentrated substrate for the thrombin action, providing in such a way a possibility of rapid, effective consolidation of the initial platelet thrombi with the fibrin fibers.