The role of adhesive proteins and membrane interactions in the processes of thrombocyte aggregation

: The reported data concerning the role of fibrinogen in platelet aggregation are reviewed and compared to the authors' experimental data obtained by electron microscopy and cytochemical techniques. Using fibrinogen and fibrinogen antibodies bound to colloidal gold, it has been shown that the presence of fibrinogen bridged between the adjoining cells is not necessary for primary platelet microaggregation stimulated by ADP or thrombin. The formation of direct interplatelet contacts resembling "pentalaminar membranes" has been shown to participate in that process. Some mechanisms are proposed to explain the enhanced adhesiveness of the activated platelet surfaces. Redistribution of phospholipids in the membrane bilayer leading to the exposure of negatively charged phospholipids may underlie that phenomenon. The clustering and internalization of "occupied" membrane receptors may also contribute to the formation of close contacts between platelets stimulated by primary agonists in the presence of exogenous fibrinogen and other adhesive proteins.