Fibrinogen internalization by ADP-stimulated blood platelets. Ultrastructural studies with fibrinogen-colloidal gold probes.

Interaction of gel filtered, ADP-stimulated human platelets with low (0.05 mg/ml) and high (1 mg/ml) fibrinogen (Fg) was examined by transmission electron microscopy. To visualize exogenous Fg in a course of its interaction with stimulated platelets, Fg coupled to 18-nm colloidal gold (Fg-Au) was employed.

[Role of exogenous fibrinogen in the processes of degranulation of thrombocytes stimulated with thrombin. Ultrastructural study using fibrinogen labeled with colloidal gold].

Using colloidal gold-conjugated fibrinogen (F-Au) it is shown that exogenous fibrinogen can participate in the platelet release reaction. In the absence of F-Au, internal secretory vacuoles readily formed in human platelets stimulated with thrombin, but extrusion of their content was delayed. Upon incubation with F-Au, endocytic channels induced by F-Au-receptor interactions, fused with internal vacuoles, thus establishing spatial communications of the latter with the outer medium.

[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.

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.

[Ultrastructural changes of platelets during ADP- stimulated aggregation (in the presence of fibrinogen)].

The ultrastructure of resting and stimulated human blood platelets (P) was studied by the transmission electron microscopy. The cells were chemically fixed (using tannic acid and OsFeCN mixture) 1, 3, 5 and 15 min after the addition of ADP and fibrinogen (F). Early changes in P ultrastructure consist in drastic reduction of the electron-dense layer of glycocalyx and in an increase of the plasma membrane permeability.