Ultrastructural studies on the binding sites of fibrinogen on platelet surface during aggregation

Activated human blood platelets show characteristic globular structures on their surface. Aggregated platelets in blood plasma form contact zones with 40-50 nm spaces between the plasmalemmata. These spaces are bridged by filamentous structures (5,000 per microns 2). To investigate whether the globules or bridges observed are caused by fibrinogen, platelets were washed and treated first with thrombin (1 U thrombin/ml) in order to remove fibrinogen from platelet storage organelles and then with plasmin (0.5-2 U/ml) in order to dissolve remnants of fibrinogen from the platelet surface. Platelets treated in this way were resuspended in tyrode-albumin buffer solution (containing hirudin and prostaglandin E1). No storage organelles were revealed but the platelets reconstituted their discoid shape and the marginal bundle of microtubules. By representing the platelet glycocalix with alcian blue it was observed that the previously homogeneous surface coat was interdispersed with holes of 60-70 nm in diameter, approx. 900 per microns 2. Fibrinogen (8 mg/ml) was then added and the suspension was stirred for 3 minutes at 37 degrees C. The platelets again aggregated and formed contact zones in blood plasma as described above. In spaces of 40 to 50 nm width filamentous bridges similar in size, structure and number to those between aggregated platelets in blood plasma were observed. In both cases the bridges appeared to adhere with small rods into the plasmalemma. Bridges and rods can be easily stained with protein stabilizing agents. In contrast, glycocalix treated with alcian blue are weakly stained. The findings strongly indicate that fibrinogen is the mediator of this type of platelet contact in aggregates. The fibrinogen binding sites are situated to the plasmalemmal outer leaf let and not on the peripheral glycocalix.