Ultrastructural analysis of αIIbβ3-dependent and -independent platelet aggregates. Washed platelets (A,C) or washed platelets pretreated with dRGDW (200 μM; B,D) were stimulated with collagen (A,C) or with collagen and coagulation factors VIIa, X, II, and I in the presence of calcium chloride (B,D) under stirring conditions (900 rpm) at 37°C. After 10 minutes of aggregation, samples were fixed and prepared for electron microscopy as described in “Materials and methods.” Samples were examined directly (A-B) or after immunogold labeling using an antibody against fibrin(ogen) followed by protein A–gold (C-D). Bars indicate 200 nm (A-B) and 500 nm (C-D).