Spreading and procoagulant activity of platelets adhering to fibrin. (A-D) Hirudinated (A-B) mouse and (C-D) human whole blood was perfused over a fibrin surface at 300 s⁻¹ for 3 minutes. The morphology of the platelets adhering to fibrin was examined by (A, C) SEM and the number of spread platelets was quantified for (B, D) each condition. Bar, 30 µm. (A-D) Platelets from (A-B) WT (n = 4) or GPVI^−/− (n = 4) mice and from (C-D) human whole blood treated with a control Fab (n = 4) or the anti-GPVI Fab 9O12 (n = 4). (B, D) Quantification was performed on at least 10 images per condition. Mean ± SEM, *** P < .001, Mann-Whitney U test. (E) Hirudinated human whole blood was perfused over fibrin for 3 minutes at 300 s⁻¹ and then the surface was rinsed with PBS for 10 minutes and perfused with annexin V-568 for 10 minutes. (E) Procoagulant platelets appeared as cells positive for annexin V staining (green) using epifluorescence microscopy. Bar, 30 µm. PBS, phosphate-buffered saline.