Time course analysis of the contact interface between platelets and collagen fibrils. Blood was obtained as described in the legend to Figure 5, with the exception that no mepacrine was added. The technique of RICM, described in “Materials and methods,” allows visualization of the larger collagen fibrils immobilized on the glass bottom of the flow chamber before the beginning of blood perfusion (time 0 s). After 5 seconds of perfusion with either normal or GP VI^null blood, comparable numbers of single platelets (arrows) are seen interacting with the surface. Their shape is round indicating that spreading following activation has not yet taken place. After 45 seconds, in the case of normal blood perfusion the adherent platelets have become spread and occupy a larger portion of the surface; in contrast, in the case of GP VI^null blood perfusion, platelets have the same morphology as after 5 seconds, indicating that they have not become activated and, thus, have not spread. The insets to the right present a larger magnification of the surface after 45 seconds of perfusion. Note that the individual boundaries of spread platelets tend to be lost. In this technique, the darker color of spread platelets compared with those that have not spread indicates a closer proximity to the collagen fibrils. Video 2 presents a more detailed view of these events and demonstrates that the spread platelets represent the base of large thrombi attached to the collagen fibrils and protruding into the flow path.