Role of platelet integrin ₂β₁ in platelet adhesion and thrombus formation under flow on type I collagens with distinct structural specificities. Blood containing PPACK as anticoagulant and treated with the fluorescent dye mepacrine for platelet visualization was perfused at 37°C over distinct type I collagen preparations, in a parallel plate flow chamber. The flow rate was set to produce a wall shear rate of 1,500 s⁻¹, 500 s⁻¹, or 100 s⁻¹ at the inlet of the flow chamber and each experiment was recorded on tape using a videomicroscopy system. The figure shows single frame images of the surface, each corresponding to an area of 65,536 μm², obtained after 3 minutes of perfusion. Blood was either untreated (control) or treated with a monoclonal antibody selectively inhibiting the platelet integrin ₂β₁ function. Note that functional inhibition of platelet ₂β₁ completely abolished stable platelet attachment and thrombus formation on pepsin-solubilized (nonbanded), but not on native insoluble collagen (banded) or collagen reconstituted from the same pepsin-solubilized collagen (from human placenta) used in these studies. These images are representative of the results obtained in at least 6 separate experiments with blood from different donors. After 3 minutes of perfusion, the total volume of platelet thrombi present in an area of 102,236 μm² was measured by confocal sectioning at 1.0-μm intervals, as described in Materials and Methods. Volume measurements represent the mean ± standard error of the mean of 4 separate experiments with different blood donors. The size of single platelets can be appreciated in the panel showing the results obtained in the presence of the anti-₂β₁ antibody with pepsin-solubilized collagen or in Figs 3 and 7, in which platelet to platelet interactions have been prevented by blocking _IIbβ₃ function.