Platelet Adhesion to the Subendothelium Under Flow

The adhesion of cells is normally studied in the absence of flow. In a classic setup, ELISA wells are coated with an adhesive protein and subsequently incubated with a cell suspension. The gravity is responsible for the transport of the cells to the adhesive surface. This setup is not appropriate for blood cells, such as platelets, because platelet adhesion occurs physiologically under flow conditions. The shear stress induced by the flow causes qualitative and quantitative differences in adhesion (1 ). Blood platelets have a relatively low density and do not settle easily under gravity. Under flow conditions, convective diffusion of platelets to the vessel wall is responsible for a strong increase of platelet number near the reactive surface, resulting in a strongly enhanced platelet adhesion. A second important effect of flow is mediated by the shear stresses exerted on the platelets. To overcome these stresses, a special protein, the von Willebrand factor, is necessary, which has a very high affinity for the platelet membrane receptor glycoprotein Ib. In the absence of flow, the presence of von Willebrand factor is not necessary for platelet adhesion, but with increasing shear rates, the presence of von Willebrand factor becomes essential. Another disadvantage of studying platelet adhesion under static conditions is that owing to the presence of red blood cells, these experiments must be performed with washed platelets, and the required centrifugation steps cause unwanted platelet activation.