Platelets play a crucial role not only in the formation of a normal hemostatic plug but also in formation of a pathologic thrombus, particularly within arteries subjected to high shear stress. Platelets circulate passively as they traverse a vascular tree lined by an intact monolayer of endothelial cells. As an initial step in hemostasis and thrombogenesis, platelets adhere to altered vascular surfaces or exposed subendothelial matrices. Following adhesion, they become activated, change shape, secrete granule contents, and aggregate to each other to form a primary hemostatic plug and to provide a catalytic surface to enhance blood coagulation. Thus, thrombus formation depends primarily on platelet adhesive interactions with both extracellular matrix proteins [e.g. von Willebrand factor (vWF), collagen, fibronectin] and cellular surfaces (e.g. platelets, leukocytes). In order to mediate these adhesive events, platelets possess numerous receptors specialized for this task. In this chapter, we will focus on the structure and function of platelet plasma membrane glycoproteins that serve as important adhesion receptors. One theme that emerges is that many, if not all, adhesion receptors also function as signalling receptors. Other chapters in this book will focus on surface membrane proteins that function primarily as receptors for excitatory and inhibitory agonists and on granule proteins, such as P-selectin and CD40 ligand, that may mediate adhesive functions concomitant with granule secretion (Chapters 9–13).

Platelet adhesion receptors

Surface labelling of intact platelets has shown that they possess numerous glycoproteins on their surface (Fig. 5.1).