The Platelet: Membrane and Surface Reactions

The platelet plays an essential role in hemostasis and in the preservation of the organism. Without platelets, catastrophic hemorrhage ensues. With full platelet activation, catastrophic thrombosis may occur. Physiologically the platelet helps maintain the integrity of the vascular system by interacting with the vessel wall and other elements of the blood, particularly the procoagulant plasma proteins of the fibrin clotting mechanism. The platelet also participates in the defense mechanism of the body, including the inflammatory and immunologic responses as well as the clearing function of the blood in removal of particulate material such as bacteria. It likewise serves as a vehicle of transport of many materials. Pathologically, the platelet plays a central role in hemorrhage and thrombosis, in the pathogenesis of altered permeability states, and in atherosclerosis. Only a few years ago, the study of the platelet changed from a quiescent field to one in which rapid strides are being made. As in other fields, recent advances have come about from studies utilizing improved methodology. Most important have been (a) ultrastructural investigations with scanning and freeze-fracture electron microscopy, (b) studies of the cell membrane and the phenomenon of storage and release of many compounds such as nucleotides and serotonin, and (c) new develop­ ments in protein chemistry and enzymology, including recognition of the role of contractile protein in platelet function and new approaches to the study of the antihemophilic factor-the von Willebrand factor macromolecular complex of plasma-and its interaction with platelets. From these and other studies, a more comprehensive concept of the role of the platelet in hemostasis has developed. The circulating platelet is the smallest of the blood cells, anuclear and disc­ shaped, with a complex metabolism common to most cells. It is sparse in mito­ chondria and almost devoid of rough endoplasmic reticulum, but is rich in lysosomal-like organelles or granules. It possesses a complex canalicular network,