The Role of Platelets in Fibrinolysis

This chapter reviews the complex manner by which platelets interact and regulate aspects of the fibrinolytic system in both a positive and negative manner. The key mechanisms are: (1) the secretion of a number of key proteins and molecules during platelet activation that influence the fibrinolytic system, the most noteworthy of which, platelet PAI-1, correlates directly with the lysability of thrombi; (2) the platelet membrane provides a surface to support plasminogen activation via binding of plasminogen, plasminogen activators, and proteins of the contact pathway; and (3) platelets modify clot structure, with dense areas of fibrin forming around platelet masses, which hinders movement of the lysis front through the clot. The net effect of these reactions is difficult to appraise and is most likely related to platelet concentration and proximity to fibrin and the endothelium, as well as the expression and binding of proteins to the platelet surface.