Up against the wall

Platelet tissue factor: how did it get there and is it important?

Recently, the presence of functionally active tissue factor (TF) in platelets has been reported by several groups. In this location, TF is postulated to play an important role in the propagation phase of thrombus formation. Although the existence of platelet TF still remains controversial to some extent, a review of the current literature proposes at least three distinct sources of "platelet-associated TF" in those laboratories that have reported its presence: (1) TF that is taken up in the form of circulating microparticles, usually derived from monocytes; (2) TF stored in the alpha-granules of platelets that may have been taken up and/or endogenously synthesized; and (3) TF that is synthesized and expressed on the plasma membrane of mature platelets. These pathways are not mutually exclusive, and the dominant mechanism may depend on the state of platelet activation and, possibly, on other host factors that differ in physiological hemostasis versus pathological thrombosis. This brief review will summarize the state-of-the-art understanding on the origins and possible role of platelet TF.

Flow Effects on Coagulation and Thrombosis

Thrombosis occurs in a dynamic rheological field that constantly changes as the thrombus grows to occlusive dimensions. In the initiation of thrombosis, flow conditions near the vessel wall regulate how quickly reactive components are delivered to the injured site and how rapidly the reaction products are disseminated. Whereas the delivery and removal of soluble coagulation factors to the vessel is thought to occur via classic convection-diffusion phenomena, the movement of cells and platelets to the injured wall is strongly augmented by flow-dependent cell-cell collisions that enhance their ability to interact with the wall. In addition, increased shear conditions have been shown to activate platelets, alter the cellular localization of proteins such as tissue factor (TF) and TF pathway inhibitor, and regulate gene production. In the absence of high shearing forces, red cells, leukocytes, and platelets can form stable aggregates with each other or cells lining the vessel wall, which, in addition to altering the biochemical makeup of the aggregate or vessel wall, effectively increases the local blood viscosity. Thus, hemodynamic forces not only regulate the predilection of specific anatomic sites to thrombosis, but they strongly influence the biochemical makeup of thrombi and the reaction pathways involved in thrombus formation.