Tissue factor storage, synthesis and function in normal and activated human platelets

The source and significance of blood-borne tissue factor (TF) are controversial. The presence of TF in platelets was initially attributed to transfer of the protein from other cells (e.g., monocytes) and/or TF-bearing microparticles. Recently, TF-mRNA, neo-synthesis of the protein and TF-dependent procoagulant activity (PCA) have been reported in human platelets. The storage of “encrypted”, potentially active TF in circulating, non-stimulated platelets remains debatable. One report strongly suggests that the starting of platelet PCA depends on de novo TF synthesis induced by platelet activation, whereas others provide persuasive evidence that platelets circulate with preformed TF, readily functional upon demand. These findings may have an impact on our current ideas of physiological hemostasis and thrombus formation. In fact, platelets would lead not only the formation of the primary plug, but in this microenvironment they would also contribute to the triggering of thrombin generation, fibrin deposition, clot consolidation and initial protection from fibrinolysis. Much research is needed to validate this platelet-based hemostasis model.