Some thoughts about localization and expression of tissue factor.
It is likely that tissue factor (TF) evolved as a haemostatic protein and, as such, it is highly concentrated in vascular tissue. Most cell surface TF is latent and simple exposure of the cell surface to circulating procoagulant proteins is not sufficient to trigger coagulation. Recently, it has been shown that an intracellular pool of TF accumulates after stimulation of vascular smooth muscle cells with growth factors. We have estimated that 20% of cellular TF is available on the surface, 30% is intracellular and 50% is latent. Since the bulk of cell surface TF is latent, staining vessels for TF does not accurately reflect their haemostatic and thrombogenic potential. It has long been thought that, in vivo, initiation of haemostasis requires only disruption of the vascular wall. We have detected vesicular TF in arterial sections raising the possibility that this pool of TF initiates thrombosis and possibly haemostasis. Much progress has been made in investigating the role and mode of action of TF, but fundamental questions remain to be answered.