Tissue factor (TF) is a glycoprotein that is secreted by cells in extravascular tissues. It stabilizes labile blood enzyme factor VII and enables its enzymatic activity. Thus stabilized, factor VII generates small amounts of thrombin that are sufficient to energize tissue maintenance and "trigger" the enzymatic interaction of factors VII, VIII, IX and X. The traumatic exposure of tissue factor to flowing blood thus initiates blood coagulation as the first event in tissue repair. The resulting viscoelastic clot substitutes for the damaged vascular endothelium and re-isolates flowing blood from damaged tissues. The viscoelastic clot then governs the penetration of factors VII and X into damaged tissues, where they interact with tissue factor to generate thrombin that energizes the cellular activities of tissue repair.
Tissue factor is found in high concentrations in brain, retina, nerves, spinal cord, lung, gonads, cervix, placenta, amniotic fluid, and peri-arterial tissue. Its presence explains the exaggerated incidence of malignancy, metastasis, and hypercoagulability in these tissues.