Tissue factor, also called platelet tissue factor, factor III, thromboplastin, or CD142 is a protein encoded by the F3gene present in subendothelial tissue and leukocytes. Its role in the clotting process is the initiation of thrombin formation from the zymogenprothrombin. Thromboplastin defines the cascade that leads to the activation of factor X - the tissue factor pathway. In doing so it has replaced the previously named extrinsic pathway in order to eliminate ambiguity.
The F3 gene encodes coagulation factor III which is a cell surface glycoprotein. This factor enables cells to initiate the blood coagulation cascades, and it functions as the high-affinity receptor for the coagulation factor VII. The resulting complex provides a catalytic event that is responsible for initiation of the coagulation protease cascades by specific limited proteolysis. Unlike the other cofactors of these protease cascades, which circulate as nonfunctional precursors, this factor is a potent initiator that is fully functional when expressed on cell surfaces. There are 3 distinct domains of this factor: extracellular, transmembrane, and cytoplasmic. This protein is the only one in the coagulation pathway for which a congenital deficiency has not been described. In addition to the membrane-bound tissue factor, soluble form of tissue factor was also found which results from alternatively spliced tissue factor mRNA transcripts, in which exon 5 is absent and exon 4 is spliced directly to exon 6.
Together with factor VIIa, tissue factor forms the tissue factor or extrinsic pathway of coagulation. This is opposed to the intrinsic (amplification) pathway which involves both activated factor IX and factor VIII. Both pathways lead to the activation of factor X (the common pathway) which combines with activated factor V in the presence of calcium and phospholipid to produce thrombin (thromboplastin activity).
an extracellular domain, which consists of two fibronectin type III modules whose hydrophobic cores merge in the domain-domain interface. This serves as a (probably rigid) template for factor VIIa binding.
Some cells release TF in response to blood vessel damage (see next paragraph) and some do only in response to inflammatory mediators (endothelial cells/macrophages).
TF is expressed by cells which are normally not exposed to flowing blood such as sub-endothelial cells (e.g. smooth muscle cells) and cells surrounding blood vessels (e.g. fibroblasts). This can change when the blood vessel is damaged by for example physical injury or rupture of atherosclerotic plaques. Exposure of TF expressing cells during injury allows the complex formation of TF with factor VII. Factor VII and TF form an equal molar complex in the presence of calcium ions and this leads to the activation of factor VII on a membrane surface.
The inner surface of the blood vessel consists of endothelial cells. Endothelial cells do not express TF except when they are exposed to inflammatory molecules such as tumor necrosis factor-alpha (TNF-alpha). Another cell type that expresses TF on the cell surface in inflammatory conditions is the monocyte (a white blood cell).
Historically, thromboplastin was a lab reagent, usually derived from placental sources, used to assay prothrombin times (PT time). Thromboplastin, by itself, could activate the extrinsic coagulation pathway. When manipulated in the laboratory, a derivative could be created called partial thromboplastin. Partial thromboplastin was used to measure the intrinsic pathway. This test is called the aPTT, or activated partial thromboplastin time. It was not until much later that the subcomponents of thromboplastin and partial thromboplastin were identified. Thromboplastin is the combination of both phospholipids and tissue factor, both needed in the activation of the extrinsic pathway, and partial thromboplastin is just phospholipids without tissue factor. Tissue factor is not needed to activate the intrinsic pathway.
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