Blood Tissue factors

There is a second means by which Factor X can be activated known as the extrinsic pathway. The name derives from the fact that the pathway involves a tissue factor (Factor III) which is not present in the blood. Tissue factor is shed by the membrane of damaged cells and is a lipoprotein complex which provides a protein cofactor and also a surface on which Factor VII can activate Factor X in the presence of Ca " " (Figure 25.14). Factor X increases its own production by activating Factor VII. 

Figure 51-1. The pathways of blood coagulation. The intrinsic and extrinsic pathways are indicated. The events depicted below factor Xa are designated the final common pathway, culminating in the formation of cross-linked fibrin. New observations (dotted arrow) include the finding that complexes of tissue factor and factor Vila activate not only factor X (in the classic extrinsic pathway) but also factor IX in the intrinsic pathway, in addition, thrombin and factor Xa feedback-activate at the two sites indicated (dashed arrows). (PK, prekallikrein HK, HMW kininogen PL, phospholipids.) (Reproduced, with permission, from Roberts HR, Lozier JN New perspectives on the coagulation cascade. Hosp Pract [Off Ed] 1992Jan 27 97.)...

Tissue factor pathway inhibitor (TFPI) is a major physiologic inhibitor of coagulation. It is a protein that circulates in the blood associated with lipoproteins. TFPI directly inhibits factor Xa by binding to the enzyme near its active site. This factor Xa-TFPI complex then inhibits the factor Vlla-tissue factor complex. 

The fluidity of blood is a result of the inhibition of a complex series of enzymic reactions in the coagulation cascade (see Fig. 10). When triggered either intrinsically (by contact with foreign surfaces ), or extrinsically (by tissue factors from damaged cells), inactive proenzymes (factors XII, XI, IX, and X) are transformed into activated pro-teinases (XHa, XIa, IXa, and Xa, respectively). Each proteinase catalyzes the activation of the following proenzyme in the sequence, up to formation of thrombin (Factor Ha), another proteinase that catalyzes partial... 

B51. Broze, G. J., Warren, L. A., Novotny, W. F., Higuchi, D. A., Girard, J. J., and Miletich, J. P The lipoprotein-associated coagulation inhibitor which inhibits the factor Vll-tissue factor complex also inhibits factor Xa Insight into its possible mechanism of action. Blood 71,335-343 (1988). 

Nemerson Y. The tissue factor pathway of blood coagulation. Semin Hematol 1992 29, 170-6. 

Two coagulation factors function uniquely in the extrinsic pathway factor III (tissue factor) and factor VII. Tissue factor is an integral membrane protein present in a wide variety of tissue types (particularly lung and brain). This protein is exposed to blood constituents only upon rupture of... 

Figure 12.1 Schematic diagram of the initial steps of the extrinsic blood coagulation pathway. See text for details (TF tissue factor)...

Simultaneously, activation of the extrinsic coagulation cascade occurs as a result of exposure of blood to the thrombogenic lipid core and endothelium, which are rich in tissue factor. This pathway ultimately leads to the formation of a fibrin clot composed of fibrin strands, cross-linked platelets, and trapped red blood cells. 

Figure 9.3. Photograph of a mouse 7 days after i.v. injection of a coaguligand formulation consisting of truncated Tissue Factor mixed with a bispecific antibody directed at the MHC Class II molecules on the tumour vasculature and at truncated Tissue Factor. The mouse carried a C1300 muy tumour measuring approximately 10 x 10 mm in diameter at the time of treatment. Within hours after treatment the tumour blood flow was blocked by generalized blood coagulation in the tumour vasculature (not shown). Seven days after treatment, the necrotic tissue was almost completely removed by the host immune cells.

Figure 9.4. Schematic representation of the mechanism of action of the coaguligand approach. Cross linking of truncated Tissue Factor to tumour endothelial cells leads to local blood coagulation via the tTF/fVIIa complex. tTF, truncated Tissue Factor fVIIa, factor Vila fX (A), factor X (A).

Extrinsic pathway This pathway has fewer steps than the intrinsic pathway and occurs rapidly, within a matter of seconds if the trauma is severe. It is called the extrinsic pathway because a protein tissue factor, also called thromboplastin or coagulation factor III, takes into the blood stream from outside and initiates the formation of prothrombinase. Tissue factor is released from the surface of the damaged cells. It activates factor VII. Factor VII combines with factor X, activating it. Factor X in the presence of Ca combines with factor V to give active enzyme prothrombinase. 

Immunoglobulins, oq-trypsin inhibitor and a2-macroglobulin,k ten or more blood clotting factors and proteins of the complement system all have protective functions that are discussed elsewhere in this book. Hormones, many of them proteins, are present in the blood as they are carried to their target tissues. Many serum proteins have unknown or poorly understood functions. Among these are the acute phase proteins, whose concentrations rise in response to inflammation or other injury. 

Factor IX (Christmas factor) is next in the intrinsic mechanism cascade. It can be activated either by XIa or by Vila of the tissue factor pathway. The absence of a functional factor IX leads to the inherited X-linked bleeding disorder hemophilia B which affects 1 in 30,000 males. The condition can be mild or very serious551552 and may be caused by a variety of mutations or by incorrect splicing of the messenger RNA for the 416-residue factor IX. The level of factor IX in blood increases with age, almost doubling by old age.552a... 

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