Cell based model of coagulation

Cell based model of coagulation and the importnace

1. Cell based model of coagulation Dr. G.D.A. Samaranayaka Registrar in Transfusion Medicine

2. Cascade model

3. Intrinsic pathway Intrinsic pathway named -> all factors required for this pathway present in blood. Activation - factor XII comes in contact with negatively charged sub-endothelial collagen exposed following injury to the blood vessel cell and platelet derived polyphosphates. factor XII is converted -> active from factor XIIa . Sequential cascade of activation of FXI, IX and X to their active forms. FXa + Fva + calcium and platelet phospholipids ->form prothrombinase Formation of fibrin through the common pathway Tested in the laboratory by measuring activated partial thromboplastin time ( aPTT ).

4. Extrinsic pathway Activated when factor VII in blood comes in contact with tissue factor (TF) that is released following tissue injury. Factor VII and factor X are converted into their active forms

5. Common pathway The prothrombinase consists of activated factor X ( Xa ), activated factor V ( Va ), calcium and platelet phospholipids. Prothrombinase activate prothrombin or factor II is converted to thrombin ( IIa ) The factors in common pathway (factors X, V, II and I) can be tested in the laboratory by measuring the aPTT and PT.

6. Limitations of cascade/waterfall model Individuals with factor XII deficiency do not suffer from bleeding in spite of the requirement of this factor for initiating the intrinsic pathway. The aPTT of these individuals are prolonged but they are asymptomatic. Deficiency of highmolecular weight kininogen (HMWK) and pre-kallikrein also do not lead to a clinical bleeding tendency Deficiency of factor XI leads to variable haemostatic deficits in human beings with bleeding seen in some individuals Individuals with factor IX or factor VIII deficiency have severe bleeding even though their extrinsic and common pathways are normal and should be sufficient to promote clotting Deficiency of factor VII also causes bleeding even though the intrinsic pathway is intact.

7. Cell based model Formation of thrombin from prothrombin is the central event Reactions occur rapidly and do not follow a neat sequence as in cascade model Divided into three phases - interwoven and overlap each other 1. Initiation phase 2. Amplification phase 3. Propagation phase

8. Initiation Tissue factor (TF) is the initiator TF Found in TF-bearing cells - smooth muscle cells, fibroblasts in the subendothelial layer Small amounts - macrophages, endothelial cells and platelets Hidden and membrane bound Expressed only after an injury 1 2 2 3 1

9. Initiation - Process Binding of TF with factor VII contact of FVII (or aFVIIa ) in the plasma with tissue factor (TF) Activation of Factor X and IX VIIa /TF complex activates FX & XI – bridge intrinsic and extrinsic paths Conversion of Prothrombin to Thrombin b y Factor Xa Only a small amount of thrombin generated – inefficient generation circulating Xa is inhibited by TFPI & antithrombin III TFPI/ Xa complex also inhibits the VIIa /TF complex

10. Amplification Activation of platelets Platelet adhere to the injured vasculature and extra-vascular tissues – vWF TF in initiation -> activate platelets Platelet activation FV, VIII, XI activation Formation of tenase complex Formation of prothrombinase Complex Thrombin formation

11. Platelet activation Platelets become irregular in shape with multiple pseudopodia that increase surface area Platelets start expressing receptors and binding sites for various clotting factors. by reversing the asymmetry of the phospholipid bi-layer anionic phosphatidyl serine (PS) found on the inner layer is brought to the outer layer - “flip-flop” PS attracts the positively charged Ca2+ Platelets start releasing factors dense granules - serotonin, calcium and ADP alpha granules - factor V, fibrinogen, von Willebrand factor ( vWF ) and platelet derived growth factor

12. FV, VIII, XI activation Initiation phase thrombin activates factors on platelets FV FVIII coupled with vWF - > vWF release -> futher plt activation FXI FVa , FVIIIa , FXIa

13. Formation of tenase complex & prothrombinase complex IXa combines with factor VIIIa (along with calcium and phospholipids) on the platelet surface form tenase complex ( VIIIa / IXa ) -> Activates Factor X -> Xa Xa with factor Va (along with calcium and phospholipids) on platelet surface -> form prothrombinase complex ( FXa-FVa )

14. Thrombin formation prothrombinase ( Xa / Va ) converts prothrombin (substrate) into large amounts of thrombin compared to factor Xa (in the initiation phase), prothrombinase is about 300,000 times more active in the formation of thrombin from prothrombin “burst of thrombin”

15. Propagation phase Factor XIa that was formed in the initial part of the amplification phase activates factor IX to factor IXa which enables the formation of tenase complex Formation of thrombin and fibrin plug prothrombinase initiates a burst of thrombin formation from prothrombin. The thrombin converts fibrinogen to fibrin monomers. These fibrin monomers interweave - factor XIIIa , Interweave with each other and platelets to form a stable fibrin plug

16. Control and Termination of clotting If the process of thrombin production is not controlled, the thrombin that is formed at the site of injury can spread to the adjacent normal areas and cause coagulation in surrounding normal areas. Tight control by T FPI natural anticoagulants - Protein C, Protein S Antithrombin III Activate the fibrinolytic system tissue plasminogen activator ( tPA ) and urokinase-type plasminogen activator ( uPA ) - activating the fibrinolytic pathway

17. Clinical implications Normal haemostasis requires normal vascular function, normal platelets and coagulation factors. Deficiency of factor VII can lead to bleeding as it is important for initiation phase Haemophilia A & B Factor VIII and IX deficiencies lead to severe bleeding important for the amplification phase Haemophilia C deficiency of factor XI - autosomal disease causes variable bleeding tendency with bleeding seen in only some probably FXI in the propagation phase may not be needed if adequate thrombin is generated in the amplification phase

18. References

19. Thank you