Teaching the cell based model of coagulation

Abstract

Coagulation reactions in vivo are localized to cellular surfaces. The endothelial cells that line the vascular space provide a non‐thrombogenic surface preventing unnecessary coagulation, thromboses and platelet activation. When this lining is disrupted, blood comes in contact with the extra vascular tissue exposing platelets to the platelet activating agent collagen. The activated platelet externalizes plasma membrane phosphatidylserines‐which provide a cell anchored scaffolding for formation of coagulation complexes, such as the prothrombinase and tenase complexes, accelerating coagulation reactions 300,000 fold. Activated platelets also form a platelet plug at the site of injury as they aggregate.This laboratory exercise emphasizes the importance of cells in the coagulation process. Coagulation triggered by the “intrinsic” pathway is assayed by addition of inhibitors, Triton‐X, and EGTA to demonstrate the role of phosphatidylserine vesicles, and Ca2+ in the formation of the coagulation complexes. Thrombin conversion of fibrinogen to fibrin, is not inhibited by Triton‐X, demonstrating that the phosphatidylserine coagulation complexes are not required for this final step in the coagulation pathwayThe activation and aggregation of platelets in response to collagen and ADP and their inhibition by prostaglandin E1 is monitored by changes in light transmission as the suspension of platelets undergo activation, first changing shape from disc to sphere and then as they form aggregates of various sizes.