Hemostatic clot formation entails thrombin-mediated cleavage of fibrinogen to fibrin. Previous in vitro studies have shown that the thrombin concentration present during clot formation dictates the ultimate fibrin structure. In most prior studies of fibrin structure, clotting was initiated by adding thrombin to a solution of fibrinogen; however, clot formation in vivo occurs in an environment in which the concentration of free thrombin changes over the reaction course. These changes depend on local cellular properties and available concentrations of pro-and anti-coagulants. Recent studies suggest that abnormal thrombin generation patterns produce abnormally structured clots associated with an increased risk of bleeding or thrombosis. Further studies of fibrin formation during in situ thrombin generation are needed to understand fibrin clot formation in vivo.
Keywordsthrombin; fibrinogen; fibrin; plasmin; fibrinolysis; clot structure; hemophilia; thrombosis
Fibrinogen and fibrin clot formationFibrinogen is a 450 Å, 340 kDa trinodular protein present at high (2 -4 mg/mL) concentrations in plasma. Fibrinogen consists of pairs of three different disulfide-linked polypeptide chains: Aα, Bβ, and γ. These six polypeptide chains are arranged with their N-termini converged in a central "E" domain of the molecule. The C-termini of the Bβ and γ chains extend outward into distal "D" domains. The C-termini of the Aα chains are globular and situated near the central E domain of fibrinogen where they interact intra-molecularly. 1 Comprehensive reviews on the biochemistry of fibrinogen and fibrin structure have been published. 2-4Mechanisms of fibrin production have been elucidated in studies conducted by adding exogenous thrombin to purified fibrinogen. These studies have shown that thrombin removes N-terminal peptides from the fibrinogen Aαl and Bβ chains, causing the spontaneous formation of half-staggered, double-stranded protofibrils followed by thickening of protofibril chains. 5 Initial formation of protofibrils occurs during a "lag" phase in which no turbidity increase is detected. Subsequent lateral aggregation of fibrin protofibrils causes a turbidity increase. The magnitude of the turbidity increase relates to the structure of the formed clot; formation of thicker fibers causes a greater increase in final turbidity. 6,7 Fibrin formation can be followed Address correspondence to: Alisa Wolberg, Ph. D., Department of Pathology and Laboratory Medicine, University of North Carolina at Chapel Hill, 815 Brinkhous-Bullitt Building, CB #7525, Chapel Hill, NC 27599-7525, phone: (919) 966-8430, fax: (919) 966-6718, email: alisa_wolberg@med.unc.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production p...