Although much is known about fibrin polymerization, because it is complex, the effects of various modifications are not intuitively obvious and many experimental observations remain unexplained. A kinetic model presented here that is based on information about mechanisms of assembly accounts for most experimental observations and allows hypotheses about the effects of various factors to be tested. Differential equations describing the kinetics of polymerization were written and then solved numerically. The results have been related to turbidity profiles and electron microscope observations. The concentrations of intermediates in fibrin polymerization, and fiber diameters, fiber and protofibril lengths have been calculated from these models. The simplest model considered has three steps; fibrinopeptide A cleavage, protofibril formation, and lateral aggregation of protofibrils to form fibers. The average number of protofibrils per fiber, which is directly related to turbidity, can be calculated and plotted as a function of time. The lag period observed in turbidity profiles cannot be accurately simulated by such a model, but can be simulated by modifying the model such that oligomers must reach a minimum length before they aggregate. Many observations, reported here and elsewhere, can be accounted for by this model; the basic model may be modified to account for other experimental observations. Modeling predicts effects of changes in the rate of fibrinopeptide cleavage consistent with electron microscope and turbidity observations. Changes only in the rate constants for initiation of fiber growth or for addition of protofibrils to fibers are sufficient to account for a wide variety of other observations, e.g., the effects of ionic strength or fibrinopeptide B removal or thrombospondin. The effects of lateral aggregation of fibers has also been modeled: such behavior has been observed in turbidity curves and electron micrographs of clots formed in the presence of platelet factor 4. Thus, many aspects of clot structure and factors that influence structure are directly related to the rates of these steps of polymerization, even though these effects are often not obvious. Thus, to a large extent, clot structure is kinetically determined.

Electron microscopy of freeze-dried, shadowed fibrin fibers has demonstrated that these structures are twisted. The pitch and radius of many fibers were measured from the micrographs. Although there is some variability, the average pitch of 1930 ± 280 (SD) nm is independent of radius. The distribution of observed radil of fibers assembled in vitro is highly skewed, suggesting that individual fibers grow to a maximum radius of about 50 nm, except when both pH and ionic strength are high; fibers aggregate to form thicker fiber bundles under some conditions. The observed twisting may be responsible for limiting the lateral growth of individual fibers. Protofibrils near the surface of a twisted fiber are stretched relative to those near the center. Consequently, the degree to which a protofibril can be stretched limits the radius of a fiber; protofibrils can be added to a growing fiber until the energy required to stretch an added protofibril exceeds the energy of binding. These properties of assembly arise directly from the intrinsic twist of the fibrinogen molecule determined from structural evidence. Simple geometric considerations lead to conclusions regarding the locations of the binding sites for assembly of the protofibril and the flexibility of the fibrin molecule.The physical properties of blood clots are determined to a large extent by properties of fibrin fibers, such as diameter, degree ofaggregation, stability, and resistance to proteolysis. Fibrinogen is the soluble precursor of the fibrin clot. It has a molecular weight of 340,000 and is made up of three pairs of polypeptide chains, (AaBI3y)2. The molecule is 45 nm in length and consists of globular domains connected by rodlike, a-helical coiled-coil domains (1, 2). All amino termini are linked together in the central domain, the disulfide knot.The carboxyl-terminal ends of the BP and y chains form the proximal and distal end domains, respectively. The carboxylterminal ends of both of the Aa chains appear to interact to form a domain adjacent to the central domain.Fibrin fibers with an axial repeat of 22.5 nm form from fibrinogen upon removal of the fibrinopeptides by thrombin. Cleavage of the fibrinopeptides in the central domain of the molecule exposes binding sites that are complementary to sites always present at each end of the molecules (1). These specific interactions result in the half-staggering of molecules to form two-stranded protofibrils, which then aggregate laterally to yield fibers. The characteristic 22.5-nm band pattern seen in electron micrographs of fibrin fibers indicates that the protofibrils align in register axially. Electron micrographs of cross sections offibers indicate that the side-to-side packing of protofibrils is disordered, although the average distance between neighboring protofibrils exhibits some regularity (ref. 3; J.W.W. and J. G. White, unpublished data).The diameter of fibrin fibers is an important component in determining the physical properties of the fibrin clot. Diameters of fibers from clots made un...

Objective-A splice variant of fibrinogen, ␥Ј, has an altered C-terminal sequence in its gamma chain. This ␥A/␥Ј fibrin is more resistant to lysis than ␥A/␥A fibrin. Whether the physical properties of ␥Ј and ␥A fibrin may account for the difference in their fibrinolysis rate remains to be established. Methods and Results-Mechanical and morphological properties of cross-linked purified fibrin, including permeability (Ks, in cm 2 ) and clot stiffness (GЈ, in dyne/cm 2 ), were measured after clotting ␥A and ␥Ј fibrinogens (1 mg/mL). ␥Ј/␥Ј fibrin displayed a non-significant decrease in the density of fibrin fibers and slightly thicker fibers than ␥A/␥A fibrin (12Ϯ2 fiber/10 Ϫ3 nm 3 versus 16Ϯ2 fiber/10 Ϫ3 nm 3 and 274Ϯ38 nm versus 257Ϯ41 nm for ␥Ј/␥Ј and ␥A/␥A fibrin, respectively; PϭNS). This resulted in a 20% increase of the permeability constant (6.9Ϯ1.7 10 Ϫ9 cm 2 versus 5.5Ϯ1.9 10 Ϫ9 cm 2 , respectively; PϭNS). Unexpectedly, ␥Ј fibrin was found to be 3-times stiffer than ␥A fibrin (72.6Ϯ2.6 dyne/cm 2 versus 25.1Ϯ2.3 dyne/cm 2 ; PϽ0.001). Finally, there was a 10-fold decrease of the fibrin fiber lysis rate. Key Words: coagulation Ⅲ fibrin Ⅲ fibrinogen Ⅲ fibrinolysis Ⅲ thrombosis H igh plasma fibrinogen is an independent predictor of cardiovascular events. 1 The resulting hypercoagulable state and the decrease of fibrinolysis rate related to the greater amount of fibrin formed are thought to be the major underlying mechanisms of thrombotic events. 2-4 Abnormal fibrin structure in vitro has been related to premature coronary artery disease in young patients and to severe thromboembolic disease, irrespective of the plasma fibrinogen concentration. [5][6][7][8] This so-called thrombogenic fibrin consists of numerous thin fibers organized in a tight three-dimensional network in which resistance to lysis arises directly from its architecture. 7,9,10 Recent in vitro investigations have revealed an association between defective fibrinolysis and the amount of ␥A/␥Ј fibrinogen incorporated into the clot. 4 This variant fibrinogen contains an altered ␥A chain termed ␥Ј and constitutes approximately 7% to 15% of the total fibrinogen found in plasma. 11 The ␥Ј chain, which arises from alternative processing of the ␥A chain mRNA, serves as a carrier for factor XIII. 12,13 Therefore, it has been suggested that ␥A/␥Ј affects the stability of the clot formed in vitro by concentrating and increasing the rate of factor XIII activation, which catalyzes the formation of isopeptide bonds between ␣ and ␥ chains of polymerizing fibrin strands. 4,13 The resulting extensive crosslinking is thought to be responsible for the lysis resistance and may account in part for the role of ␥A/␥Ј fibrinogen as an independent risk factor for coronary artery disease. 14,15 In addition, a recent study of ␥A/␥Ј fibrinogen levels in patients undergoing coronary angiography also showed that ␥A/␥Ј fibrinogen levels were higher, on average, in coronary artery disease patients than in patients without coronary artery disease; this association was independent ...