Influence of factor XIII and fibronectin on fiber size and density in thrombin-induced fibrin gels.

The possible role of fibronectin as a molecular anchor for fibrin to ground substance, the ability of factor XIII to cross-link fibronectin to fibrin, and the demonstrated differences between purified fibrin and fibrin formed in plasma prompted this study of the effect of both proteins on fibrin assembly and structure. The influences of activated factor XIII (FXIIIa) and fibronectin were studied by use of turbidity techniques. Fibronectin over a concentration range of 0 to 800 micrograms/ml produced no change in either the mass-length ratio or density of fibrin fibers in 1 mg/ml fibrin gels. FXIIIa at concentrations as high as 6.5 mumol/L, although not altering fiber mass-length ratio, caused a 30% increase in fiber density. Gels formed in the presence of fibronectin plus FXIIIa demonstrated increasing fiber mass-length ratios and increasing fiber density with increasing fibronectin concentration. Mass-length ratios for gels formed in the presence of 3.24 mumol/L FXIIIa increased from 3.25 to 5.20 x 10(13) daltons/cm as the fibronectin concentration increased from 0 to 800 micrograms/ml. Fiber density increased from 13.6 to 19.1 x 10(22) daltons/cm3 over the same fibronectin range. These results imply that FXIIIa-induced structural changes such as increased elastic modulus may be mediated in part by increased fiber density. Fibronectin alone has virtually no impact on fibrin assembly, but in the presence of FXIIIa becomes incorporated into fibrin and results in increased fiber size and density.