Investigation of Fibrin Fiber Internal Structure

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The major structural component of a blood clot is a meshwork of about 150 nm thick fibrin fibers. It is well understood how fibrin monomers assemble into the double-stranded, half-staggered protofibrils. However, how these protofibrils laterally assemble to form mature fibers is poorly understood. There is evidence that fibrin fibers are very porous with a protein content of only 20-30%. We performed two types of experiments to investigate the internal structure of fibrin fibers. We formed fibrin fiber from fluorescently labeled fibrinogen and determined the light intensity of a fiber, which is proportional to the number of monomers, as a function of fiber diameter. We found that the intensity, I, scaled as I∼D1.43±0.2 (wet fibers) and as I∼D1.2±0.14 (dry fibers). This implies that cross-sectional monomer density also scales as D1.4, and not as D2, as would be expected for fiber with a solid, homogeneous cross-section. We also determined the Young's modulus, E, as a function of fiber diameter, and found that E scales as E∼D−1.4. Thus, E decreases dramatically with diameter. These modulus data suggest that the number of bonds per cross-section scales as D0.6, consistent with a fiber model that has a dense core and a very loosely connected periphery.In summary, our data suggest that fibrin fibers have a very inhomogeneous cross-section with a dense core and a very loose periphery.This work was support by the Wake Forest University Translational Science Center, grant numbers: U01508, U01078.