BPS2026 – Fibrin fiber progression from monomers to mature fibers

Understanding fibrin polymerization is central to explaining how clots form and stabilize. Classical models posit that monomers first assemble into linear protofibrils that later aggregate laterally, but much of this view comes from highly diluted or extensively washed preparations that disrupt the native network and obscure the gel point. Here, we combine minimal-wash fixation with critical-point drying to preserve intact fibrin networks for atomic force and scanning electron microscopy at defined time points. In parallel, we performed dynamic fluorescence light microscopy with optical sectioning to identify the gel point and quantify a post-gel rise in network intensity. Time-stamped snapshots show that polymerization begins as a disordered, loosely connected mesh rich in free ends and stochastic branching, where transient local clusters promote cooperative growth and enable fibers to reorganize into straighter, longer, and thicker structures, while others remain relatively isolated. As gelation approaches (∼4 min under our conditions), the network acquires a more continuous and straighter backbone; after gelation, the continued increase in intensity and organization is consistent with ongoing lateral aggregation, via incorporation of fibrin oligomers that thicken and align fibers. This framework does not require protofibrils to reach a set length before branching or aggregation. Early branching and transient clustering organize a dynamic, loosely connected mesh with ongoing fiber exchange that guides growth toward the gel point. At more physiological concentrations, fiber-fiber interplay appears pivotal for network formation and helps explain clot mechanics and thrombosis.