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fibers of the extracellular matter of connective tissue in animals and humans, consisting mainly of the protein collagen. Collagen fibers 1–12 microns (μ) thick are represented by unbranched threads, or fibrils (diameter 0.3–0.5 μ). The fibrils consist of microfibrils (400 angstroms [Å] thick), which have periodically alternating (640 Å) dark and light sections as a result of the regular distribution along them of molecules of procollagen, or tropocollagen. Microfibrils, from which the fibrils and collagen fibers are made up, form outside the cells by polymerization of tropocollagen, which is secreted by collagen-forming cells (fibroblasts, cartilage, bone, and so on). Collagen fibers are distinguished by great tensile strength and low elasticity; they fulfill a mechanical function. Collagen fibers in cartilage are called chondrin fibers, and fibers from bone are called ossein fibers.