Subunits of type I collagen, the most abundant collagen, assemble to form extremely strongfibrils, which are then bundled together further by other collagens into much larger structurescalled collagen fibers.(a) TEM shows fibrils cut longitudinally and transversely. In longitudinal sections fibrilsdisplay alternating dark and light bands; in cross section the cut ends of individual collagenmolecules appear as dots. Ground substance completely surrounds the fibrils. X100,000.(b) The large bundles of type I collagen fibrils (C) appear as acidophilic collagen fibers inconnective tissues, where they may fill the extracellular space. Subunits for these fibers weresecreted by the fibroblasts (arrows) associated with them
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