Collagen fibrils are the major mechanical component in the extracellular matrix of a broad range of multicellular animals from echinoderms to vertebrates where they provide a stable framework for tissues. They form the key tension-resisting element of a complex fiber-composite system that has a tissue-specific hierarchical structure linked to mechanical demands. Remarkably, these tissues are self-maintaining and avoid fatigue failure over the lifetime of the animal. Collagen fibrils can assemble spontaneously from purified solutions of collagen molecules. In developing tissues, however, in addition to the intrinsic self-assembly properties, there is cellular machinery that regulates fibril nucleation, spatial orientation, and fibril size, according to the tissue and stage of development. The intricate mechanisms underlying the generation of a collagen fibril network of defined architecture and mechanical properties are now becoming apparent. Impairment of this system leads ultimately to mechanical failure or tissue fibrosis.
Keywords: Connective tissue; Cornea; Extracellular matrix; Serial block face-scanning electron microscopy; Tendon.
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