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A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone[1] and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other muscles. Tendons and muscles work together and can only exert a pulling force.



Normal healthy tendons are mostly composed of parallel arrays of collagen fibres closely packed together. The dry mass of normal tendons, which makes up about 30% of the total mass with water, is composed of about 86% collagen, 2% elastin, 1–5% proteoglycans, and 0.2% inorganic components such as copper, manganese, and calcium.[2][3] The collagen portion is made up of 97-98% type I collagen, with small amounts of other types of collagen. These include type II collagen in the cartilaginous zones, type III collagen in the reticulin fibres of the vascular walls, type IX collagen, type IV collagen in the basement membranes of the capillaries, type V collagen in the vascular walls, and type X collagen in the mineralized fibrocartilage near the interface with the bone.[2][4] Collagen fibres coalesce into macroaggregates. After secretion from the cell, the terminal peptides are cleaved by procollagen N- and C-proteinases, and the tropocollagen molecules spontaneously assemble into insoluble fibrils. A collagen molecule is about 300 nm long and 1-2 nm wide, and the diameter of the fibrils that are formed can range from 50-500 nm. In tendons, the fibrils then assemble further to form fascicles, which are about 10 mm in length with a diameter of 50-300 μm, and finally into a tendon fibre with a diameter of 100-500 μm.[5] Groups of fascicles are bounded by the epitendon and peritendon to form the tendon organ.

The collagen in tendons are held together with proteoglycan components, including decorin and, in compressed regions of tendon, aggrecan, which are capable of binding to the collagen fibrils at specific locations.[6] The proteoglycans are interwoven with the collagen fibrils and that their glycosaminoglycan (GAG) side chains have multiple interactions with the surface of the fibrils, showing that the proteoglycans are important structurally in the interconnection of the fibrils.[7] The major glycosaminoglycan (GAG) components of the tendon are dermatan sulfate and chondroitin sulfate, which associate with collagen and are involved in the fibril assembly process during tendon development. Dermatan sulfate is thought to be responsible for forming associations between fibrils, while chondroitin sulfate is thought to be more involved with occupying volume between the fibrils to keep them separated and help withstand deformation.[8] The dermatan sulfate side chains of decorin aggregate in solution, and this behavior can assist with the assembly of the collagen fibrils. When decorin molecules are bound to a collagen fibril, their dermatan sulfate chains may extend and associate with other dermatan sulfate chains on decorin that is bound to separate fibrils, therefore creating interfibrillar bridges and eventually causing parallel alignment of the fibrils.[9]

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