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The fibroblast cells in the tendon ground substance produce collagen, but first, they produce a large intracellular collagen precursor, procollagen. Once the procollagen has been secreted into the extracellular matrix, it can be cleaved by procollagen peptidases to form the triple-helix molecule known as tropocollagen.

The tropocollagen molecules are then aligned in an orderly fashion in the extracellular environment and undergo polymerization with each other to form collagen fibrils.

The architecture of collagen has been studied extensively and is responsible for the physical properties and tensile strength of tendon. When viewed with electron microscopy, collagen fibrils exhibit a regularly repeating, band-like pattern every 64 to 68 nm.

This banding pattern is a result of an organized, repetitive packing of individual triple-helix collagen molecules into an overlapping arrangement that results in a rigid, rodlike fibril. Type I collagen accounts for the vast majority of human tendon mass.

The collagen molecule spans approximately 280 nm and is 1.5 nm in thickness. Individual collagen molecules are composed of three primary amino acids: glycine, which accounts for every third amino acid in the chain (~33%); proline (15%); and hydroxyproline (15%) .

Three α-peptide chains make up the triple-helix structure of the Type I collagen molecule: two α1 chains, and a single α2 chain. The polymerization of these three chains into a triple-helix arrangement gives the molecule greater rigidity than either of the two types of collagen α-chains alone. The collagen fibrils are arranged into fibers that run parallel to the long axis of the tendon, imparting excellent tensile strength to the tissue.

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