Tendon

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Overview of dense connective tissue.

Tendons are dense connective tissue structures, composed of an hierarchy of longitudinally arranged collagen fibers, elastin, glycoproteins, proteoglycans and a lesser amount of specialized fibroblast cells. 

Type 1 collagen is the most abundant form of collagen identified within tendinous structures and is directly responsible for its strength and durability. Collagen fibres lie in a longitudinal fashion, parallel to the longitudinal axis of the tendon. They are packed firmly together and arrange in an undulating pattern, forming crimp-like fibrils which produce a robust structure of high-tensile strength.

Located adjacent to the collagen fibers are the specialized cells of the tendon: the tenoblasts and tenocytes. While tenoblasts and tenocytes are vitally important in the maintenance of tendon health, they are relatively few and far between. Tendons are considered to be hypocellular tissues with tenoblasts and tenocytes residing in <5% of the total volume of a tendon.

The hierarchal organization of tendons from smallest to largest is as follows. A group of collagen fibrils forms a collagen fiber. A group of collagen fibers forms a primary fiber bundle. A group of primary fiber bundles then forms a secondary fiber bundle. A group of secondary fiber bundles then forms a tertiary bundle. Finally, all of the tertiary fiber bundles grouped together make up the tendon.

Surrounding bundles of collagen fibers and forming the primary outer covering of the tendon is a thin connective tissue sheath known as the epitenon. This sheath aids in facilitating gliding movements between collagen bundles and contains the nerves and blood vessels which provide the extrinsic neurovascular supply to the tendon. The epitenon extends deep into the varying hierarchies of collagen fibers, forming the endotenon which is a fine sheet of connective tissue that wraps around the individual bundles of collagen fibers.