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Tendon Structure and Architecture

Tendon is juxtaposed between muscle and bone and is responsible for transmitting muscular forces to the skeletal system during limb locomotion. It is a dense, regularly arranged connective tissue that is well suited for resisting tensile loads with minimal elongation during muscle contraction.

Approximately 85% of a tendon’s dry weight is composed of Type I collagen, which serves to provide the tendon with most of its tensile strength. Tendon gets most of its elasticity from the protein elastin, which accounts for less than 5% of a tendon’s total mass.

Surrounding the collagen fibers and elastin proteins is an extracellular matrix, or ground substance, that is rich in proteoglycans, glycosaminoglycans (GAGs), and a variety of other lipids and proteins. The extracellular matrix is important, both because it provides a structural support for the collagen framework and because its viscoelastic properties help to reduce the frictional and shear stresses that are placed on the tendon.

Fibroblasts can be found within the tendon ground substance and also are important, because they are capable of producing new collagen and ground substance materials, which are essential for maintenance and repair of the tendon.

The hierarchical arrangement of collagen fibers is similar to that of muscles described earlier in this chapter. Tendons are composed of densely organized collagen at various levels of complexity.

Tropocollagen is the basic building block of the Type I collagen found in tendon. The triple-helix polypeptide chain of tropocollagen is further organized into microfibrils, fibrils, fascicles, tertiary bundles, and finally, tendon itself. The entire tendon is sheathed in its epitenon that carries the nerve’s rich neurovascular supply.

A mesh of loose areolar connective tissue known as the paratenon often surrounds the epitenon, and together, they are known as the peritendon. Occasionally, tendons that traverse areas of increased frictional stress will possess a double-layer covering that is lined by synovial cells instead of the paratenon.

This synovial-lined covering is known as a tenosynovium. The flexor tendons in the forearm are examples of tendons possessing a tenosynovium. The endotenon lining of the tendon bundles is continuous with the perimysium of muscle at the myotendinous junction, and it adjoins the periosteum at the OTJ. Four zones in the

  • Tendon (fibers of Sharpey)
  • Fibrocartilage
  • Mineralized fibrocartilage
  • Bone

These four distinct zones are thought to reduce tendon loads by distributing the forces over a greater surface area of bone.

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