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Tendons

Spot Knowledge

  • Role: Transmit muscle force to bone → joint motion, stability, elasticity, energy storage

  • Composition:
    Water:     65–80%
    Collagen: ~86% dry weight (mainly type I) → tensile strength
    Proteoglycans: ~1% (decorin, biglycan, fibromodulin) → fibril organization, load-bearing
    Elastin: <2% → elasticity, key in Achilles tendon

  • Cells: Mainly tenocytes (fibroblasts), few tenoblasts; ECM synthesis

  • Structure: Hierarchical collagen arrangement (molecule → fibril → fascicle → tendon)

  • Vascularity: Limited; nutrition via diffusion; low healing capacity

Tendon: Structure, Components, and Functions


General Features


Tendons transmit muscle force to bone, enabling joint motion. Their primary roles include force transmission, energy storage, and providing elasticity. They also contribute to joint stability.


Structural Composition


Tendons are composed of 65–80% water. Approximately 86% of the dry weight is collagen. Proteoglycans constitute 1%, and elastin fibers account for less than 2%, contributing to elasticity.


Cellular Composition


Tendons are sparsely cellular tissues. The main cell type is the fibroblast (tenocyte), which synthesizes collagen, proteoglycans, and other ECM components. Tenoblasts are immature, more metabolically active cells. Surrounding the tendon are the epitenon and endotenon, which contain blood vessels, nerves, and lymphatics.


Collagen


Type I collagen is the predominant fibrillar type. Its molecular structure is rich in glycine (33%), proline (15%), and hydroxyproline (15%). Collagen fibrils are organized hierarchically: molecule → microfibril → subfibril → fibril → fascicle → tendon. This organization provides high tensile strength. Type III collagen increases during repair.


Proteoglycans


Proteoglycans constitute ~1% of the dry weight. Due to their water-binding capacity, they enhance mechanical strength. Decorin, biglycan, and fibromodulin are the major proteoglycans, playing roles in fibril organization and load-bearing.


Elastin


Elastin accounts for less than 2% of tendon content. Located between fascicles, it provides elasticity. It is particularly important in energy-storing tendons such as the Achilles tendon.


Vascularity


Tendons are largely avascular, with nutrition primarily achieved via diffusion. Blood vessels usually enter through the epitenon and endotenon. This limited vascularity contributes to their restricted healing capacity.


Functional Properties

  • Force      transmission: Transfers muscle contraction forces to bone.

  • Energy      storage: Stores and releases energy during movement due to elastic      properties.

  • Mechanical      strength: Resists tensile and loading forces.

  • Flexibility      and adaptation: Supports continuous motion and joint stability.

Clinical Relevance

  • Tendon      injuries heal slowly.

  • Limited      vascularity restricts nutrition and repair.

  • Alterations      in collagen and proteoglycan content are associated with degenerative      tendon disorders.

  • Repetitive      microtrauma can lead to tendinopathy.

References

1. Zhang H, Wang Y, Li B, Chen S. Tendon mechanobiology in the context of tendon biofabrication. Front Bioeng Biotechnol. 2025;13:1560025. doi:10.3389/fbioe.2025.1560025

2. Massey JH, Shearer T, Hazel A. A microstructural model of tendon failure. arXiv. 2021;2103.04844.

3. Mackey AL, Kjaer M. Structure-function relationships in tendons: a review. Int J Mol Sci. 2023

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