Notes on Tendon Injuries and Structure

Tendon Injuries and Structure

Learning Objectives

  • Define, explain, describe, and compare the following key components of tendons:
    • Structural components of tendons:
    • Protein fibers (collagen, elastin)
    • Non-fiber proteins (glycoproteins)
    • Proteoglycans (PGs) and cells
    • Extracellular matrix (ECM):
    • Composition and function
    • Role in tendon structure and loading response
    • Elastin and elastin fibers:
    • Function and significance
    • Ground substance composition:
    • Proteoglycans and glycosaminoglycans (GAGs)
    • Proteoglycan types:
    • Differences between aggrecan and versican
    • Function in tendon protection and structure
    • Tenocytes:
    • Function and reaction to mechanical load

Extracellular Matrix (ECM)

  • Definition: Space surrounding cells that provides structural and functional support.
  • Categories:
    • Protein Fibers:
    • Collagen (65-80% of dry weight)
    • Elastin (1-2% of dry weight)
    • Non-Fiber Proteins:
    • Glycoproteins that do not form fibers.
    • Ground Substance:
    • Contains proteoglycans, glycosaminoglycans (GAGs) (0.2-0.5% of dry weight), and inorganic components (0.2% of dry weight).
  • Cells:
    • Tenocytes (90-95% of cellular content): Responsible for ECM production and maintenance.
    • Other cell types (5-10% of cellular content): Less involved in ECM production.
  • Function of ECM: Connective tissue support, slower healing due to metabolism.

Elastin and Collagen

  • Elastin:
    • Comprises 1-2% of dry weight; provides flexibility and resilience in tendons.
  • Collagen:
    • Predominantly type I collagen in tendons; provides tensile strength.

Tendon Glycoproteins

  • Function:
    • Adhesive roles (e.g., fibronectin, laminin)
    • Regulates cell-matrix interactions (e.g., tenascins, thrombospondins)

Ground Substance

  • Characteristics:
    • Gel-like medium, consisting of 60-80% water, extremely hydrophilic.
    • Major macromolecules include proteoglycans and GAGs.

Proteoglycans (PGs)

  • Definition: Macromolecules with core proteins and long chains of glycosaminoglycans (GAGs).
  • Function:
    • Form a gel-like substance and are vital for holding water, providing resilience and hydration in tissues.
  • Family Types:
    • Hyaluronan: Non-covalent and lacks sulfates.
    • Chondroitin sulfate: Most prevalent GAG, essential for cartilage lubrication.

Glycosaminoglycans (GAGs)

  • Structure: Long unbranched chains composed of repetitive disaccharide units; allow binding of water which provides swelling pressure.
  • Examples:
    • Uronic acids (glucuronate, iduronate)
    • N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc)

Regional Composition of Tendons

  • Different regions: Mid-tendon, myotendinous junction (MTJ), and osteotendinous junction (OTJ).
    • Mid-Tendon: Dominantly type I collagen, with minimal type III, V, and VI.
    • Myotendinous Junction (MTJ): Increased expression of type II collagen.
    • Osteotendinous Junction (OTJ): Contains significant amounts of proteoglycans such as aggrecan.

Cells in Tendons

  • Tenocytes:
    • Roles: Maintain ECM, respond to mechanical load, and involved in matrix synthesis.
    • Types: Tenoblasts (immature, active) and tenocytes (mature).

Mechanical Loading Response

  • Pathological Response:
    • Dysregulated signaling leading to matrix degradation and injury.
  • Adaptation Response:
    • Repair and healing through appropriate loading and regeneration mechanisms.
  • Key Elements:
    • Mechanical loads initiate cellular responses, including cytokine release and neovascularization.

Chronic Tendinopathy

  • Features of Dysregulation:
    • Normal collagen architecture altered, increased ground substance, neovascularization.
  • Pathology Continuum:
    • Normal tendon to degenerative changes characterized by disruptions in the ECM and collagen structure.