Cartilage

Composition of Articular Cartilage

Articular cartilage is composed of different components that contribute to its structure and function. The table below outlines the wet and dry weight distribution of these components:

Component Wet Weight Dry Weight
Water 75% -
Collagen 20% 70%
Chondrocytes 5% -
Proteoglycans 10% -
Other - Glycoproteins, enzymes, extracellular ions

Structure of Articular Cartilage

Articular cartilage consists of individual chondrocytes embedded in an extracellular matrix, forming a 3D cartilaginous framework. It is organized into four layers:

  1. Superficial Gliding Zone
    • Thinnest layer with the highest collagen and water content.
    • Lowest proteoglycan content.
    • Collagen fibers run parallel to the surface, providing resistance to shear stress and high tensile stiffness.
    • Covered by Lamina Splendens, a protective film.
  2. Middle Transitional Zone
    • Collagen fibers arranged obliquely.
    • Acts as a transition zone for shear forces and compressive forces.
  3. Deep Radial Zone
    • Thickest zone with the highest proteoglycan concentration and low collagen content.
    • Collagen fibers are arranged perpendicular to the surface, making it resistant to compressive forces.
  4. Calcified Zone
    • Contains calcified cartilage and is anchored to subchondral bone by hydroxyapatite crystals.
    • Virtually no type 2 collagen; mostly type 10 collagen.

Collagen in Articular Cartilage

Collagen synthesis occurs both inside and outside chondrocytes, leading to the formation of collagen fibers. The synthesis process involves:

  • Formation of polypeptide chains within chondrocytes.
  • Assembly of a triple helix structure called Pro-collagen in the Golgi apparatus.
  • Release of pro-collagen into the extracellular matrix, where it forms Tropo-Collagen.
  • Tropo-Collagen crosslinks to form collagen fibers.

Type 2 Collagen is the most abundant, making up 90% of cartilage collagen. It is stable with a half-life of 25 years and provides tensile strength.

Other Types of Collagen: - Type 10 Collagen: Found in the calcified zone, associated with cartilage calcification. - Type 11 Collagen: Regulates fiber diameter and holds the lattice together. - Type 9 Collagen: Present on the cartilage surface. - Type 6 Collagen: Carries shear stresses and increases in osteoarthritis.

Proteoglycans

Proteoglycans are large, hydrophilic molecules that interact with water, providing cartilage with compressive strength. The most common proteoglycan is Aggrecan, which forms aggregates with hyaluronic acid, enhancing water retention and interactions with collagen.

Glycosaminoglycans (GAGs) are proteoglycan subunits: - Chondroitin Sulfate: Decreases with age but increases in osteoarthritis. - Keratin Sulfate: Increases with age.

Chondrocytes

Chondrocytes are derived from mesenchymal stem cells and differentiate into articular or physeal chondrocytes. Their primary functions include: 1. Collagen production. 2. Proteoglycan production. 3. Enzyme secretion for cartilage metabolism (metalloproteinases).

Nutrition and Nerve Supply

  • Avascular: Articular cartilage lacks blood vessels and receives nutrients through diffusion from the synovial fluid.
    • Loading of joints helps squeeze out waste, while unloading allows nutrient diffusion.
  • Aneural: Cartilage has no nerve supply.

Function of Articular Cartilage

  1. Joint Lubrication
  2. Shock Absorption

Articular cartilage has excellent biomechanical properties: - Low friction (more effective than joint replacements). - 10 times more efficient at shock absorption than bone.

Cartilage Lubrication

  • Fluid Film Lubrication: Dominant during movement, particularly elasto-hydrodynamic lubrication.
  • Boundary Lubrication: Acts during rest.

Pathological States of Cartilage

  • Superficial Lacerations: Do not heal due to avascularity.
  • Deep Lacerations: Heal by fibrocartilage formation but cannot bear repetitive loading as effectively as type 2 collagen.

Changes in Cartilage with Aging and Osteoarthritis

Parameter Aging Osteoarthritis
Water Content Decreases Increases
Synthetic Activity Decreases Increases
Collagen Content Decreases Decreases (concentration increases)
Proteoglycan Content Decreases Decreases
Chondrocyte Size Increases -
Chondrocyte Number Decreases -
Chondroitin: Keratin Ratio Decreases Increases
Stiffness (Young’s Modulus) Increases Decreases (due to high water content)

Osteoarthritis

Osteoarthritis develops due to two primary mechanisms: 1. Increased degrading enzyme concentration. 2. Direct trauma to cartilage.

The condition leads to:

  • Damaged collagen meshwork.
  • Increased water content and proteoglycan synthesis.
  • Compromised load
  • bearing ability, leading to cartilage fatigue and eventual degeneration.
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