Cartilage Biology and Physiology

Cartilage Types & Basics - The Cartilage Crew

• Specialized connective tissue: avascular, aneural, alymphatic.
• Cells: Chondrocytes (reside in lacunae).
• Extracellular Matrix (ECM):
  • Water (~75% by weight).
  • Collagens (mainly Type II for hyaline & elastic; Type I & II for fibrocartilage).
  • Proteoglycans (e.g., aggrecan, which binds hyaluronan).
• Three Main Types:
  • Hyaline Cartilage (Type II collagen): Most abundant. Found in articular surfaces, nose, larynx, trachea, costal cartilages. Provides smooth, low-friction surfaces.
  • Elastic Cartilage (Type II collagen + Elastin fibers): Provides flexible support. Found in external ear, epiglottis, Eustachian tube.
  • Fibrocartilage (Type I & Type II collagen): Resists compression and shear forces.

    ⭐ Fibrocartilage, characterized by Type I and Type II collagen, is found in intervertebral discs, pubic symphysis, and menisci of the knee.

Hyaline Cartilage Structure - Articular Ace

• Primary Cell: Chondrocytes (in lacunae).
• Extracellular Matrix (ECM):
  • Fibers: Mainly Type II collagen (tensile strength).
  • Ground Substance:
    • Proteoglycans (Aggrecan, Hyaluronic Acid).
    • 80% water (resilience).
• Key Features: Avascular, Alymphatic, Aneural. Nutrition by diffusion (synovial fluid).
• Articular Cartilage Zones (Superficial to Deep):
  • Superficial (Tangential): Flat chondrocytes; parallel collagen (shear resistance).
  • Middle (Transitional): Round chondrocytes; random collagen.
  • Deep (Radial): Columnar chondrocytes; perpendicular collagen (compression resistance).
  • Calcified Zone: Tidemark; anchors to bone.

⭐ Aggrecan, a large proteoglycan in hyaline cartilage, binds to hyaluronic acid and is responsible for the tissue's osmotic properties and compressive stiffness.

Cartilage Metabolism & Nutrition - Chondrocyte Chow

⭐ Cartilage is avascular, aneural, and alymphatic; chondrocytes receive nutrition primarily via diffusion from synovial fluid, facilitated by joint loading.

• Nutrient Delivery:
  • Mainly from synovial fluid (glucose, amino acids, O₂).
  • Diffusion through matrix, enhanced by intermittent joint loading ("sponge effect").
• Chondrocyte Energy Source:
  • Predominantly anaerobic glycolysis due to low O₂ tension: $Glucose \rightarrow Pyruvate \rightarrow Lactate + ATP$.
• ECM Balance (Turnover):
  • Anabolism (Synthesis): Chondrocytes produce Type II collagen, aggrecan. Key stimulators: TGF-β, IGF-1.
  • Catabolism (Degradation): Mediated by MMPs & ADAMTS enzymes. Key inducers: IL-1, TNF-α.

Cartilage Biomechanics & Repair - Stress & Distress

• Biomechanics: Articular cartilage is a specialized connective tissue designed to withstand and distribute loads.
  • Type II collagen: Provides tensile strength and structural integrity.
  • Aggrecan (proteoglycan): Confers compressive stiffness by binding $H_2O$, creating turgor pressure.
  • Viscoelasticity: Exhibits creep (deformation under constant load) and stress-relaxation (stress reduction under constant deformation), crucial for shock absorption.
  • Lubrication: Boundary (lubricin) and fluid-film mechanisms minimize friction.
• Repair Limitations: Cartilage has a very limited capacity for self-repair.
  • Avascular, aneural, and alymphatic nature restricts access to progenitor cells and nutrients.
  • Chondrocytes: Low mitotic activity and are fixed within the matrix, preventing migration to injury sites.
  • Partial-thickness defects: Generally do not heal.
  • Full-thickness defects: May heal with fibrocartilage (Type I collagen), which is mechanically inferior to native hyaline cartilage (Type II).
• Stress & Distress (Degeneration):
  • Physiological loading: Essential for maintaining cartilage health and matrix homeostasis.
  • Abnormal/excessive loading or trauma: Leads to chondrocyte apoptosis, increased matrix metalloproteinases (MMPs) activity, and net matrix degradation, progressing to Osteoarthritis (OA).

⭐ The limited repair capacity of articular cartilage is primarily due to its avascular nature and the inability of chondrocytes to migrate to defect sites and proliferate effectively.

High‑Yield Points - ⚡ Biggest Takeaways

• Articular cartilage is hyaline, with Type II collagen (tensile strength) and aggrecan (compressive strength).
• It's avascular, aneural, alymphatic; nutrition via diffusion from synovial fluid.
• Chondrocytes are the sole cells, maintaining the extracellular matrix (ECM).
• Limited repair capacity due to avascularity and low chondrocyte division.
• Four zones: Superficial (tangential fibers), Middle, Deep (radial fibers, highest proteoglycans), Calcified.
• The tidemark separates deep uncalcified from calcified cartilage.