Cartilage Histology and Classification

Cartilage Histology

• Cartilage is a specialized connective tissue with a rubbery, rigid, and flexible matrix, distinctive cells, and a unique fiber population.
• Cartilage develops from mesenchyme.
• Unlike other connective tissues, cartilage lacks blood vessels, nerves, and lymphatics, making it susceptible to pressure and relying on diffusion for nutrients.
• Cartilage's primary functions include supporting soft tissues and assisting in the development and growth of long bones.

Cartilage Classification

• Cartilage is categorized into three types based on fiber abundance and type within the ground substance:
  • Hyaline cartilage
  • Elastic cartilage
  • Fibrocartilage

Hyaline Cartilage

• The most common type of cartilage.
• Perichondrium: A connective tissue membrane surrounding hyaline cartilage except in joint cavities. The perichondrium has two layers:
  • Outer fibrous layer: Dense connective tissue rich in type I collagen fibers, fibroblasts, and a few blood vessels.
  • Inner vascular and cellular layer (chondrogenic layer): Contains chondroblasts, responsible for new cartilage formation.
• Cells:
  • Chondroblasts:
    • Immature cartilage cells originating from undifferentiated mesenchymal cells.
    • Found in the inner chondrogenic layer of the perichondrium, arranged singly and parallel to the surface.
    • They synthesize the specialized extracellular matrix of cartilage.
    • As chondroblasts mature, they become enclosed within lacunae, transforming into chondrocytes.
  • Chondrocytes:
    • Mature cartilage cells found within lacunae.
    • Superficial chondrocytes are small, flat, and aligned parallel to the surface.
    • Deep chondrocytes (mature or old) are larger, rounded, oval, or triangular and arranged perpendicular to the surface.
    • Chondrocytes can divide once or twice within lacunae, forming cell nests. Mature chondrocytes lack the ability to divide.
    • Chondrocytes synthesize and secrete the extracellular matrix.
    • Old chondrocytes secrete alkaline phosphatase.
• Matrix:
  • Appears homogeneous and basophilic under the light microscope.
  • Composed of a complex mixture of glycosaminoglycans (hyaluronic acid and chondroitin sulfate) and structural glycoproteins (chondronectin).
  • The basophilia is attributed to the presence of glycosaminoglycans.
  • Contains type II collagen and elastic fibers, but they are too thin to be visible with a light microscope.
• Sites of hyaline cartilage:
  • Costal cartilage
  • Long bones of the fetal skeleton
  • Articular cartilage
  • Trachea and bronchi

Elastic Cartilage

• Resembles hyaline cartilage, with a perichondrium present.
• However, the matrix contains abundant yellow elastic fibers.
• Sites:
  • Ear pinna
  • External auditory meatus
  • Eustachian tube
  • Epiglottis

Fibrocartilage

• Lacks a perichondrium (intra-articular).
• The matrix contains abundant parallel collagen fibers type I, separated by rows of chondrocytes.
• Provides significant strength and flexibility.
• Sites:
  • Symphysis pubis
  • Intervertebral discs
  • Interarticular cartilages

Cartilage Histogenesis

• Cartilage growth is influenced by hormones and vitamins.
• Occurs through two processes:
  • Interstitial growth: Cell division of pre-existing chondrocytes.
    • Occurs primarily during the early stages of cartilage formation and in articular cartilage.
  • Appositional growth: Differentiation of chondrogenic cells in the perichondrium.
    • Results in the formation of chondroblasts and new chondrocytes, which secrete new cartilage matrix at the periphery.

Cartilage Degeneration

• Occurs when chondrocytes undergo hypertrophy and die, and the matrix becomes calcified.
• A common part of endochondral bone formation.

Cartilage Repair

• Cartilage has a limited ability to repair after injury due to:
  • Avascularity
  • Immobility of chondrocytes
  • Limited ability of mature chondrocytes to proliferate.
• Extensive damage results in fibrous tissue repair instead of cartilage regeneration.

Medical Application: Osteoarthritis

• A degenerative joint disease commonly characterized by chronic joint pain and varying degrees of joint deformity.
• Involves the destruction of articular cartilage.
• Characterized by a reduction in proteoglycan content, leading to decreased intercellular water content within the cartilage matrix.
• Chondrocytes contribute to osteoarthritis pathology.
  • They produce interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α), stimulating the production of metalloproteinases.
  • This process simultaneously inhibits the synthesis of type II collagen and proteoglycans by chondrocytes.