Cartilage: Structure and Function

Cartilage Definition

• A specialized connective tissue that develops from mesenchyme.
• Comprised of cells, an extracellular matrix (ECM), and ground substance.
• Avascular, meaning it receives nutrients through diffusion from the surrounding ground substance.
• Provides tensile strength, resilience against compression, and structural support for soft tissues.

Cartilage Cells

• Chondroblasts produce the ECM.
• Mature chondrocytes are entrapped within lacunae within the matrix.
• Perichondrium, a surrounding connective tissue layer, is chondrogenic, responsible for the generation of new chondroblasts.

Cartilage Matrix

• Produced and maintained by chondroblasts and chondrocytes.
• Ground substance contains hyaluronic acid, sulfated glycosaminoglycans (GAGs), and proteoglycans, making it highly hydrated.
• The proportion of collagen (Type I or II) and elastic fibers determines the cartilage type:
  • Hyaline (Type II collagen)
  • Elastic (Type II collagen)
  • Fibrocartilage (Type I collagen)
• Collagen and elastic fibers contribute to the cartilage's firmness.
• Acts as a shock absorber due to its semi-rigid nature.

Perichondrium

• Most hyaline and elastic cartilage is enclosed by perichondrium, a layer of vascularized dense irregular connective tissue.
• Has two layers:
  • Outer fibrous layer containing type I collagen fibers and fibroblasts.
  • Inner cellular layer containing chondrogenic cells.
• Chondrogenic cells differentiate into chondroblasts, responsible for secreting the cartilage matrix.
• Fibrocartilage and hyaline cartilage on joint surfaces lack perichondrium.

Hyaline Cartilage

• Most prevalent cartilage type.
• In embryos, it forms the skeletal model for bones, undergoing endochondral ossification during growth.
• In adults, it persists in articular surfaces of bones, rib ends, larynx, nose, trachea, bronchi, and other locations, without calcification.
• Matrix contains type II collagen fibers, proteoglycans, and glycoproteins.
• Contains chondronectin, an adhesive glycoprotein that links glycosaminoglycans and collagen fibers, facilitating chondroblast and chondrocyte adherence to collagen.
• Surrounded by perichondrium with fibroblasts.
• The inner chondrogenic layer produces chondroblasts, which differentiate into chondrocytes.
• Chondrocytes, responsible for maintaining cartilage, reside in lacunae.
• Lacunae can house individual chondrocytes or groups of chondrocytes (isogenous groups).
• Two types of matrix between lacunae exist:
  • Territorial matrix (1) stains darker.
  • Interterritorial matrix (2) stains lighter due to a lower concentration of proteoglycans.