Cartilage Biology

Cartilage Characteristics

• Cartilage is a tough, resilient type of connective tissue that provides structural support and cushioned, low-friction surfaces in joints.
• Cartilage is mainly composed of a flexible mass of extracellular matrix (ECM), with cells (chondrocytes) making up a small percentage of the tissue's mass.

Cartilage Structure

• Chondrocytes are embedded within lacunae surrounded by the ECM.
• Cartilage ECM typically includes collagen and abundant proteoglycans, such as aggrecan, which bind a large amount of water.

Cartilage Features

• Cartilage lacks blood vessels, lymphatics, and nerves.
• Cartilage is usually surrounded by a dense connective tissue perichondrium that is vascularized.

Types of Cartilage

• There are three major forms of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage.

Hyaline Cartilage Structure

• ECM is homogenous and glassy, composed of type II collagen fibrils and aggrecan complexes with bound water.
• Territorial matrix around lacunae has less collagen and more proteoglycan, resulting in slight staining differences.

Chondrocytes

• Occur singly or in small, mitotically derived isogenous groups.

Perichondrium

• Typically present, except in hyaline cartilage of articular surfaces and epiphyses of growing long bones.

Elastic Cartilage

• Similar to hyaline cartilage in terms of chondrocytes and major ECM components
• Matrix contains abundant elastic fibers, visible with special stains
• Elastic fibers increase the tissue's flexibility
• Provides flexible support for:
  • External ear
  • Certain structures of the middle ear
  • Larynx
• Always surrounded by perichondrium

Fibrocartilage

• Composed of varying combinations of hyaline cartilage and dense connective tissue
• Histologically characterized by small chondrocytes in a hyaline matrix
• Features layered structure with larger areas of bundled type I collagen
• Scattered fibroblasts present throughout the tissue
• Provides strong, tough support in specific locations

Functions and Locations

• Found at tendon insertions
• Present in intervertebral discs
• Found in certain other joints

Embryonic Mesenchyme and Cartilage Development

• Embryonic mesenchyme is the precursor tissue of all types of cartilage.
• Mitotic proliferation of mesenchymal cells leads to early differentiation, resulting in a tissue with condensations of rounded cells called chondroblasts.
• Chondroblasts produce various matrix components, which collectively swell with water and form a large amount of extracellular matrix (ECM).
• The matrix components secreted by chondroblasts separate them from one another.
• The multiplication of cartilage cells gives rise to isogenous aggregates.
• Each isogenous aggregate is surrounded by a condensation of territorial matrix.