Classification and Structure of Cartilage

• Glycosaminoglycans (GAGs): These are large carbohydrates made of repeating disaccharide units. Because GAGs are hydrophilic (water-attracting), they pull water into the matrix, creating a gelatinous environment.
• Proteoglycans: These are formed when GAGs (such as chondroitin sulfate or keratin sulfate) bind to protein molecules.
• Hyaluronic Acid (Hyaluronate): This is a specific, very long GAG that acts as a backbone. Many proteoglycans associate with the hyaluronic acid to form a complex. This complex binds massive amounts of water, giving the matrix a stiff, viscous quality similar to "Jello".

Classification of Cartilage Types

Cartilage is classified into three distinct types based on its structure, fiber content, and location.

1. Hyaline Cartilage

Hyaline cartilage is the most abundant type in the body. It is characterized by a "glassy" or translucent appearance because its collagen fibers form an almost imperceptible network.

• Locations: It covers the articular surfaces of synovial joints, forms the fetal skeleton, makes up the respiratory ducts (trachea, larynx, nose), and is found in the costal cartilages of the ribs.
• Structure: It is surrounded by a perichondrium, a layer of dense irregular connective tissue. The matrix is divided into the territorial matrix (rich in GAGs, immediately surrounding chondrocytes) and the interterritorial matrix (rich in collagen, located further away).

2. Elastic Cartilage

Elastic cartilage is very similar to hyaline cartilage but contains a much higher concentration of elastic fibers.

• Function & Location: It is designed to maintain the shape of a structure while allowing for great flexibility. It is found in the external ear (pinna) and the epiglottis.

3. Fibrocartilage

Fibrocartilage is the toughest of the three, acting as a hybrid between dense regular connective tissue and cartilage. It contains thick Type I collagen fibers and chondrocytes often aligned in rows.

• Key Feature: Unlike the other two, fibrocartilage lacks a perichondrium.
• Function & Location: It provides immense tensile strength and absorbs compressive shock. It is found in the intervertebral discs, the pubic symphysis, and the menisci of the knee.

Growth and Clinical Relevance

How Cartilage Grows

Cartilage increases in size through two distinct processes:

1. Appositional Growth: New cartilage is added to the surface. Chondroblasts in the inner layer of the perichondrium secrete new matrix against the existing cartilage.
2. Interstitial Growth: New cartilage is formed from within. Chondrocytes inside the lacunae divide and secrete new matrix, causing the cartilage to expand from the inside out.

Exam Tip: Remember that articular cartilage (the cartilage on the ends of bones in joints) lacks a perichondrium. Therefore, it cannot grow via appositional growth and must rely on nutrition from synovial fluid through joint movement.

Clinical Application: Osteoarthritis

Osteoarthritis is a chronic condition often associated with aging. It involves the gradual loss or degradation of the hyaline cartilage that lines the articular ends of bones.

In weight-bearing joints like the hips and knees, "wear-and-tear" can release fragments of the cartilage into the joint space. These fragments trigger macrophages in adjacent tissues to release inflammatory factors and matrix metalloproteinases, which further damage the cartilage and cause pain.