Corpus: Cartilage

Cartilaginous tissue, or simply cartilage, is a type of connective tissue made up of specialized cells called chondrocytes and an extracellular ground substance.

Cartilage is found in various parts of the human body. As articular cartilage, it covers the surfaces of bones in movable joints (e.g., the knee and hip joints), enabling smooth, low-friction movement. Cartilage also forms the structure of intervertebral discs and menisci. In growing organisms, cartilage is present in the growth plates of bones (epiphyseal plates).

In humans, cartilage is located in the following regions:

Cartilage tissue is composed of cartilage cells (chondrocytes) and an extracellular matrix (ECM). The ECM consists of a ground substance and an organized network of collagen fibers.

Chondrocytes are surrounded by a highly basophilic matrix known as the "territorial matrix" or "cartilage courtyard." The matrix further away from the cells is called the "interterritorial matrix." The material immediately surrounding chondrocytes within the same group is known as the "pericellular matrix."

3.1. Cartilage cells

Chondrocytes are rounded cells responsible for maintaining the cartilage matrix. They are often found in small groups of 3 to 5 cells, referred to as isogenic groups, which originate from a single precursor cell (chondroblast). An isogenic group and its surrounding matrix are collectively known as a chondron.

Chondroblasts are derived from undifferentiated mesenchymal cells and are responsible for secreting the extracellular matrix. Once they stop dividing, they mature into chondrocytes. During cartilage growth, chondrocytes retain the ability to divide, but this capacity is lost in mature cartilage.

Chondroclasts are specialized cells that break down cartilage. Due to the isolation of chondrocytes within the ECM, damaged cartilage cannot be repaired by neighboring cells.

3.2. Cartilage matrix

The cartilage matrix contains up to 70 % water and is nearly devoid of blood vessels, nerves, or pain receptors. Nutrient exchange occurs through diffusion. The matrix primarily consists of collagen, elastin, and proteoglycans, though the exact composition varies among different types of cartilage.

Proteoglycans, which have numerous negatively charged components, attract sodium ions and, in turn, retain water within the cartilage tissue.

3.3. Cartilage types

Cartilage is categorized into three types based on the composition and structure of the extracellular matrix:

• Hyaline cartilage
• Elastic cartilage
• Fibrous cartilage

Cartilage is a flexible and elastic tissue that resists tearing and maintains stability under pressure. In hyaline cartilage, compressive elasticity arises from the formation of proteoglycan aggregates (aggrecan bound to hyaluronic acid) linked to collagen fibers (typically type II collagen).

Hyaluronic acid has a strong water-attracting property due to its negative charge and occupies a large volume in aqueous solutions. When combined with collagen fibers, it is compressed to about 20 % of its natural volume, creating a spring-like structure. Under mechanical pressure, water is expelled from the matrix, allowing the cartilage to compress. When the pressure is released, the spring-like structure decompresses and regains its original shape. However, with aging, the composition of proteoglycans and collagen changes, reducing cartilage elasticity.

Nutritional deficiencies or metabolic disorders affecting the cartilage can result in degeneration of the matrix and death of chondrocytes. This leads to the exposure of collagen fibers, a process known as "unmasking." Clinically, this manifests as osteoarthritis, a condition characterized by progressive cartilage breakdown and loss of joint function. Due to the significant impact of osteoarthritis on mobility, cartilage repair and regeneration are important areas of clinical research.