Cartilage: Structure, Types, and Function

Introduction to Cartilage

Cartilage is a specialized form of connective tissue that provides support and flexibility to various parts of the body. It is characterized by its firm, yet flexible matrix and the presence of specific cell types. Cartilage plays essential roles in the skeletal system, respiratory tract, and other organs.

• Key Functions: Provides structural support, reduces friction in joints, and serves as a template for bone development.

• Major Components: Cells (chondrocytes, chondroblasts, mesenchymal cells), fibers (collagen, elastic), and ground substance (proteoglycans, glycosaminoglycans).

General Description of Cartilage

Cells, Fibers, and Matrix

Cartilage consists of specialized cells embedded in an extracellular matrix composed of fibers and ground substance.

• Cells: Chondrocytes (mature cartilage cells), chondroblasts (immature, matrix-secreting cells), and mesenchymal stem cells (precursors).

• Fibers: Mainly type II collagen in hyaline cartilage, elastic fibers in elastic cartilage, and type I collagen in fibrocartilage.

• Matrix: Rich in proteoglycans and glycosaminoglycans, providing resilience and the ability to withstand compression.

Principal Cells Found in Cartilage

• Mesenchymal Cells: Undifferentiated stem cells that give rise to chondroblasts during development.

• Chondroblasts: Actively secrete cartilage matrix; found at the periphery of growing cartilage.

• Chondrocytes: Mature cartilage cells located in lacunae within the matrix; maintain the cartilage matrix.

Growth of Cartilage

Cartilage grows by two main mechanisms:

• Appositional Growth: New layers of cartilage are added to the surface by chondroblasts in the perichondrium.

• Interstitial Growth: Chondrocytes within the cartilage divide and secrete new matrix, expanding the tissue from within.

Types of Cartilage and Their Locations

There are three main types of cartilage, each with distinct structural and functional properties.

Hyaline Cartilage

Hyaline cartilage is the most common type, providing smooth surfaces for joint movement and structural support in the respiratory tract.

• Matrix: Contains type II collagen fibers and abundant proteoglycans.

• Locations: Articular surfaces of bones, nose, trachea, larynx, and fetal skeleton.

• Appearance: Glassy and translucent under the microscope.

• Perichondrium: Present except at articular surfaces.

Elastic Cartilage

Elastic cartilage provides flexible support due to the presence of elastic fibers in addition to type II collagen.

• Matrix: Contains elastic fibers and type II collagen.

• Locations: External ear (auricle), epiglottis, and Eustachian tube.

• Properties: More flexible and resilient than hyaline cartilage.

• Perichondrium: Present.

Fibrocartilage

Fibrocartilage is designed to resist compression and tension, combining properties of dense connective tissue and cartilage.

• Matrix: Contains dense bundles of type I collagen and some type II collagen.

• Locations: Intervertebral discs, pubic symphysis, menisci of the knee.

• Properties: Toughest type of cartilage; lacks perichondrium.

Cartilage Matrix: Structure and Composition

The extracellular matrix of cartilage is crucial for its mechanical properties and function.

• Collagen Fibrils: Provide tensile strength; type II in hyaline and elastic cartilage, type I in fibrocartilage.

• Proteoglycans: Large molecules composed of a core protein and glycosaminoglycan (GAG) chains; attract water, providing compressive resistance.

• Proteoglycan Aggregates: Formed by proteoglycans binding to hyaluronic acid; increase matrix viscosity and resilience.

• Territorial Matrix: Area immediately surrounding chondrocytes, rich in GAGs and less collagen.

• Interterritorial Matrix: Area between groups of chondrocytes, contains more collagen fibers.

Key Definitions

• Proteoglycan: A protein core with covalently attached GAG chains (e.g., chondroitin sulfate, keratan sulfate).

• Proteoglycan Aggregate: Multiple proteoglycan molecules attached to a hyaluronic acid backbone.

Nutrition and Metabolism of Cartilage

Cartilage is avascular, meaning it lacks blood vessels. Nutrients and waste products diffuse through the matrix.

• Nutrition: Diffusion from surrounding tissues, especially the perichondrium.

• Articular Cartilage: Lacks perichondrium; receives nutrients from synovial fluid in joints.

Perichondrium

The perichondrium is a dense layer of connective tissue that surrounds most cartilage types, providing nutrients and a source of new chondroblasts.

• Present in: Hyaline (except articular cartilage) and elastic cartilage.

• Absent in: Articular cartilage and fibrocartilage.

Cartilage Growth and Repair

• Appositional Growth: New cartilage is added to the surface by cells in the perichondrium.

• Interstitial Growth: Chondrocytes divide and secrete new matrix from within the cartilage.

• Stimulation: Growth factors, mechanical stress, and hormones can stimulate cartilage growth.

• Repair: Limited due to avascularity; mainly occurs via appositional growth.

Comparison of Cartilage Types

The following table summarizes the main differences among the three types of cartilage:

Clinical and Developmental Aspects

• Calcification: Cartilage can become calcified with age or during endochondral ossification (bone formation).

• Chondrosarcoma: A malignant tumor of cartilage cells, often found in the epiphysis of long bones.

• Development: Cartilage serves as a template for the formation of long bones during fetal development (endochondral ossification).

Example: Articular Cartilage

Articular cartilage covers the ends of bones in synovial joints, providing a smooth, lubricated surface for movement and absorbing shock. It lacks a perichondrium and receives nutrients from synovial fluid.

Example: Intervertebral Disc

The intervertebral disc is composed of fibrocartilage, providing both flexibility and strength to the vertebral column.

Additional info: Cartilage is essential for the growth and development of the skeletal system, and its unique properties make it critical for joint function and movement.