Biochemistry of connective tissue (cartilage, bone).

Introduction

Connective tissue provides structural support, connects tissues and organs, and plays a role in storage, repair, and defense mechanisms. Two specialized types of connective tissue—cartilage and bone—have unique biochemical compositions and functions. Their extracellular matrix is rich in specific proteins and polysaccharides that define their mechanical and physiological properties.

Cartilage

Cartilage is a flexible, avascular connective tissue composed of chondrocytes embedded in an extracellular matrix.

Main Components:

- **Collagen (mostly type II)**: Provides tensile strength.

- **Proteoglycans (especially aggrecan)**: Contain glycosaminoglycans (GAGs) like chondroitin sulfate and keratan sulfate that retain water and resist compression.

- **Hyaluronic acid**: Forms aggregates with proteoglycans to enhance hydration and resilience.

Types of Cartilage:

- Hyaline cartilage: Found in joints, trachea, and nose.

- Elastic cartilage: Contains elastin fibers; found in ear and epiglottis.

- Fibrocartilage: Rich in type I collagen; found in intervertebral discs and menisci.

Bone

Bone is a rigid, mineralized connective tissue. It serves mechanical, protective, and metabolic functions including calcium and phosphate storage.

Matrix Composition:

- **Organic matrix (~30%)**:

  - Type I collagen: Provides tensile strength.

  - Proteoglycans (e.g., decorin) and glycoproteins (e.g., osteonectin, osteocalcin).

- **Inorganic matrix (~70%)**:

  - Hydroxyapatite crystals [Ca₁₀(PO₄)₆(OH)₂]: Provide compressive strength.

Bone Cells:

- Osteoblasts: Synthesize bone matrix.

- Osteocytes: Maintain bone tissue.

- Osteoclasts: Resorb bone, important for remodeling.

Biochemical Processes in Cartilage and Bone

Cartilage:

- Limited regenerative capacity due to lack of blood supply.

- ECM synthesis regulated by chondrocytes and influenced by mechanical load, cytokines, and growth factors.

Bone:

- Bone remodeling is a dynamic process involving formation by osteoblasts and resorption by osteoclasts.

- Regulated by hormones (PTH, vitamin D, calcitonin), cytokines (RANKL), and mechanical stimuli.

- Alkaline phosphatase from osteoblasts is essential for mineralization.

Pathological conditions like osteoarthritis and osteoporosis involve disturbances in these biochemical pathways.

Conclusion

The biochemical composition and metabolism of cartilage and bone are crucial for their structural and functional roles in the body. Understanding these processes is essential for diagnosing and treating disorders such as arthritis, osteoporosis, and metabolic bone diseases.

References

1. Guyton and Hall Textbook of Medical Physiology, 14th Edition

2. Lehninger Principles of Biochemistry, 7th Edition

3. Lippincott Illustrated Reviews: Biochemistry, 7th Edition