Lecture Notes: Bone Formation and Remodeling

Introduction to Bone Formation

• Bone, like skin, forms before birth and continuously renews itself.

Bone Remodeling

• Definition: Ongoing replacement of old bone tissue with new bone tissue.
• Processes Involved:
  • Bone Resorption: Removal of minerals and collagen by osteoclasts.
  • Bone Deposition: Addition of minerals and collagen by osteoblasts.
• At any time, about 5% of total bone mass is being remodeled.
• Renewal rates:
  • Compact bone: ~4% per year
  • Spongy bone: ~20% per year
• Remodeling occurs at different rates in different body regions.
  • Influenced by injury, exercise, hormones, and diet.

Importance of Bone Remodeling

• New bone is more resistant to stress and fractures.

Factors Affecting Bone Metabolism

• Minerals:
  • Large amounts of calcium and phosphorus are essential.
  • Small amounts of magnesium, fluoride, and manganese are needed.
  • Minerals form hydroxyapatite crystals in the bone matrix.
• Hormones:
  • Parathyroid hormone (PTH) increases osteoclast activity to regulate blood calcium.
  • Calcitonin inhibits osteoclast activity when blood calcium is high.

Role of Vitamins

• Vitamin A: Stimulates osteoblast activity.
• Vitamin C: Essential for collagen synthesis.
• Vitamin D: Promotes calcium absorption in the gastrointestinal tract.

Hormonal Influence on Bone Growth

• Growth Hormone: Stimulates osteoblasts and cell division for bone lengthening.
• Sex Hormones (Estrogen and Testosterone):
  • Stimulate osteoblast activity and bone matrix production.
  • Responsible for growth spurts in adolescence.
  • Contribute to growth plate closure at adolescence end.
  • Post-menopause decrease in estrogen increases osteoporosis risk.

Regulation of Calcium

• PTH is released when blood calcium is low, stimulating bone resorption.
• Calcitonin is released when blood calcium is high, inhibiting resorption.

Mechanical Stress and Bone Adaptation

• Wolf's Law: Bone adapts to mechanical loads placed on it.
• Increasing mechanical stress stimulates osteoblast activity.
• Lack of stress can lead to increased bone resorption.

Examples of Mechanical Stress Impact

• Astronauts experience bone density reduction after no-gravity exposure.
• Weightlifters show improved bone density due to training.
• Tennis players have higher bone density in the dominant arm.