Bone Remodeling

Overview

• Bone Mass Recycling: 5-7% recycled weekly.
  • Spongy bone replaced every 3-4 years.
  • Compact bone replaced every 10 years.
• Processes Involved:
  • Resorption: Breaking down bone.
  • Deposit: Building up new bone.

Bone Remodeling Components

• Osteoblasts: Mediate bone deposit.
• Osteoclasts: Mediate bone resorption.
• Occur at surfaces of periosteum and endosteum.
• Remodeling Units: Packets of osteoblasts and osteoclasts coordinating the process.

Bone Deposit

• New bony matrix deposited by osteoblasts.
• Osteoblasts create new osteoid.
• Calcification Front: Mineralized osteoid due to calcium-binding proteins and collagen.
• Triggers:
  • Mechanical signals.
  • Increased calcium and phosphate concentrations.
  • Matrix proteins and alkaline phosphatase.

Bone Resorption

• Breakdown of bone mediated by osteoclasts.
• Osteoclasts dig grooves in the matrix.
• Use lysosomal enzymes and protons to digest bone material.
• Acidity: Converts calcium salts to soluble forms.
• Digested products released into blood after transcytosis.
• Osteoclasts undergo apoptosis after resorption.

Hormonal Regulation

• Parathyroid Hormone (PTH): Stimulates osteoclasts for bone resorption.
• Calcitonin: Stimulates bone deposit at high doses.
• Leptin and Serotonin:
  • Leptin inhibits osteoblasts.
  • Serotonin, primarily affecting mood, also influences osteoblast activity.

Calcium Homeostasis

• Importance: Nerve transmission, muscle contraction, blood coagulation, and cell division.
• Major calcium storage is in the bones.
• Intestinal absorption of calcium requires vitamin D.

Mechanical Stress

• Wolf's Law: Bones grow/remodel in response to stress demands.
• Stress results in thicker regions where stress is greater.
• Example: Thicker femur bones due to body weight stress.
• Handedness: Results in thicker bones on the dominant side.
• Curved Bones: Thickest at points likely to buckle.

Additional Notes

• Fetal and Bedridden Bones: Featureless due to lack of stress.
• Electrical Signals in Remodeling: Produced by movement of fluids in bones.
• Hormones affect when and how remodeling occurs.
• Mechanical stress affects where remodeling occurs.