Notes on Bone Growth and Development

Notes on Bone Growth and Development

Bone Growth and Remodeling:

  1. Balance of Bone Formation and Resorption:

    • Bone growth relies on a balance between osteoblast (formation) and osteoclast (resorption) activity.

    • Osteocytes in adults maintain the matrix, replacing calcium salts regularly.

    • Remodeling ensures almost 20% of the skeleton is replaced annually in young adults.

    • Turnover rates vary regionally, e.g., spongy bone in the femoral head replaces multiple times yearly, while compact bone in the shaft changes less frequently.

  2. Role of Remodeling:

    • Regular mineral turnover adapts bones to stresses:

      • Stressed bones: Become thicker and stronger.

      • Unstressed bones: Thin and brittle.

    • Exercise is critical for maintaining bone health.

    • Periods of inactivity (e.g., using a crutch) can cause significant bone mass loss, up to 1/3 in a few weeks

The Skeleton as a Calcium Reserve:

  1. Calcium Storage:

    • Bones store 99% of the body’s 1-2 kg calcium supply.

    • Calcium ions are crucial for physiological processes like neuron and muscle function.

    • Calcium levels are tightly regulated:

      • 30% increase = unresponsive neurons/muscles.

      • 35% decrease = convulsions.

      • 50% decrease = death.

  2. Hormonal Regulation:

    • Increase calcium levels: Parathyroid hormone (PTH) and calcitriol.

    • Decrease calcium levels: Calcitonin from the thyroid gland.

    • Skeleton acts as a calcium reserve, directly affecting bone strength.

Repair of Fractures:

  1. Fractures and Classification:

    • Bone can break under extreme stress or impacts.

    • Fractures are classified by appearance, location, and break type.

  2. Healing Process:

    • Healing requires blood supply and functional periosteum/endosteum cells.

    • Steps of repair:

      1. Formation of a fracture hematoma (large blood clot) due to broken blood vessels.

      2. Mitotic activity in periosteum/endosteum forms external and internal calluses.

      3. Hyaline cartilage forms at the external callus center.

      4. Osteoblasts replace cartilage with spongy bone, creating a brace.

      5. Remodeling removes calluses, leaving compact bone.

    • Healed bone may become slightly thicker at the fracture site.