Bone Growth and Repair Notes

Review of Bone Structure and Remodeling

  • Histology of Compact and Spongy Bone:

    • Compact bone: Dense, forms the outer layer, provides strength and support.

    • Spongy bone: Lighter, contains trabecular structure, found at the ends of long bones and in the interior of others.

  • Chemical Composition of Bone:

    • Organic components: Collagen fibers (provide flexibility and tensile strength), proteins.

    • Inorganic components: Hydroxyapatite (calcium phosphate crystals, provide rigidity and strength).

    • Advantages: The combination provides a strong yet lightweight structure that is essential for function and movement.

  • Osteoblasts, Osteocytes, Osteoclasts:

    • Osteoblasts: Bone-forming cells; found on the bone's surface; function in bone formation and mineralization.

    • Osteocytes: Mature osteoblasts that maintain bone; located within bone matrices.

    • Osteoclasts: Bone-resorbing cells; responsible for bone breakdown; found in areas of bone remodeling.

  • Hormonal Regulation of Bone Remodeling:

    • Hormones (like parathyroid hormone and calcitonin) regulate activity in osteoblasts and osteoclasts, influencing bone density and strength according to physical stress.

Bone Development

  • Ossification (Osteogenesis):

    • Process of bone tissue formation.

    • Begins in the second month of fetal development; replaces fibrous membranes and hyaline cartilage with bone.

    • Continues until early adulthood.

Intramembranous Ossification

  • Bone formation from fibrous membranes (no cartilage).

    • Commonly results in formation of the skull and clavicle.

  1. Development of Ossification Centre:

    • Osteoblasts secrete organic extracellular matrix.

  2. Calcification:

    • Calcium and minerals deposited; extracellular matrix hardens.

  3. Formation of Trabeculae:

    • Matrix develops into trabecular structures to form spongy bone.

  4. Development of Periosteum:

    • Mesenchyme develops into the outer periosteum layer.

Endochondral Ossification

  • Bone formation by replacing hyaline cartilage; forms most bones except clavicles and parts of the skull.

  • Begins at the primary ossification centre in the shaft of long bones.

    • Blood vessels change perichondrium into periosteum; mesenchymal cells specialize into osteoblasts.

Postnatal Bone Growth

  • Hormonal Regulation:

    • Growth hormone: Stimulates epiphyseal plate activity, essential for growth during infancy and childhood.

    • Sex hormones: Testosterone and estrogen promote growth spurts and end growth by inducing epiphyseal plate closure.

    • Imbalances can result in abnormal skeletal growth.

  • Growth in Length:

    • Occurs at the ossification zone through division of chondrocytes; replacement of cartilage with bone happens at the bottom of columns.

    • Thinning of epiphyseal plate occurs near the end of adolescence, leading to fusion of epiphysis and diaphysis.

  • Growth in Width (Appositional Growth):

    • Involves deposition of bone matrix by osteoblasts beneath the periosteum; osteoclasts remove inner bone to prevent excessive weight.

Bone Fractures

  • Types:

    • Youth: Traumatic injuries.

    • Old Age: Weakness due to bone thinning.

Healing Process:

  1. Hematoma Formation:

    • Blood vessels hemorrhage; a hematoma (mass of blood outside a vessel) forms at the fracture site.

  2. Fibrocartilaginous Callus Formation:

    • Capillaries grow, cells clear debris, and fibroblasts secrete collagen fibers connecting the broken ends.

  3. Bony Callus Formation:

    • Trabecular formation converts soft callus to bony callus of spongy bone.

  4. Bone Remodeling:

    • Occurs 3 to 5 years post-fracture; excess material is removed, and compact bone is formed to reconstruct shaft walls.