19 Bone Growth, Remodeling & Repair

Histology of Compact Bone

  • Osteon: the structural unit of compact bone.

    • Composed of onion-like layers of bone surrounding a central canal.

    • Present in the shafts of long bones and covering all bones.

Force Resistance in Bone

  • Concept: Situational comparison of force resistance.

    • Scenario 1: Holding a paper straw on both ends and pushing ends toward the center.

    • Scenario 2: Folding a paper straw in half.

  • Osteons: Align with forces on bones for strength resilience.

Bone Remodeling

  • Osteons: Align with stress lines to enhance bone strength.

    • Remodeling occurs as forces change over time.

    • Matrix is continuously broken down and rebuilt.

  • Regulation of Remodeling: Understanding which cells are involved in remodeling.

Types of Bone Cells

  • Four Cell Types Involved in Remodeling:

    1. Osteogenic Cells: Stem cells that develop into bone-forming cells.

    2. Osteoblasts: Immature bone cells that secrete organic components of the matrix.

    3. Osteocytes: Mature bone cells that maintain bone tissue.

    4. Osteoclasts: Multinucleate cells that dissolve bone matrix using acids and enzymes.

  • Structures:

    • Endosteum: Lining of bony canals.

    • Trabeculae and circumferential lamellae: Features of compact and spongy bone structure.

Process of Bone Remodeling

  • Steps: Activation of osteoclasts followed by the function of osteoblasts.

    • Osteoblasts become trapped and mature into osteocytes.

Anatomy of Long Bones

  • Osteon Alignment: Osteons remodeled parallel to lines of force.

    • Osteocytes in lacunae sense force direction and secrete chemicals activating osteoclasts.

Osteon Remodeling Mechanics

  • Process:

    • Osteoclasts break down matrix parallel to force lines.

    • Osteoblasts fill tunnels with new matrix leading to new osteons.

Osteon Location Changes During Remodeling

  • Observation: Cross-section showcasing changes in osteon location over time (first vs. second vs. third generation osteons).

Factors Affecting Bone Remodeling

  • Influences: Combination of forces on bones, hormonal changes, and nutrition impact matrix turnover.

    • Inorganic matrix primarily consists of calcium hydroxyapatite (Ca5(PO4)3(OH)), affecting remodeling when calcium levels fluctuate.

Bone Remodeling Problems

  • Acknowledgment of issues such as:

    • Plantar fascia complications.

    • Effects of conditions like osteoporosis.

Osteogenesis Imperfecta

  • Recognized as “brittle bone disease”, impacting remodeling.

Cartilage and Bone Development

  • Framework: Cartilage serves as a scaffold for osseous tissue formation during fetal development.

    • Some bones, like the skull, develop without cartilage.

Growth Zones in Long Bones

  • Zones: The epiphyseal plate for growth consists of:

    • Resting Zone: Site of stem cell activity towards the epiphysis.

    • Proliferating Zone: Active chondrocyte division and cartilage production.

    • Calcification Zone: Chondrocyte hypertrophy leads to the creation of large spaces and ultimately cell death.

Appositional vs. Interstitial Growth

  • Appositional Growth: Growth at the surface, increasing bone thickness or diameter.

  • Interstitial Growth: Occurs at epiphyseal plates, contributing to increased bone length.

Fracture Repair Process

  • Stages:

    1. Hematoma Formation: Clotted blood forms around the break.

    2. Callus Formation: By fibroblasts and chondrocytes.

    3. Ossification: Callus becomes ossified into bony callus of spongy bone.

    4. Remodeling: Aligns trabeculae and osteons, correcting shape and minimizing bulges.