Bone Formation and Growth

Overview of Bone Formation (Osteogenesis)

• Osteogenesis is the process of bone formation.
• Occurs in 4 main occasions:
  1. Initial formation in embryo and fetus.
  2. Growth during infancy, childhood, and adolescence.
  3. Bone remodeling (replacement of old bone tissue with new).
  4. Repair of fractures or breaks.
• Initial formation and growth during early life are the focus here.

Initial Bone Formation in Embryo

• Embryonic Skeleton: Made of mesenchyme in bone shape.
• Week 6 Development: Ossification begins through two processes:
  • Intramembranous Ossification: Bone forms directly within mesenchyme.
  • Endochondral Ossification: Bone forms within hyaline cartilage.

Intramembranous Ossification

• Occurs in: Flat bones (skull, sternum, ribs, clavicle).
• Process:
  1. Development of ossification center in mesenchyme.
  2. Mesenchymal cells cluster, differentiate to osteoprogenitor cells, then to osteoblasts.
  3. Osteoblasts secrete osteoid, become osteocytes once surrounded.
  4. Calcium and phosphate from blood form hydroxyapatite crystals, hardening tissue.
  5. Spongy bone forms and red bone marrow develops.
  6. Periosteum forms around the bone; compact bone replaces surface layers.

Endochondral Ossification

• Occurs in: Most bones of the body except flat bones.
• Process:
  1. Development of cartilage model from mesenchymal cells.
  2. Chondroblasts form hyaline cartilage, perichondrium surrounds it.
  3. Cartilage model grows in length; chondrocytes hypertrophy.
  4. Nutrient artery stimulates osteoprogenitor cells to form osteoblasts, forming periosteum.
  5. Primary ossification center develops, forming spongy bone.
  6. Medullary cavity formed by osteoclasts breaking down spongy bone.
  7. Secondary ossification centers develop at epiphyses.
  8. Remaining hyaline cartilage becomes articular cartilage and growth plate.

Bone Growth in Infancy, Childhood, and Adolescence

• Growth in Length (Interstitial Growth):

  • Involves growth of cartilage at the epiphyseal plate.
  • Cartilage replaced by bone via endochondral ossification.

• Epiphyseal Plate Zones:

  1. Resting Cartilage: Anchors epiphysis, no growth.
  2. Proliferating Cartilage: Chondrocytes divide, lengthen cartilage.
  3. Hypertrophic Cartilage: Chondrocytes enlarge.
  4. Calcified Cartilage: Chondrocytes die, calcified matrix replaced by bone.

• Growth in Thickness (Appositional Growth):

  • Osteoblasts form new bone on outside.
  • Osteoclasts break down bone on inside, maintaining thickness.

Conclusion

• Growth in length ceases when epiphyseal plates close (females ~18, males ~21).
• Growth can continue in thickness through appositional growth.