1. Ch 6.4-Bone Formation

2. When does bone formation occur?
During embryonic development
During infancy, childhood, and adolescence
Remodeling of bone
Repair of fractures

3. Embryonic Bone Formation
Two Patterns
Intramembranous Ossification
Endochondral Ossification
Forms directly within mesenchyme
(Simple; few # of bones)
Forms within hyaline cartilage that develops from mesenchyme
(Complex; most bones)

4. Intramembranous Ossification
STEP 1: Development of ossification center
Blood capillary
Ossification center
Mesenchymal cell
Osteoblast
Collagen fiber
Mesenchymal cells cluster
Form osteogenic cells → osteoblasts
Secrete organic bone EC matrix

5. Intramembranous Ossification
STEP 2: Calcification
Osteocyte
Osteoblast
Newly calcified bone EC matrix
Osteocytes become fully developed
Extend cytoplasmic projections
Surrounding osteoblasts deposit calcium and mineral salts

6. Intramembranous Ossification
STEP 3: Formation of trabeculae
Mesenchyme condenses
Blood vessel
Spongy bone
trabeculae
Osteoblast
Ossification centers fuse to form trabeculae
Blood vessels grow in between

7. Compact bone replaces surface layer of spongy bone
Intramembranous Ossification
STEP 4: Development of the periosteum
Periosteum
Spongy bone tissue
Compact bone tissue
Mesenchyme condenses
Forms periosteum
Compact bone replaces surface layer of spongy bone

8. Endochondral Ossification
STEP 1: Development of cartilage model
Proximal epiphysis
Diaphysis
Distal epiphysis
Hyaline
cartilage
Perichondrium
Mesenchymal cells form shape of future bone
Develop into chondroblasts
Secrete cartilage EC Matrix

9. Endochondral Ossification
STEP 2: Growth of cartilage model
Perichondrium
Uncalcified extracellular matrix
Calcified extracellular matrix
Chondroblasts deeply buried in cartilage
Now called chondrocytes
Mid-region chondrocytes grow;
Surrounding EC matrix calcifies
Chrondrocytes begin to die, leaving small cavities

10. Endochondral Ossification
STEP 3: Development of primary ossification center
Nutrient artery
Periosteum
Primary ossification center
Spongy bone
Nutrient artery penetrates perichondrium
Osteogenic cells become osteoblasts
Blood vessels induce growth of primary ossification center
Osteoblasts turn cartilage into bone

11. Osteoclasts breakdown newly formed spongy bone
Endochondral Ossification
STEP 4: Development of the medullary cavity
Uncalcified extracellular matrix
Calcified extracellular matrix
Periosteum
Medullary cavity
Osteoclasts breakdown newly formed spongy bone

12. Blood vessels enter epiphyses Secondary ossification centers develop
Endochondral Ossification
STEP 5: Development of secondary ossification centers
Secondary
ossification
center
Uncalcified
extracellular
matrix
Nutrient artery and vein
Epiphyseal artery and vein
Blood vessels enter epiphyses
Secondary ossification centers develop
Proceeds outward

13. Endochondral Ossification
STEP 6: Formation of articular cartilage and epiphyseal plate
Articular cartilage
Spongy bone
Epiphyseal plate
Outer cartilage becomes articular cartilage
Cartilage remains between diaphysis and epiphysis
Forms epiphyseal plate