Steps of Endochondral Ossification - I (Fig. 6.10)
1. Bone collar formation
a)
Chondrocytes  Osteoblasts  Compact bone  Bone collar
2. Cartilage ossification in shaft (Primary ossification)
a)
b)
c)
Chondrocytes within the shaft enlarge (hypertrophy)
Enlarged chondrocytes starts calcifying the surrounding matrix
Calcified matrix becomes impermeable to the diffusing nutrients
leading to:
• Death of chondrocytes  Deterioration of matrix  Formation of
large cavities
Steps of Endochondral Ossification - II (Fig. 6.10)
3. Periosteal bud invasion
•
Cavities are invaded by nearby periosteal bud containing:
• Osteoclasts  chew up some of the calcified matrix
• Osteoblasts  forms spongy bone
4. Medullary cavity formation
•
•
Osteoclasts  breakdown the newly formed spongy bone
Causes medullary cavity in the center of shaft
5. Epiphyseal ossification (secondary ossification)
•
•
Occurs shortly before or after birth
Same events as primary ossification except:
• Spongy bone is retained
• No medullary cavity formation
Steps of Intramembranous Ossification (Fig. 6.11)
1. Mesenchymal cells (embryonic tissue) clusters and differentiates into
capillaries and osteoblasts  ossification center
2. Osteoblasts secrets and calcifies osteoid (matrix)
3. Clusters of osteoid unite around the mesh of capillaries to form trabecular
matrix  Spongy bones
4. External surface condenses into periosteum
5. Spongy bone next to the periosteum remodels into compact bone