A. Intramembranous Ossification

• Mesenchyme condenses and becomes highly vascular.
• Some C.s differentiate into osteoblasts and begin to deposit matrix (= osteoid) @ primary ossification centers.
• Calcium phosphate is deposited in the matrix (ossification).
• Osteoblasts become osteocytes.
• New bone coalesces into lamellae (layers) surrounding blood vessels forming Haversian systems.
• Peripheral osteoblasts continue to lay down layers forming plates of compact bone while osteoclasts absorb bone in the middle forming spongy bone.
• In the spongy area (diploe), the mesenchyme differentiates into bone marrow.
• Bone growth and remodeling is a balance of osteoblastic and osteoclastic activity; osteoblastic activity slows down in women after menopause becoming a factor in osteoporosis.

B. Intracartilagenous Ossification

• Mesenchyme condenses into hyaline cartilage forming a "prebone" model.
• A primary ossification center develops in the center of the diaphysis; the chondrocytes hypertrophy, the matrix calcifies and the C.s die.
• A periosteal bone collar develops (from the osteogenic layer of the periosteum) around the mid 1/3 of the shaft.
• A vascular bud penetrates the periosteum carrying osteogenic osteoblasts into the spongy "marrow cavity"; these C.s differentiate into hemopoietic C.s and osteoclasts.
• A long bone grows in width by the deposition of compact lamellar bone on the outside (appositional growth), while spongy bone is being laid down and resorbed in the developing medullary cavity.
• A long bone grows in length by the same processes occurring in the epiphyses as long as the epiphyseal plate continues chondrogenesis; these are the secondary ossification centers.

C. Fracture Repair

• Immediately after the fracture, bleeding into the area causes a fracture hematoma.
• The hematoma is ossified internally by spongy bone and chondrified externally by the osteogenic layer of the periosteum.
• The callus is completely ossified leaving a swelling around the fracture site. This will be remodeled over time leaving little evidence of the fracture.