Ossification

• Chondroblasts form a hyaline cartilage model of the future bone
• Once surrounded my cartilage matrix they change into chondrocytes
• Perichondrium is formed over the bone except where it will articulate
• Perichondrium is continuous with the joint capsule

• Blood vessels supply the perichondrium
• Osteoprogenitor cells from the perichondrium change to osteoblasts
• Osteoblasts produce a woven bone collar surrounded by periostium
• Interstitial and appositional cartilage growth causes the cartilage model to lengthen and broaden
• Chondrocytes start to hypertrophy in the diaphysis
• The matrix between them is mineralized with calcium carbonate forming calcified cartilage
• Chondrocytes trapped in their calified tombs die leaving lacunae with thin calcified matrix walls

• Blood vessels invade lacunae in the calcified cartilage
• Osteoclasts and osteoblasts travel into the calcified cartilage via the connective tissue of blood vessels
• Osteoblasts then produce bone trabeculae in diaphysis forming cancellous bone
• This part of the future bone is called the primary ossification center

• More growth of the cartilage model
• Bone collar thickens and lengthens
• Mature bone is produced from woven bone by remodeling
• Medullary cavity forms when osteoclasts remove bone from the diaphysis
• Bone marrow is produced in the newly fromed medullary cavity

• Secondary ossification centres are established in the epiphysis of long bones.
• These apear late in fetal development and a baby is considered to be full term if a secondary ossification center has appeared at either the head of femur, head of tibia, of head of humerus. The last to appear is the medial epiphysis of the clavical which does not develop until 18 or 20 years
• No medulary cavity occurs in a secondary ossification center

• Cartilage is increasingly replaced by bone leaving only the epiphyseal growth plate which remains untill the bone growth is complete.
• Articular cartilage remains throughout develpoment and in adulthood on all articular surfaces

• Compact bone and cancelous bone are completely developed and the epiphyseal growth plate has fused at completion of the bone growth, leaving just the epiphyseal line
• The only cartilage remaining is on the articular surfaces
• All of the perichondriun is now periostium

Useful Link