MHS 287 Bone

Endochondral Bone Development

Endochondral ossification is a process where bone replaces cartilage. The various stages can be seen in these front and rear legs of a fetal rat.

The upper front leg shows the early stages of the formation of primary centers of ossification.

Cartilage models form in the approximate shape of the adult bone.

Steps in the formation of primary centers of ossification:

• Periosteum forms a bony collar in the diaphyseal region of the cartilage
• Chondrocytes show early hypertrophy
• Chondrocytes calcify their matrix and die
• Osteoblasts differentiate from the periosteum and begin to lay down the bone matrix

The lower rear leg shows the later stages of bone development. The femur shows the formation of the epiphyseal plates.

The primary site of ossification can be seen in the diaphysis.

The epiphyseal plates (#1,#2) continuously provide for the lengthening of the bone.

• Resting Zone (Reserve) - quiescent chondrocytes
• Proliferation Zone - rapidly dividing chondrocytes that form longitudinal columns of stacked cells
• Zone of Hypertrophy - chondrocytes cease dividing, grow in size, and secrete cartilage matrix
• Zone of Calcification - calcification of the cartilage matrix inhibits diffusion of nutrients and the chondrocytes undergo apoptosis
• Zone of Ossification - osteoprogenitor cells migrate into the cavities with new blood vessels, differentiate into osteoblasts, and deposit new bone (eosinophilic) on the scaffold of calcified cartilage (basophilic)

Secondary center of ossification is seen in one of the epiphyses