In mammals, longitudinal bone growth occurs at the growth plate by endochondral bone formation. The growth plate consists of three principal zones: resting, proliferative, and hypertrophic. The resting zone lies adjacent to the epiphyseal bone and contains infrequently dividing chondrocytes. The proliferative zone contains replicating chondrocytes arranged in columns parallel to the long axis of the bone. The proliferative chondrocytes located farthest from the resting zone stop replicating and enlarge to become hypertrophic chondrocytes (1). These terminally differentiated cells maintain a columnar alignment in the hypertrophic zone. The processes of chondrocyte proliferation, hypertrophy, and cartilage matrix secretion result in chondrogenesis.
