Remodelling of bone and bones: growth of normal and transplanted caudal vertebrae

Changes in the rate of growth, shape and structure of the 8th, 16th and 22nd caudal vertebrae of 4 and 24-27 days old Sprague-Dawley rats were studied in situ and in three different non-functional transplantation sites for 12 weeks. With increasing size, maturity and age the three vertebrae showed progressively decreasing growth, changes in shape and structural abnormalities. The smallest anlages grew faster and matured sooner than normal, so that their length equalled that of controls. Central endochondral necrosis in older bones was associated with decreased longitudinal growth but in some younger ones, despite a perforation of the cartilage and herniation of the nucleus pulposus into the marrow cavity of the shaft, growth proceeded at near normal rates. The free ends of older, larger transplants grew faster than the abutting ends joined by joint connective tissue, indicating that central necrosis of cartilage resulted from impaired nutrient diffusion. The results suggest that the cartilage model may possess an inherent capacity to produce a certain limited amount of bone tissue which may be distributed either in the form of long and thin or short and inwaisted bones, depending on the balance of forces between interstitial cartilage expansion and the restraining ensheathing periosteal-perichondrial tissues. This basic form may be modified further by functional forces.