ABSTRACT
The postnatal development of long bones was demonstrated in the femur of albino rat at the level of the growing epiphyseal plates and middle of the diaphysis. This was achieved by using decalcified sections of the femur of few hours newborn rats, 4 weeks old rats, 8 weeks old rats and 12 weeks old rats. Growth of the long bones was found to pass into two directions, increase in length and increase in diameter. The results showed that the proliferation of the cartilage cells of the growing epiphyseal plate started in few hours newborn rat and gradually increased to be well marked in younger rat [4 weeks old]. In 8 weeks old rat's femur, the proliferation of the cartilage cells showed decreased activity. This finally ceased and was replaced by bone at the diaphyseal end of the cell columns, resulting in obliteration of the epiphyseal plate in 12 weeks old rats femur. Also, the growing trabeculae of the newborn rat's femur gradually progressed and developed till they reached their maximum development at the end of 8 weeks old rat Then they decreased in growth and changed into compact bone at the end of 1 2 weeks old rat's femur. In transverse sections, the diaphysis showed a thin layer of bone collar beneath the periosteum and bone trabeculae connected together forming marrow cavity in new born femur's rat. In 4 weeks old rat, the diaphysis of the femur became thicker and more developed having bone lamellae, primary osteons and bone trabeculae. The diaphysis of the femur became well developed and was differentiated into periosteum, outer circumferential lamellae, inner circumferential lamellae, interstitial lamellae, and mature Haversion system. So the increase in diameter of the long bone was the result of successive generation of osteoblasts which arose from osteogenic membrane beneath the periosteum forming the bone matrix