Abstract
Cortical bone is porous in infants but compacts with growth. In that process, circumferential lamellae are formed on the endosteum (inner) side of cortical bone first, and also the periosteum (exterior) side, after that. A mature rat shows higher bone strength per unit area of the cortical bone, and this means that not only an increase in the bone mass, but also improvement of the bone structure contributes to enhancing the bone strength. External morphology of the bone hardly changes by immobilization, but the thickness of the cortical bone decreases, and disorder of the running direction of the blood vessels in the cortical bone also occurs. Bone formation and resorption are caused simultaneously at the early stage of an exercise period, and it is supposed that factors embedded in the bone matrix beforehand are released into bone marrow by exercise, differentiating and activating the osteoblasts. Thus, it is thought that bone adapted to mechanical stress is formed by repeating active remodeling in the growing period.
