Purpose: Decreased mechanical stress causes disuse bone atrophy characterized by reduced bone mass and weakened bone. However, few studies have measured the mechanism behind such changes in different areas of bone. The present morphological study investigated the effects of decreased mechanical stress on bone mineral density in different areas of the femur by measuring bone mineral density and assessing the microstructure of osteoblasts. Methods: Twenty-one 9-week-old male Sprague-Dawley rats were acclimatized for one week, and then were divided into control, exercise, and cast immobilization groups. The study was conducted over an 8-week period, from age 10 weeks to 17 weeks in the rats. Bone mineral density was measured by dual energy X-ray absorption (QDR-2000) in the proximal epiphysis/metaphysis, diaphysis, and distal epiphysis/metaphysis of the femur. The microstructure of osseous cells was examined by Scanning Electron Microscopy (SEM). Results: Bone mineral density of the distal epiphysis/metaphysis was significantly lower in the cast immobilization group than in the exercise or control groups (p<0.001). In the cast immobilization group, scanning electron microscopy of the distal epiphysis/metaphysis revealed no boundary between small osteoblast and smooth neighboring cells; however, in the exercise group, actively osteoblastic osteoblasts covered the bone surface, resembling a stone wall.Conclusion: Decreased mechanical stress caused a decrease in bone mineral density that varied in different areas of the femur. The largest decrease in density occurred in the distal epiphysis/metaphysis. Osteoblast microstructure played an important role.

To investigate the effects of exercise on femoral regional bone mineral density and osteocyte micromorphology in young ovariectomized (OVX) rats, animals were divided into an ovariectomized group (OVX group), an ovariectomized + exercise group (OVX + exercise group), and a sham surgery group (SHAM group). Femoral bone mineral density was significantly lower in the OVX and OVX + exercise groups than in the SHAM group. A large difference in bone mineral density in the distal femoral epiphysis was observed between the OVX and OVX + exercise groups. In the OVX group, bone lacunae showed less immunostaining for DMP-1(dentin matrix protein-1) and osteocyte processes were fewer than in the SHAM group. In the OVX + exercise group, osteocyte processes were thicker (0.44 to 0.66 μm) than in the OVX group (0.22 to 0.26 μm). These results indicate that exercise induced structural changes in the femur, including formation of osteocyte processes and strengthening of the communication network between osteocytes.