The effect of reloading on bone volume, osteoblast number, and osteoprogenitor characteristics: studies in hind limb unloaded rats.

Skeletal unloading during space flight results in bone loss. In astronauts the extent to which bone is lost varies greatly between different bones of the skeleton as well as between different individuals. Following return to earth, recovery of bone mass during reloading also varies between different bones and different individuals. Due to this variability and the limited number of subjects it is difficult to study the effects of unloading/reloading on bone in humans. A viable alternative is to use the rat model of hind limb unloading developed at NASA. We have previously demonstrated that, in 6-week-old male rats, 14 days of unloading result in a decrease in osteoprogenitor number in cell populations isolated from the proximal femur. The goal of the present study was to determine the number of osteoprogenitor cells present in cell populations derived from the proximal femur of young rats after 14 days of unloading followed by 14 days of reloading and to characterize their proliferative capacity. To do this, we determined the number of alkaline phosphatase-positive colony forming units (CFU-AP) and osteoblast CFU (CFU-O). To establish whether the effects of unloading and reloading were specific for cells of the osteoblast lineage, we also determined the number of fibroblastic CFU (CFU-F). Effects on proliferation were evaluated by measuring the size of CFU-O. Unloading resulted in a 66% reduction in CFU-AP. CFU-O numbers were decreased by 76% and mean colony size was 33% less than controls. The decrease in osteogenic and osteoprogenitor cells in vitro paralleled the decrease in bone volume (- 50%), osteoblast number (- 35%), and bone formation rate (- 46%) observed in the proximal tibial metaphysis of unloaded rats. Unloading had no effect on osteoclast number or surface. Subsequent reloading for 14 days restored CFU-AP. CFU-O numbers were only partially restored at 14 days (83% of controls) but nodule size was 1.2-fold greater than controls. Neither unloading nor reloading had an effect on the total number of progenitors (CFU-F). Reloading restored bone volume back to control values, but osteoblast number and bone formation rate were still lower than those in corresponding controls. Both osteoclast number and surface were lower in reloaded animals than in age-matched controls. Our results indicate that 14 days of unloading result in a decrease in osteoprogenitor number and that reloading for 14 subsequent days completely restores CFU-AP and bone volume to control levels, while the number of CFU-O in vitro and osteoblasts in vivo were partially recovered but still lower than corresponding controls. Strikingly, osteoclastic bone resorption after 14 days of reloading was greatly reduced compared to controls, suggesting a significant contribution of this to the recovery process.