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Bone Adaptation to Disuse in Aging Rodents: Age-Related Differences in Response to Mechanical Unloading and Subsequent Reloading


Adaptation of the bones of older subjects to disuse is largely unknown and bone recovery following disuse in older subjects has not been studied. Up to half of all people over the age of 50 suffer from osteoporosis or low bone mass and older patients are more vulnerable to extended hospitalizations. With advancing age, bone experiences an age-related decline through trabecular bone loss, cortical thinning, and decreased osteocyte density. Decreased osteocyte density could contribute to the impaired adaptation of older bones to increased loading. This could affect an older subject’s ability to recover from disuse-induced bone loss. In this research we sought to quantify bone adaptation of young and old rats and mice in response to mechanical unloading via tail suspension and subsequent reloading. In addition, we aimed to compare the bone adaptation of old mice to a transgenic mouse model of low osteocyte density to determine the effect of osteocyte density on bone adaptation in response to mechanical unloading and reloading. We found that young rats lost trabecular bone quickly during unloading but recovered after only a short period of reloading. Meanwhile, old rats did not lose bone during unloading but did lose trabecular bone during early reloading that did not recover with continued reloading. In mice, we found that young, old, and transgenic mice all lost trabecular bone during unloading but young mice recovered more bone during reloading than old or transgenic mice. Old mice also lost cortical bone during unloading and reloading while transgenic mice lost no cortical bone. These results illustrate a difference in bone adaptation between different ages of mice and rats and further research into this difference may provide valuable insights for protecting the skeletal health of the elderly.

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