Age-related reactions of rat bones to their unloading.

The effect was investigated of continued growth of animals on bone formation and resorption under conditions when their musculoskeletal systems were unloaded. Studied were rats aged 1, 2.5, and 6 months. To provide the unloading, the rats were suspended at an angle so that their hindlimbs did not touch the floor. After 22 days, the animals were sacrificed, and the length and width of the diaphysis of the femoral bone and its calcium content were measured. Bone reactions to the suspension were found to be age-dependent. The rats of 2.5 months showed the highest and the rats of 6 months the lowest increase in bone reabsorption at the side of the bone-marrow canal and in reduction of the cortical layer. With age, calcium decrease in the femur of suspended rats became less pronounced compared to controls. The age-related bone reactions to the musculoskeletal unloading must be taken into consideration when planning and conducting rat studies.