Increased bone marrow adiposity in murine femoro-tibial epiphyses exposed to 30 days of microgravity

Bone marrow mesenchymal stem cell (BM-MSC) differentiation in long bones is sensitive to mechanical loading. Increased loading promotes osteogenesis and reduces adipogenesis while reduced loading tips MSC differentiation away from the bone formation in favor of adipogenesis. To examine the effects of the unloading on epiphyseal bone and adipocyte content, subchondral femoro-tibial bones isolated from mice flown for 30 days in microgravity were assessed for evidence of altered bone area and adipocyte number. Consistent with the known response of bone to microgravity, 30 days of spaceflight resulted in approximately 25% less subchondral bone area. Concurrently, 10-fold more adipocytes were present in the bone marrow cavities of femur and tibia in flight compared to ground control samples. These data support the hypothesis that biomechanical unloading promotes adipogenic differentiation and confirms earlier studies in rat vertebrae of increased adipogenesis during 14 days of microgravity. The potential long-term effects of increased bone marrow adipocyte formation on flight personnel health is unknown and warrants further investigation.