From the beginning of life on earth, gravity has shaped skeleton from cellular to biological function and modelling activity. After a 16-year hiatus, spaceflights onboarding rodents were resumed with the Bion-M1 satellite (April 19-Mai 19 2013) offering a unique opportunity to investigate the effects of gravity on bone tissue. A group of 15-weeks-old C57/Bl16 male mice exposed to spaceflight (Flight, n=5) was compared with a ground synchronous conditions control group for which the environmental conditions of the spaceflight were simulated (Syn, n=6) and a ground standard conditions control group (Ctr, n=6). In femur, animals from Flight and to a less extent from Syn have damaged micro architecture parameters such as BV/TV (%, mean±SEM) (Flight: 1.69±0.4; Syn: 4.80±0.89; Ctr: 11.4±1.72; p<0.05) or cortical thickness (μm, mean±SEM) (Flight: 194±4.8; Syn: 205±2.7; Ctr: 215±3; p<0.05). At the cortical level, we observe in the Flight group a periosteal resorption, an osteocytic sclerostin expression increased as well as an osteocytic lacunae disorganisation. We also notice an impairment of cortical bone material level properties in the Flight animal vs Ctr; the plastic and elastic properties parameters assessed by nano-indentation are decreased. In T12 vertebrae, BV/TV was also adversely altered (Flight: 22.9±1.9; Syn: 28.5±0.63; Ctr: 30.9±1.46; p<0.05), and resorption was highly increased in L3 vertebrae (Oc.S/BS: %, mean±SEM) (Flight: 0.62±0.12; Syn: 0.13±0.03; Ctr: 0.11±0.01; p<0.01 between Flight and other groups). In L2 vertebrae, the Flight and Syn animals have a less mature collagen (measured by FTIRM: Fourier Transform InfraRed Microspectroscopy). The degree of mineralisation analysed on micro radiography was unchanged. Intervertebral distances were affected neither by microgravity nor by synchronous conditions. These first data show a major contribution of gravity on skeleton integrity and could contribute to better understand and anticipate the spaceflight-related bone loss in humans.
Disclosure: The authors declared no competing interests. This work was supported by the Centre National dEtudes Spatiales.