A Japanese experiment on ISS, comparing the development of fish in weightlessness with those on the ground, has provided` scientists more information about bone density loss in weightlessness.

Akira Kudo at Tokyo Institute of Technology, together with scientists across Japan, have shown that medaka fish reared on the International Space Station for 56 days experienced increased osteoclast activity – bone cells involved in the re-absorption of bone tissue – likely leading to a subsequent reduction of bone density. They also found several genes that were upregulated in the fish during the space mission. The team generated fish with osteoclasts that emit a fluorescent signal. They sent 24 fish into space as juveniles, and monitored their development for 56 days under microgravity. The results were compared with a fish control group kept on Earth.

Kudo and his team found that bone mineral density in the pharyngeal bone (the jaw bone at the back of the throat) and the teeth of the fish reduced significantly, with decreased calcification by day 56 compared with the control group. This thinning of bone was accompanied by an increase in the volume and activity of osteoclasts. The team conducted whole transcriptome analysis of the fish jaws, and uncovered two strongly upregulated genes (fkbp5 and ddit4), together with 15 other mitochondria-related genes whose expression was also enhanced. Reduced movement under microgravity also has an influence. The fish began to exhibit unusual behavior towards the latter stages of their stay in space, showing motionless at day 47.

What the data mostly confirms is that long-term weightlessness is a bad thing for the development of bones, and not just in humans. Whether scientists can use these results to counter these harmful effects is not clear, however.