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Fish reveal details of bone density loss during space missions

Studies of medaka fish raised on the International Space Station shed light on how bone responds to sustained exposure to microgravity.

Spending time in space in a reduced gravity environment can have lasting effects on the body. For example, it is known that gravity plays a key role in the correct formation and maintenance of bone structure. Studies have shown that astronauts experience a significant drop in bone mineral density when they have been on space missions, but the exact molecular mechanisms responsible for this are unclear.

Now, 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.

Image showing comparison of fluorescent signals for osteoclasts and osteoblasts
Comparison of fluorescent signals for osteoclasts and osteoblasts
Scientists at Tokyo Institute of Technology have shown how osteoclast volume and activity is enhanced in the upper and lower jaw bones of medaka fish after 56 days spent on the International Space Station. The team found subsequent reduction in bone density in the space fish compared with a control group of fish kept on Earth
Credit: Tokyo Institute of Technology