Space: Fish Stand in For Human Astronauts in Bone-Density Test

When astronauts are in spaceflight, they lose bone mineral density. While this is known, the reason for this drop in mass has not yet been sussed out. To find the answers, scientists from the Graduate School of Bioscience and Biotechnology at the Tokyo Institute of Technology brought 24 medaka fish to the International Space Station and examined the fish over 56 days.

Scientists have previously known that spending time in microgravity stimulates the cells in charge of absorbing bone tissue, which are called osteoclasts. Therefore, this experiment looked at how the rise in osteoclast activity related to the reduction in bone mineral density. The result was just as expected: the greater the volume and activity of the osteoclasts, the greater the reduction in bone density.

The researchers also noted that there were some anomalies in the osteoclast mitochondria -- the organelles that generate their energy -- and an uptick in genetic activity among the two genes associated with mitochondrial function. The mitochondria's abnormal functioning and the fact that the two genes related to the functioning are becoming more animated proves that there is likely a connection between the incitement of the osteoclasts and the mitochondria's reaction to microgravity. 

According to Akira Kudo, the principal investigator on the study and a professor at the Tokyo Institute of Technology, it may be possible to use medicine to correct the mitochondria dysfunction, so that the astronauts will regain any bone density lost while in space.

The medaka fish in the study had fluorescent osteoclast cells, allowing the scientists to better observe their elevated activity. Despite normal body growth, the fish lost mineral density in their teeth and bones. Towards the end of the experiment, the fish began swimming less and ultimately became motionless, according to the study published in Scientific Reports. The fish becoming immobile informs researchers that microgravity's effect on bone density is related to adjustments in mechanical force that diminish physical activity.

 "It has been difficult to understand the mechanism of age-related bone loss on Earth," Kudo said in a press release. "The Medaka experiment finding of new genes affected by microgravity provides a good animal model to clarify this mechanism."

According to the researchers, these findings would help to progress the advancement of osteoporosis drugs--and also help the astronauts losing bone mass when they are up in space for lengthy periods of time.  

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