Medaka fish raised on the International Space Station developed lighter, weaker bones, researchers in Japan report.

Previous studies have found the bones of astronauts become less dense after spending extended periods of time in microgravity, but researchers haven't been able to determine why.

The new study, published in the journal Scientific Reports, offers insights as to how bones react to life in space.

According to researchers at the Tokyo Institute of Technology, time spent in microgravity encouraged osteoclast activity. Osteoclasts are the bone cells responsible for bone resorption. Osteoclasts dismantle bone tissue, while osteoblasts form bone tissue.

Together, these cells play a vital role in the maintenance, repair, remodeling, and overall amount of bone tissue.

Researchers were also able to identify a series of unregulated genes responsible for boosting osteoclast activity.

The research was made possible by genetic engineering. The Mendaka specimens were modified so that their osteoclasts and osteoblasts produced two different types of fluorescence when activated, allowing researchers to monitor the fishes' bones in real time.

Of the 312 modified fish, only the healthiest 24 juvenile specimens were sent to space. The rest stayed behind to serve as a control group.

Weightlessness has a variety of effects on human health. In addition to resulting in bone and muscle loss, time spent in microgravity has also been shown to affect an astronaut's vision, blood pressure, ability to heal and much more.

In preparation for future deep-space missions, NASA scientists are working with researchers around the world to better understand the physiological consequences of lengthy periods of time spent in space.