The research, published June 30 in the journal PNAS, looked at female mice that had spent extended periods on the International Space Station (ISS). These mice had similar levels of fertility as Earth-bound mice. However, their children and grandchildren produced smaller-than-average litters, had fewer litters overall and had lower levels of key reproductive hormones, among other measurable differences.
As the study was conducted in mice, it’s difficult to determine whether the effects might carry over to human astronauts. However, mice are good biological models for understanding the health impacts of space travel, given that they are mammals with a similar reproductive system to humans, experts told Live Science.
A "void" in research
Past research, such as the NASA Twins Study, has suggested that short-term spaceflight has modest effects on human health. That study compared astronaut Scott Kelly, who completed a year-long mission on the ISS, to his identical twin brother, Mark Kelly, who remained on Earth.
A 2019 study looked at male mice that spent time aboard the ISS and found they could father healthy offspring. The new study, by contrast, was conducted to "fill a void" in research investigating how space travel might impact the female reproductive system, Christenson said.
For the new study, 20 female mice lived on the ISS for 42 days. The female mouse reproductive cycle is very short, only about four to five days long, while the human menstrual cycle is 28 days on average, Christenson noted. "But a lot of the molecular background is very similar" in terms of how hormones fluctuate each cycle, he explained.
The current study focused on lab mice, so it remains to be seen how the effects might translate to people. (Image credit: dra_schwartz via Getty Images)
Researchers saw clearer differences in the next generation of mice. The offspring of the spacefaring mice had about 15% less muscle strength than the control groups' offspring. Additionally, the female mice in that second generation were more prone to stress than the female offspring in the control groups, based on a behavioral experiment.
The second generation of mice could also get pregnant, but these mice had fewer litters and much smaller litters than the control mice did. Further testing also showed that the offspring of spacefaring mouse mothers had lower levels of anti-müllerian hormone (AMH), a hormone whose levels correlate to overall egg count. Higher AMH means higher egg reserves and vice versa. The subsequent third generation also had lower levels of AMH compared with mice in the control groups.
This theoretical mechanism and its relationship to mice's reproductive capacity still needs to be confirmed, though.
Related storiesFor now, "this study adds to the growing body of literature on the effects of space travel in general on the reproductive system," said Dr. Ulrike Luderer, director of the Center for Occupational and Environmental Health at the University of California, Irvine, who was not involved in the research.
She hopes that further research can focus on additional reproductive markers, such as hormonal levels, to pinpoint the long-term consequences of deep-space travel. Those will be especially relevant if "humans spend long periods of time in [space] or colonize some other planet."
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