Outer space conditions hamper sperm’s ability to navigate toward an egg

By Will Dunham

WASHINGTON, March 30 (Reuters) – As humanity moves closer to establishing a long-term presence on the Moon and Mars, the question of whether reproduction is possible in extraterrestrial environments may become more than hypothetical. And new research using simulated microgravity conditions has identified major challenges.

Experiments by scientists in Australia have revealed that microgravity conditions, simulated in the laboratory, disrupt sperm navigation, reduce fertilization rates and, when exposure is prolonged, compromise the quality and survival of early embryos.

They found that human and mouse sperm were about 50% less efficient at swimming in a channel mimicking the female reproductive tract under these conditions compared to normal gravity. In mouse eggs, this resulted in an approximately 30% drop in fertilization success. Research has also revealed complications during early embryo development.

The human body has evolved over millions of years to function optimally in Earth’s environment, including its gravity, and travel beyond Earth’s boundaries causes many physiological changes that affect human health.

The United States, with NASA’s Artemis program, plans to send astronauts to the Moon in the coming years, as does China.

“With the Artemis program actively working to return humans to the Moon and serious plans underway for crewed missions to Mars, the ability to reproduce beyond Earth is fundamental to any long-term settlement,” said reproductive scientist Nicole McPherson, who leads the sperm and embryo biology group at the Robinson Research Institute at the University of Adelaide in Australia, and lead author of the study published in the journal Communications Biology.

“This includes not only human reproduction, but also the animals and agricultural species that any self-sustaining habitat would depend on,” said McPherson, who is also director of research and diagnostic laboratories at Genea, one of Australia’s leading IVF providers.

Fertilization occurs when a man’s sperm navigates the female reproductive tract and enters an egg, with the genetic material from the two cells combining. The new study is the first to show that gravity plays a critical role in the ability of sperm to travel toward an egg.

“It’s not the ability to swim that is affected. Sperm in microgravity are still moving, they just can’t find their way. The function that appears to be disrupted is navigation, the ability to orient and move purposefully toward a destination. We think this happens because many proteins on the surface of sperm act as mechanosensors, tiny molecular devices that sense physical forces, including gravity,” McPherson said.

“When this gravitational pull is removed, these sensors appear to be ejected, leaving the sperm without a reliable frame of reference for going up, down or in which direction to move. It’s a bit like trying to navigate a maze blindfolded and turning,” McPherson said.

Adding progesterone, a female hormone naturally released around the time of ovulation as a chemical signal that helps sperm find their way, helped more human sperm overcome the negative effects of microgravity.

MICROGRAVITY SIMULATION

To simulate microgravity, the researchers used a device that puts cells through a condition resembling continuous free fall in weightlessness in space. To test navigation, they used a plastic chamber with narrow channels open at both ends, with sperm having to navigate from one end to the other.

There was an approximately 50% reduction in the number of human and mouse sperm that successfully navigated microgravity compared to normal gravity.

For mouse eggs, there was a 30% reduction in fertilization success after four to six hours in microgravity compared to normal gravity. The embryos that managed to form in microgravity appeared to be of higher quality, with more cells that would later form the fetus.

“This suggests that brief exposure to microgravity might act as a sort of selective filter, with only the most resilient sperm and embryos succeeding,” McPherson said.

But when developing mouse embryos were exposed to microgravity during the 24 hours after fertilization – when genetic material from both parents first comes together – fewer embryos formed, and those that did showed signs of developmental delay and a reduction in cell numbers in critical early stages.

Similar results were obtained on embryos using pig cells.

“The clearest conclusion is that replication in space will be considerably more difficult than most people think, and that challenges will appear at multiple stages, not just one,” McPherson said.

(Reporting by Will Dunham; editing by Daniel Wallis)