Lost in space: Human sperm can’t find their way without gravity

Having kids is a decision with a lot of gravity, literally. Scientists have found that sperm in space can’t find their direction during their pursuit to fertilize an egg. Such digressions could pose a problem in the future as the possibilities of human colonies in space become more likely.

Spacing out

A lack of gravity “impaired directional navigation and fertilization capacity” of human sperm cells, said a study published in the journal Communications Biology. “This is the first time we have been able to show that gravity is an important factor in sperm’s ability to navigate through a channel like the reproductive tract,” Nicole McPherson, a senior lecturer at Adelaide University’s Robinson Research Institute and the senior author of the study, said in a release.

Researchers tested human, pig and mouse sperm by putting them into a “microgravity simulation chamber designed to mimic the female reproductive tract and tested the swimmers’ ability to navigate,” said Scientific American. The results showed a “significant reduction in the number of sperm that were able to successfully find their way through the chamber maze in microgravity conditions compared to normal gravity,” said McPherson. The results repeated “across all models, despite no changes to the way sperm physically move.” The sperm’s directional loss “was not due to a change in motility but other elements.” In the case of human sperm, “less than 20% of them reached the finish line in near weightlessness,” compared to 50% in Earth’s conditions,” said Smithsonian Magazine.

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There may be a way to lead sperm in the right direction: the hormone progesterone. “Progesterone works as a chemical signal, a kind of biological homing beacon that the egg releases around the time of ovulation,” McPherson said to Scientific American. “Sperm have receptors on their surface that detect this signal and use it to orient themselves and swim toward the source.” However, the progesterone only helped at concentrations “considerably higher” than those found in nature, so it is not yet a “simple fix for fertility in space.”

A new home

Having babies in space may be a necessity in the future as humans aim to establish human settlements on the moon and Mars. The human body “evolved over ​millions of years to function optimally in Earth’s environment, including its gravity,” said Reuters. “Trekking beyond Earth’s confines causes many physiological changes that affect human health.” Until now, research had not been done about “whether the cells can successfully navigate the female reproductive tract and fertilize eggs,” said Smithsonian Magazine.

Despite the potential to get lost in space, “many healthy embryos were still able to form even when fertilized under these conditions,” McPherson said. “This gives us hope that reproducing in space may one day be possible.” The next research step is to investigate “how varying gravitational environments, such as those on the moon, Mars and proposed artificial gravity systems, impact sperm navigation and early embryo development,” said the release. As of now, “NASA and other governmental space agencies maintain that no one has ever had sex in space,” said Scientific American. “But future human spacefarers may want to have families and reproduce while in a microgravity environment.”