Mammalian sperm cells struggle to navigate the female reproductive tract in microgravity, reaching and fertilizing eggs 30 percent less often than in normal gravity, according to a University of Adelaide study. The findings reveal unexpected biological hurdles for space reproduction as commercial tourism and settlements advance.
Simulating Weightlessness in an Adelaide Laboratory
Researchers led by reproductive biologist Nicole McPherson at the University of Adelaide set out to test whether mammalian reproduction could succeed beyond Earth. The study investigated sperm cells from humans, mice, and pigs inside a 3D clinostat—a high-tech centrifuge acquired through the founder of space medicine company Firefly Biotech that spins sample vials around two axes to simulate microgravity by confusing cells about their spatial position. Within two minutes, the device reduces gravity to less than one-tenth of Earth’s normal pull.
Inside a standard incubator mirroring fertility-clinic conditions with low-oxygen settings and culture fluids, human and mouse sperm were placed at one end of a tiny fluid-filled channel. They were given an hour to swim toward an egg at the other end. Computer analysis revealed that human sperm swam with normal speed, forward motion, and vigor, yet fewer cells successfully reached the far end compared to tests run in normal gravity. According to the reporting by Space, the cells’ engines ran fine, but their directional compass failed.
Navigational Failure and Embryonic Deterioration
While sperm rely on chemical cues like progesterone concentrations to find eggs, gravity also governs how they swim near surfaces. When the absence of gravity scrambles that spatial awareness, fertilization rates drop. Four hours of simulated weightlessness cut mouse fertilization by 30 percent, and six hours lowered pig fertilization.
The study also tracked what happened to embryos after fertilization occurred. Blastocysts formed after four hours of microgravity exposure initially appeared stronger, which researchers attributed to a natural selection process allowing only the fittest sperm to reach the target. However, when microgravity exposure continued for up to 24 hours, that initial quality deteriorated. Mouse embryos divided more slowly and carried fewer epiblast cells—the cells responsible for building the fetus’s body—as the absence of gravity interfered with maternal and paternal DNA coming together and early epigenetic remodeling.
Implications for Lunar and Martian Settlements
With SpaceX expecting to send its first crews to Mars by 2030 and NASA targeting a lunar return by 2029, the question of long-term human habitation takes on renewed urgency. McPherson noted that maintaining space settlements without constantly colonizing from Earth requires successful in-space reproduction.
“When you think about the future of space exploration and space settlements, it’s happening. It’s happening now,” Nicole McPherson, a reproductive biologist at the University of Adelaide, Australia, and lead author of the paper, told Space.com. “I think people forget that for us to maintain these settlements without having to continually colonize them from Earth, we need to be able to reproduce in space.”
Nicole McPherson, reproductive biologist at the University of Adelaide, Australia
Despite the hurdles, the experiments offered a potential workaround: researchers discovered that a high dose of progesterone successfully restored navigation in human sperm under space-like conditions. The team next hopes to test reduced-gravity environments like those of the moon and Mars to see if partial gravity mitigates the problem.