Astronauts may have a hard time reproducing in outer space, research suggests, but what does that mean for the future of space colonies?

Zero gravity means zero game when it comes to fertilization success, new research suggests. Scientists observed sperm and eggs in a simulated microgravity environment and found that the environment hinders sperm migration, fertilization, and embryo development, posing serious challenges to the future of space colonization.

The human, mouse, and pig study, published Thursday (March 26) in the journal Communications Biology, found that microgravity led to misorientation of sperm, reduced fertilization success in mouse eggs, and delayed development in pig embryos.

This discovery has great implications for establishing a permanent human presence beyond Earth. Planned long-term settlements on the Moon and Mars depend not only on keeping astronauts alive, but on eventually allowing humans to reproduce there.

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Sperm in microgravity

Previous studies have shown that microgravity can inhibit estrogen production and reduce sperm counts in mice. However, it is still unclear what happens at the cellular level when sperm and eggs float in near-weightless conditions.

To simulate microgravity, the researchers used a device called a clinostat machine, which “continuously rotates a cell or sample in multiple directions, essentially turning the cell or sample into a “It works by randomizing the direction of the force so rapidly that the cells never get a chance to settle or orient themselves,” Nicole McPherson, a researcher who runs the Sperm and Embryo Biology Group at the University of Adelaide’s Robinson Institute and lead author of the study, told Live Science by email. “From the cell’s point of view, there is no consistent ‘rise’ or ‘fall’; instead, it experiences a kind of continuous free fall, which closely mimics what living cells experience in weightless space.”

Using a space simulator, researchers placed human and mouse sperm into a small maze designed to mimic the female reproductive system. In both cases, fewer sperm were able to successfully navigate the maze in microgravity compared to those that traveled under Earth’s gravity.

“Many of the proteins in sperm act as mechanosensors, small molecular devices that detect physical forces,” McPherson said. “It stands to reason that if you remove gravity, these sensors will be thrown out and the sperm’s ability to orient and move will be disrupted.”

Under normal conditions due to the Earth’s gravitational pull, a woman’s reproductive tract releases a chemical signal called progesterone after ovulation that helps sperm make their way toward the egg, MacPherson said. To increase the chances of human sperm reaching eggs in microgravity, researchers added this hormone to the system.

“It had some effect, but the concentrations needed to produce the effect were much higher than those that occur naturally in the female reproductive tract,” MacPherson said.

In theory, it is possible to administer high doses of progesterone, but McPherson cautioned that more research is needed into safety and effectiveness before the hormone is prescribed to space travelers as a fertility enhancer.

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Reduced fertilization and developmental delay

The researchers then examined fertilization and embryonic development in mouse and pig eggs. Under simulated microgravity, fertilization success rates were 30% lower for mouse eggs and about 15% lower for pig eggs compared to Earth’s gravity.

Six days after insemination, the pig embryos showed signs of developmental retardation. “After fertilization, the embryo still needs to implant itself in the uterine wall, a process that uses gravitational cues,” MacPherson said. “The fetal cells then have to organize correctly to eventually form every organ in the body, are maintained by the placenta, and function properly throughout the pregnancy. Microgravity can disrupt some or all of these steps.”

While these results pose significant challenges for the future of space colonization, they also give scientists great insight into how gravity influences the development of life on Earth.

“The role that gravity plays from the moment the sperm begins its journey to the moment the embryo begins to develop seems to be just beginning to be understood,” Macpherson said. “Gravity is not just a background of life; it is deeply integrated into the biological processes that give rise to life.”

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