When a SpaceX supply mission recently visited the International Space Station, its 5,800 pounds (2,631 kilograms) of cargo included equipment for studying thunderstorms from the vantage point of space and growing vegetables in orbit — the sort of stuff that you'd expect astronauts to need for their research. But tucked in with those items was something more unusual — containers with samples of frozen sperm from 12 anonymous human donors, obtained from a California sperm bank, along with six samples of sperm from breeding bulls.
Those samples will be used in a planned experiment, Micro-11. Astronauts will thaw the sperm and add chemicals that trigger activation of movement and preparation for fusing with an egg. Then, they'll put the sperm under a microscope, so they can shoot video footage of the sperm swimming. In addition, they'll perform chemical tests. Meanwhile, back on Earth, other researchers will be performing the same tests on identical samples of sperm for comparison.
Capacitation is Key
The experiment is designed to study sperm motility — that is, the ability to move and swim — in microgravity conditions, and how successful they are in triggering the physiological changes known as capacitation, which enables them to penetrate and fertilize eggs, according to this article from the NASA website. That information could help to answer the question of whether humans would be capable of reproducing if they settle in future colonies on the Moon or Mars, or if they venture on even longer journeys into the cosmos.
"If you don't have healthy sperm, you've got real risk to multi-generational survival in space," explains Joe Tash, an emeritus professor in the Department of Molecular and Integrative Physiology at the University of Kansas Medical Center. Tash was the lead scientist in developing the experiment, and continues to be involved as a consultant.
NASA has been conducting research on reproductive and developmental space biology since the 1990s, according to an email from researchers at NASA's Ames Research Center in California's Silicon Valley, who are involved in the Micro-11 research. In one study published in the March 14, 1995 issue of Proceedings of the National Academy of Sciences, for example, female frogs aboard a space shuttle were induced to ovulate so that their eggs could be fertilized. The study demonstrated that gravity-induced rotation of the zygote, the cell formed by the fusing of a sperm and egg, wasn't necessary to develop a free-swimming tadpole with a normal body.
Micro-11 marks the first time that human sperm — that is, aside from the ones inside male astronauts' bodies — have been launched into space, though there have been previous research efforts involving animal sperm. Back in the 1980s, German researchers launched bull semen into space on a suborbital rocket that briefly subjected them to a minute and a half of weightlessness, which was recorded on video, Tash says. Then, in 1997, Tash and colleagues sent samples of sea urchin sperm on two space shuttle missions. In those two experiments, it wasn't possible to shoot video under a microscope, so instead they looked at signal transduction, the molecular mechanism within the sperm cell that tells it when to start wriggling its tail.
The results of the German study and Tash's 1997 research both found that the sperm actually swam faster in microgravity than they do back on Earth. But in one of Tash's experiments, he detected what may be a big hindrance to procreating in space. When sea urchin sperm were exposed to chemical compounds called peptides that would be released by eggs, the signaling that triggers capacitation — an essential part of reproduction — occurred much more slowly.
The Micro-11 experiment will gather data on the effect of microgravity on swimming and capacitation in separate trials, according to Tash.
Radiation Exposure Is a Problem
But microgravity isn't the only factor that could influence reproduction in space. Outside of the Van Allen Belts that shield the Earth from high-energy particles from space, radiation exposure could create serious problems. A study published in the May 20, 2017 issue of Reproduction found that exposure to charged particles of the sort found in space damaged the ovaries of female mice.
"Our research found that the finite ovarian follicle reserve is depleted by exposure to charged oxygen or iron particles, which are components of galactic cosmic rays," the study's corresponding author, Ulrike Luderer, writes in an email. She is a professor of Medicine, Developmental and Cell Biology, and Public Health at the University of California, Irvine. "This damage is irreversible and will result in early onset of ovarian failure (premature menopause)."
Additionally, Luderer said human and animal studies provide evidence that exposure to gamma radiation and charged iron particles decreases sperm motility.
But even if it's possible to conceive in space, pregnancy and childbirth could be problematic. Kris Lehnhardt, a senior faculty member at the Baylor College of Medicine's Center for Space Medicine, says in an email that it's not known how well an embryo would develop in the environment of space, or whether natural birth would be possible.