After almost six years on the space station, frozen mouse sperm produces healthy pups

Space radiation didn't cause excessive damage to freeze-dried sperm stored on the station for over 2,000 days.

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Healthy mice were born from frozen sperm that spent almost six years exposed to space radiation on the ISS.

Teruhiko Wakayama, University of Yamanashi

The most well-travelled mouse sperm in history left the Earth in 2013 on a return journey to the International Space Station (ISS). After spending almost six years on the station, the freeze-dried sperm were returned to Earth in a SpaceX cargo capsule in 2019 and used to breed litters of healthy "space pups."

The study, published in the journal Science Advances on Friday, details the space sperm experiments, which were conducted by a team of Japanese researchers aiming to understand the long term effects of space radiation on mammalian sperm. The freeze-dried sperm were sent to the ISS and spent nearly six years on the orbital laboratory, which zips around the Earth at a distance of around 250 miles. 

What did the researchers do? The researchers collected sperm from male mice and placed them in ampules -- small glass vials -- before freeze-drying them to remove all the water. They stored the freeze-dried (FD) sperm on both the International Space Station and, in parallel, in freezers on Earth. Some sperm were returned after nine months on the ISS, to test everything was working as planned, but two other groups of samples spent 1010 and 2129 days on the station. 

Once returned, the sperm were rehydrated and a type of mouse IVF was performed to impregnate female mice with space sperm and Earth sperm. Females then delivered their litters and the space pups were compared to "ground control" pups.

"Space pups did not show any differences compared to the ground control pups, and their next generation also had no abnormalities," the team wrote.  

The researchers also assessed whether the space sperm differed to the sperm stored on Earth by examining damage to their DNA and gene expression. Under a microscope, space sperm looked identical to those from Earth and the team also report no extra DNA damage occurred to space sperm exposed to radiation. Gene expression profiles were unchanged.

Why does it matter? "Space wants to kill you" is a catchphrase that gets thrown around a lot -- and for good reason. Space radiation blasts through just about everything in the cosmos and, without adequate protection, it can collide with DNA causing breakages and mutations. The researchers don't see DNA damage in the freeze-dried sperm, which is a big win. 

The researchers say that the freeze-drying process, which removes the water from their sperm samples, may have a protective effect against DNA damage because some of this damage is generated as a result of water within sperm cells.

However, the ISS is quite close to the Earth and is shielded from particularly dangerous space radiation by the planet's magnetic field. Whether deep space exploration will pose more trouble for freeze-dried sperm is an open question.

In the distant future, perhaps, we could also build a kind of frozen Noah's Ark in space, with freeze dried sperm from a number of species stored underground on the moon in case we experience some terrifying, apocalyptic crash in biodiversity (climate change might wreak that kind of havoc).

What's next? NASA and other space agencies from across the world have plans to build the "Gateway," a space station that will orbit the moon and act as an outpost for human travel deeper into the solar system. In their concluding remarks the researchers suggest freeze-dried sperm experiments could also be carried out on the Gateway to test the effect of space radiation further from the Earth.