Healthy mice are born from frozen sperm that have spent nearly six years exposed to space radiation on the ISS.

Teruhiko Wakayama, Yamanashi University

The most traveled mouse sperm in history left Earth in 2013 on a return trip to the International Space Station (ISS). After spending nearly six years on the station, the freeze-dried sperm was returned to Earth in a SpaceX cargo capsule in 2019 and used to rear litters of healthy “space puppies”.

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

What did the researchers do? The researchers collected the sperm from male mice and placed them in ampoules – small glass vials – before freeze-drying them to remove all the water. They stored the freeze-dried (FD) sperm both on the International Space Station and, in parallel, in freezers on Earth. Some sperm were returned after nine months on the ISS, to test that everything was working as expected, but two other groups of samples spent 1,010 and 2,129 days on the station.

Once turned over, the sperm were rehydrated and a type of mouse IVF was performed to impregnate female mice with space sperm and terrestrial sperm. The females then delivered their litters and the space puppies were compared to “ground control” puppies.

“The space puppies showed no difference from the ground control puppies, and their next generation also had no abnormalities,” the team wrote.

The researchers also assessed whether space sperm differed from sperm stored on Earth by examining damage to their DNA and gene expression. Under the microscope, space sperm looked identical to Earth’s, and the team also reported that no additional DNA damage was done to space sperm exposed to radiation. The gene expression profiles were unchanged.

Why is it important? “Space wants to kill you” is a catchphrase that’s used often – and for good reason. Space radiation passes through just about everything in the cosmos, and without proper protection it can collide with DNA causing breaks and mutations. Researchers don’t see DNA damage in freeze-dried sperm, which is a big win.

Researchers say the freeze-drying process, which removes water from their semen samples, may have a protective effect against DNA damage, as some of this damage is generated by the water in the sperm.

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

In the distant future, perhaps, we could also build some 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 a terrifying doomsday collapse in biodiversity (climate change could wreak such havoc).

And after? NASA and other space agencies around the world have plans to build the “Gateway”, a space station that will orbit the moon and serve as an outpost for human travel deeper into the solar system. In their concluding remarks, the researchers suggest that freeze-dried sperm experiments could also be conducted on the gateway to test the effect of space radiation further from Earth.

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