It’s always a delight to receive a thoughtful letter in the mail, but scientists in Japan have added a whole new layer to the experience by sending each other postcards containing freeze-dried mouse sperm. What’s more, the researchers were able to produce viable mouse offspring with the sperm that landed in their mailboxes after days in the post.
The unprecedented experiment could transform the way that sperm from many different species is transported, pioneering applications for “infertility treatments, livestock production, maintenance of strains of genetically modified individuals, and conservation of genetic resources, including those of endangered species,” according to a study published on iScience on Thursday.
“This is the first report in the world [to show] that freeze-dried mouse sperm can be preserved in a thin plastic sheet (0.2 millimeters) instead of conventional glass ampoules,” said Daiyu Ito, a PhD student at the University of Yamanashi who led the new study, in an email.
“We went through various trials and errors and finally succeeded,” he continued. “When we were able to develop a method of preservation of mouse sperm freeze-drying on a sheet, we thought that mouse sperm should be able to be mailed on a postcard by this method” which is the “absolute cheapest” technique ever developed to transport sperm.
Ito works in the laboratory of Teruhiko Wakayama, a biologist at the University of Yamanashi who has led innovative research into sperm transportation for years. Wakayama and his colleagues previously developed a technique for shipping mouse sperm in small glass capsules, or ampoules, which provided a simpler alternative to sperm cryopreservation in liquid nitrogen, an expensive and high-maintenance process.
Using these glass ampoules, the team was able to send mouse sperm to the International Space Station, where they were exposed to years of outer space radiation in what became the station’s longest biological experiment. After the sperm was returned to Earth and transplanted into female mice, healthy pups were born—a promising sign that long-term space travel may not prevent reproduction.
Though the ampoules enabled the ISS experiment, glass containers are also fragile and bulkier than paper, inspiring Wakayama and his colleagues to keep searching for even simpler methods to transport sperm. The new study documents the team’s efforts to preserve freeze-dried sperm on many different surfaces that could be easily mailed or stored in massive “sperm books” that collate samples from thousands of individuals.
Placing the sperm between sheets of weighing paper, which were then sealed in plastic, emerged as the best option in the laboratory, so the team set out to test the efficacy of the method in Japan’s postage system. Samples were kept at low temperatures prior to being dropped in the mailbox, but the sperm-laden postcards spent up to three days traveling at room temperature through the mail system.
Amazingly, this “sperm air mail,” as the study calls it, produced healthy mouse pups after arriving at new destinations.
“We had confidence that we could make offspring from postcard sperm, but we were really impressed when the mouse offspring were actually born, because the sperm was stored in very thin sheets and sent by postcard without any protection,” Ito said.
The feat is even more impressive considering that the team mailed sperm samples during the New Year season, a particularly hectic time for the post office. One of the scientists even sent a festive “Happy New Year” card to a colleague, with mouse sperm as an extra token of good luck.
“As you know, ‘Happy New Year’ greeting cards are sent by many people,” leading to what “may be the most busy time at the post office,” Ito explained. “So, if we can succeed with this card, we can expect to succeed no matter what time of year we mail them.”
“In addition, since New Year's greeting cards are used to write one's hopes for the next year, we conducted the New Year's greeting card mailing experiment in the hope that it would be a good year, i.e., that we would be able to successfully mail sperm and publish a paper,” he added. “Then, our hope was fulfilled and we were able to publish our paper in a good journal: iScience.”
These sperm postcards could have big implications for a wide variety of fields that rely on sperm transportation and storage. The new study notes that “genetic resources are an asset to humanity’s future” that must be preserved in the event that species are endangered by diseases or environmental changes, including those linked to the climate crisis. Mailed sperm could also simplify the process of livestock reproduction, assisted reproductive technologies, and other collaboration between laboratories.
“Several species have succeeded in producing offspring from freeze-dried sperm, such as rats, hamsters, and rabbits,” Ito said. “In the case of humans, the application of this technology will be in the distant future because it requires a lot of experimentation and a guarantee of absolute success for humans.”
At the same time, Ito and his colleagues warned that the convenience of their method could also lead to an increase in illegal transportation of sperm. Ito cited recent attempts to smuggle the sperm of Wagyu bulls from Japan to China as one example of a crime that could be harder to detect if sperm postcards are widely implemented.
“If plastic sheets containing freeze-dried sperm are used, they will be easy to hide and the current transportation system will not be able to detect the freeze-dried sperm and will cause the illicit outflow of genetic resources,” he said. “Therefore, for the establishment of this sheet preservation technique for freeze-dried sperm, it is essential to not only improve the success rate of offspring and extend the preservation period at room temperature, but also to prepare international laws to prevent their use for purposes other than legitimate purposes, even if this technology was misused.”
In the near-term, the researchers plan to anticipate these undesirable contingencies, while also improving on the reliability of their technique. The team also hopes to successfully freeze-dry eggs, which could open up similarly convenient modes of transportation of female reproductive cells.
“So far nobody has succeeded in preserving eggs in a freeze-drying condition” and “we believe that we can do it,” Ito concluded.