Freeze-dried mouse sperm sent via postcard

“When I developed this method for preserving mouse sperm by freeze drying it on a sheet, I thought that it should be able to be mailed on a postcard and so when offspring were actually born after being mailed, I was very impressed,” said the University of Yamanashi’s Professor Daiyu Ito, first author of the iScience paper.

“The postcard strategy was easier and cheaper compared to any other method. We think the sperm never expected that the day would come when they would be in the mailbox.”

Ito belongs to the laboratory of mouse-cloning expert Professor Teruhiku Wakayama. The group had previously become the first to freeze dry and preserve mammalian sperm, which was sent to the ISS for studies into the effect of space radiation on young mice. The semen samples were originally preserved in a glass ampule (a small vial) which were prone to breaking and thus ruining the contents. This forced the researchers to use cushions to prevent breakage during transport, limiting the amount of sperm they could have flung into orbit.

These setbacks encouraged the scientists to search for an alternative method for preserving mouse sperm: one that would be less bulky or prone to breaking.

Plastic sheets were the most promising medium for transportation as they are compact, light, relatively strong and suitable for posting. The sheets were toxic for the sperm, however, so the researchers set about testing various materials to go inside the plastic sheets. Eventually, they identified weighing paper as the best option, being easy to handle and having the highest offspring rate.

This new method of preservation allows thousands of mouse strain’s sperm to be stored in a single book, which the scientists refer to as the “sperm book”. The sperm book was stored in a freezer at -30°C until further use.

Ito, Wakayama and their colleagues tested the limits of the sperm book by posting it tens of miles and found that it remained potent even after the cross-country journey. They were even able to post the semen from the sperm book as postcards by attaching the plastic sheet to the postcard, with no further protection necessary. One scientist even sent one of their peers a 'Happy New Year' card with mouse sperm attached as a unique gift.

The scientists suggest that this approach, once perfected, will have a real impact in their field. The next goal is to be able to store mouse sperm for at least one month at room temperature. In the future, they also hope to develop a method that will allow the freeze-dried sperm to “come back to life” through rehydration and then fertilise on their own.

“It is now recognised that genetic resources are an asset to humanity’s future. Even though many genetic traits are not needed for survival, depending on the environmental context, it is necessary to preserve them,” said senior author Wakayama. “The plastic sheet preservation method in this study will be the most suitable method for the safe preservation of a large amount of valuable genetic resources because of the resistance to breakage and less space required for storage.”