A major obstacle to human colonization of the galaxy, or at least the Solar System, looks a touch less intimidating, thanks to some freeze-dried mouse sperm. Mouse sperm stored on the International Space Station for nine months has been returned to Earth and used to successfully inseminate female mice. The resulting mice had only slightly more mutations than controls born from sperm that stayed on the ground.
“Radiation on the International Space Station is more than 100 times stronger than that on Earth,” notes a paper in Proceedings of the National Academy of Sciences. The situation is similar on the Moon, and little better on Mars, given the near absence of protective atmospheres and magnetic fields. This has often been raised as an obstacle to expansion beyond Earth. Although humans can survive this radiation, there is a question of how well our germlines – eggs and sperm – would do over an extended period.
Following the tradition of sending animals to space to test what we are unready to risk ourselves, Dr Sayaka Wakayama and Professor Teruhiko Wakayama of the University of Yamanashi, Japan, froze mouse sperm to -95ºC (-139ºF) and had it circle the Earth for nine months. Although they found a small increase in DNA damage compared to Earth-bound sperm, the Wakayamas and their co-authors report that “fertilizations and birth rates are similar to those of controls.”
Damage to germlines sometimes shows up, not in the immediate offspring, but the following generation, so the team allowed some of the mice produced in this way to mate, finding they were fertile. The paper does not record any increase in genetic ill-health among mice descended from the cryogenic astronauts.
Testing the viability of frozen sperm is important because there is a high chance future space colonies will use it, at least to some extent. The authors note that assisted reproductive technology is on the rise. Space colonies are likely to start small, and face problems of insufficient genetic diversity, which could most easily be addressed using frozen sperm and eggs from those left behind. Yet frozen germ cells are unable to repair DNA damage in the way mammalian cells can do in a living body.
Although mice make useful subjects for such research, given the short gestation time once the sperm is returned to Earth, replication with a wider variety of animals may be required before we risk humans. There have already been a number of experiments on the reproduction of invertebrates, birds and most famously geckos in microgravity, with additional radiation an inevitable part of the research. Presumably, the next step is larger mammals, a topic on which the Muppets had something to say.