By Paloma Salmeron-O’Brien
Deciding whether or not to have kids is an important life decision, and there are a lot of bases you have to cover before making that leap. You have to think about your relationship, your financial stability, and oh, space radiation of course. Given the rapid evolution of technology and the growing interest in space exploration, scientists predict that humanity might orchestrate manned missions into deep space in the relatively near future (keyword here being “relatively”, Musk). Despite our current technological limitations, scientists are already evaluating the many risks associated with long-term space travel, one of which is radiation. Without the protection of Earth’s atmosphere and magnetic field, organisms are subject to various forms of strong radiation that permeate the vacuum of space. These include solar radiation from the sun as well as galactic cosmic rays originating from outside our solar system. Such high-energy radiation is difficult to shield against and poses the risk of damaging an organism's genetic material. The accumulation of radiation damage over long-term space exploration could lead to a variety of health issues, including cancer. Not only does radiation pose a risk to organisms directly exposed to it, but should the radiation cause damage to the organism’s reproductive cells (germ cells), potentially harmful mutations could arise and be passed on to their offspring as well. It gets even worse as, given the enormity of space, exploratory voyages could extend beyond one or two generations of crew. Continued exposure to radiation over the course of several generations could lead to the accumulation of heritable mutations. As a result, the genetic composition of a space-voyaging species could be drastically altered over a relatively short period of time, essentially leading to species-wide transformation. Needless to say, the dangers posed by space radiation exposure are extensive.
Knowing all this, a large team of Japanese researchers set out to examine the effects of space radiation on mammalian reproduction. Now, previous experiments subjecting cells to various types of radiation have been conducted in an effort to mimic the effects of space. However, scientists have been unable to replicate the qualities specific to space radiation, limiting the predictive capabilities of these tests. Scientists have also sent live animals and cells to the ISS to examine their growth in space, however, these experiments have all been short-term. Live subjects require constant maintenance and monitoring on the ISS, so live experiments aren’t viable over extended periods of time. Hence why the researchers decided to experiment with freeze-dried mammalian cells instead of live subjects. Cryopreserved cells have their metabolism halted and thus don’t require intensive upkeep, making them much better subjects to study the effects of space radiation. Freeze-dried mouse spermatozoa were specifically chosen because of their resistance to higher temperatures, meaning they could survive the rocket journey to the ISS without the need for special freezing equipment. Not only that, but were the experiment to be a success, it would set a precedent for using mouse cells to evaluate the effects of space on humans in future experiments.
As you can likely imagine, this experiment had a lot of different components. To prepare the sperm samples, the 12 best males were selected from an initial population of 66 mice by examining the quality of the sperm they produced. Of the spermatozoa produced, 24 of the best ampoules (sealed glass capsules) of sperm were selected from each male. These ampoules were then divided into six boxes, and from this finalized sample group, three boxes of ampoules were sent to the ISS while the remaining three became the ground-control (puns aside, that is the actual term used in the article). All spermatozoa were subjected to the same freeze-dry treatment to preserve them. Of the boxes sent to space, the first box was returned after 9 months as a means to see whether the experiment was proceeding correctly. The second box was returned 2 years and 9 months after the initial launch, and the third box remained on the ISS for a record-breaking 5 years and 10 months. When the space-treatment ampoules were retrieved, four ampoules (two from the second and third return groups) were re-hydrated and used to fertilize mouse oocytes. During this time period, the sperm ampoules from the three ground-control boxes were subjected to varying doses of x-ray radiation, the highest being 30 grays. Doses were administered over the course of an artificial fertilization process. Fresh sperm was used as a control in determining how much damage the radiation treatments caused.
In visually comparing the space and ground samples using a microscope, researchers observed no difference in the morphology of the sperm between samples. In surveying the produced mice pups, as anticipated, scientists found that radiation damage increased on a dose-dependent basis. Despite this, after fertilization, some viable offspring were still obtainable even after exposure to the highest radiation dosage. The proportion of these viable offspring from the freeze-dried spermatozoa was also significantly higher than that of the fresh control sperm. Additionally, within the freeze-dried sample groups, the proportion of viable space-treatment offspring was similar to that of the ground treatment. The viable “space-pups” exhibited no differences in fitness from the ground-pups, and their offspring showed no abnormalities. Production of a significant proportion of viable offspring even after the high-radiation dosage of 30 grays indicates that the freeze-dried sperm have a strong tolerance to radiation. As radiation is known to generate damage by interacting with water molecules inside cells, the tolerance of the treated sperm is likely due to the freeze-drying process of removing water from the cells’ interiors. Taking into account the radiation dosage administered and the average rate of damage that results, researchers predict that freeze-dried sperm could be preserved on the ISS for approximately 201 years, with or without protection from radiation. That said, the ISS orbits Earth at a distance of approximately 400km, meaning it is partially shielded from radiation by the Earth’s magnetic field. Deep space exploration may then yield a higher rate of radiation damage than seen in these experiments due to the complete lack of protection.
Being able to reliably transport preserved germ cells will be integral in the event of long-term deep space missions in order to sustain genetic diversity and organic resources. In the event that humanity was to set up habitation on an extraterrestrial planet, preserved germ cells would allow for safer means of reproduction on those outposts and would allow us to sustain populations of domestic animals and pets. Being able to preserve the genetic integrity of germ cells in space would allow humanity to maintain the sort of genetic diversity integral to weathering diseases, environmental changes, and other survival challenges that we might encounter on and beyond our planet. All of this brings humanity another step closer to freely exploring the stars.
S. W. https://orcid.org/0000-0001-8700-8993, Ito, D., Kamada, Y., https://orcid.org/0000-0003-0343-8035, T. S., Suzuki, T., https://orcid.org/0000-0001-8122-2307, A. N., https://orcid.org/0000-0003-3972-3153, R. A., Ishikawa, T., https://orcid.org/0000-0002-5615-2674, S. K., Hirose, N., Kazama, K., Yang, L., Inoue, R., Kikuchi, Y., Hayashi, E., Emura, R., https://orcid.org/0000-0002-7483-2120, R. W., Nagatomo, H., Suzuki, H., … Teruhiko Wakayama* https://orcid.org/0000-0002-6151-2603 [email protected]Advanced Biotechnology Center, U. of Y. (2021, June 11). Evaluating the long-term effect of space radiation on the reproductive normality of mammalian sperm preserved on the International Space Station. Science Advances. Retrieved February 27, 2022, from https://www.science.org/doi/10.1126/sciadv.abg5554#