Space mice babies

Kyoto, Japan -- As space programs evolve and we continue to mistreat our own planet, human dreams of space tourism and planetary colonization seem increasingly common. However, features of spaceflight such as gravitational changes and circadian rhythm disruption -- not to mention radiation -- take a toll on the body, including muscle wasting and decreased bone density. These may even affect our ability to produce healthy offspring.

Studying the impact of spaceflight on germ cells -- egg and sperm precursor cells -- is particularly important because they directly influence the next generation, and any irreversible damage done to these will likely be transmitted to offspring. Previous examinations of embryonic stem cells that have undergone spaceflight have revealed abnormalities, but the exact cause of the damage has remained unknown.

This inspired a team of researchers at Kyoto University to test the potential damage to spermatogonial stem cells during spaceflight and the resulting offspring. The team utilized stem cells from mice, which have a much shorter reproductive life span than humans.

The research team first cryopreserved the stem cells and then sent them to the International Space Station, where they were stored in a deep freezer for six months. The cells were then returned to Kyoto, where the team observed no apparent abnormalities. After thawing and in vitro expansion, the research team transplanted the cells into mouse testes.

Within three to four months, offspring from these frozen cells were born through natural mating. When examining the newborn mice, the research team observed that they were healthy and exhibited normal gene expression. These results suggest that cryopreserved germ cells maintain fertility for at least six months.

"It is important to examine how long we can store germ cells in the ISS to better understand the limits of storage for future human spaceflight," says first author Mito Kanatsu-Shinohara.

Stem cells from many species can be cryopreserved and still produce sperm, so these findings contribute to the laying of a foundation for the development of germ cell preservation during future long-haul space missions.

The research team originally predicted that spaceflight would be more harmful to spermatogonial stem cells than cryopreservation, due to their sensitivity to radiation. However, the results actually revealed the opposite: while the concentration of hydrogen peroxide used in cryopreservation was sufficient to kill off some of the cells, the research team observed minimal differences between the pre- and post-spaceflight germ cells.

Additional assessments are nonetheless required. The mice offspring appear normal and do not exhibit abnormal DNA patterns, but long-term health issues cannot be ruled out until the lifespan and fertility of these mice and subsequent generations of mice are properly analyzed.

"We still have some spermatogonial stem cells frozen on the ISS, so we will continue to conduct further analysis," says Kanatsu-Shinohara.

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The paper "Germline transmission of cryopreserved mouse spermatogonial stem cells maintained on the International Space Station" appeared on 15 August 2025 in Stem Cell Reports, with doi: ___

About Kyoto University

Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at undergraduate and graduate levels complements several research centers, facilities, and offices around Japan and the world. For more information, please see: http://www.kyoto-u.ac.jp/en