Space travel accelerates stem cell aging – threatening astronaut health

For years, doctors and scientists have tracked how space changes the human body. Astronauts often come back with weaker muscles, thinner bones, and immune systems that do not respond as well as before. Now, a new discovery shows that the problem goes even deeper. The very stem cells that build blood and fuel the immune system start to age faster in orbit.

A recent paper in Cell Stem Cell shows that hematopoietic stem and progenitor cells—called HSPCs—lose their ability to renew themselves when exposed to microgravity and radiation aboard the International Space Station. These bone marrow cells normally give rise to red blood cells, white blood cells, and platelets. Without them, the body cannot replace worn-out blood or defend itself against infections.

Lead study author Catriona Jamieson, director of the Sanford Stem Cell Institute at UC San Diego, summed it up simply: “In space, stem cells decline in function. They actually reduce their ability to renew themselves or regenerate, and that’s an important thing to be able to know for long-term space missions.”

The project was part of the NASA-supported Integrated Space Stem Cell Orbital Research team. Instead of studying whole astronauts, researchers engineered tiny devices called nanobioreactors that mimic bone marrow. Each reactor was about the size of a cell phone and was seeded with human stem cells taken from hip replacement surgery patients.

To monitor activity in real time, the cells were equipped with a fluorescent reporter system, FUCCI2BL, which lights up depending on where the cell is in its cycle. The reactors were loaded into CubeLabs, small autonomous systems that can run experiments in orbit without constant human input. These units traveled on four resupply flights operated by SpaceX between late 2021 and early 2023.

Some stem cells spent as long as 45 days in space, while matching controls stayed on Earth. Afterward, scientists compared both sets using sequencing, gene expression profiling, and cytokine analysis.

Healthy stem cells spend about 80 percent of their time “asleep.” This quiet state preserves their ability to produce new blood and immune cells over a lifetime. But that pattern broke down in orbit. Cells woke up, stayed active, and quickly burned through their energy stores. By the time they returned to Earth, they showed signs of functional exhaustion.

Jamieson explained, “The stem cells woke up, and they didn’t go back to sleep, and they became functionally exhausted. If our stem cells become exhausted under conditions of stress like microgravity, then they won’t function to make a proper immune system.”

Tests confirmed that space-traveled HSPCs could not replate—or renew themselves—as often as ground controls. The problem was partly fixed when the cells were grown on a younger supportive stromal layer, but recovery was weak when placed back on their original bone marrow stroma. That suggests the environment around stem cells plays a major role in how well they bounce back.

Beyond the loss of renewal, the space samples showed molecular stress. Telomeres—the protective caps on chromosome ends—shortened. Mitochondria lost both gene activity and copy number, reducing energy capacity. A key self-renewal gene, ADAR1 p150, was turned down. Inflammatory signaling molecules spiked.

Even more concerning, the study revealed activation of so-called repetitive DNA elements, sometimes referred to as the “dark genome.” These sequences, which make up more than half of human DNA, are usually kept quiet. Under stress, they can switch on, behaving like ancient viral remnants inside the genome. Jamieson compared it to a “death spiral,” similar to what she sees in preleukemic cells that risk turning into cancer.

Whole-genome sequencing also revealed more mutations in blood-forming cells, including clonal hematopoietic mutations that could set the stage for long-term immune dysfunction. Enzymes like APOBEC3, which can cause genetic editing, appeared deregulated. Together, these changes painted a clear picture of accelerated cellular aging.

Another layer of stress came from inflammation. Cytokine analysis showed elevated levels of pro-inflammatory signals in the space cells compared with ground samples. Chronic inflammation is a well-known driver of stem cell aging, suggesting the orbit environment pushes these cells closer to exhaustion.

The effects depended on the supporting stromal environment. When grown on young stromal cells, space-traveled HSPCs downregulated inflammatory genes and activated protective ones, showing some recovery potential. But when placed on their own aged stroma, they lost protective gene activity, further weakening immune defenses.

This new work builds on NASA’s famous Twins Study, which followed astronaut Scott Kelly during a year in orbit compared with his twin brother Mark on Earth. That project revealed telomere changes, clonal hematopoiesis, and immune system shifts. The current research provides cellular-level evidence that blood stem cells themselves are at the root of many of these effects.

Spaceflight stem cell research actually began in 2010 with an experiment aboard the shuttle Discovery that tested how microgravity influenced mouse embryonic stem cells. What makes this new study different is the use of human blood-forming cells, monitored in real time during orbit rather than only after return.

The findings may sound dire, but there is good news. According to preliminary results from another study, stem cells can recover after astronauts return to Earth, though it may take up to a year. That suggests that damage is not always permanent, and with the right strategies, the risks could be managed.

Jamieson and her team plan to continue testing countermeasures, including medications that may block harmful genome activity. She sees bioreactors as “avatars for stem cell health” that can predict which astronauts might withstand space better and help identify treatments before missions.

Several scientists not involved in the study have praised its clarity. Arun Sharma of Cedars-Sinai Medical Center called the evidence “strong” and said it could help in designing therapies that slow or reverse aging. Luis Villa-Diaz at Oakland University agreed, noting that while the results reveal risks, they also give scientists a clear direction to develop protective strategies.

Elena Kozlova of Uppsala University added that her own research showed different results with other types of stem cells, where microgravity sometimes promoted growth genes. This highlights the complexity of space biology, where outcomes may depend on cell type and experimental conditions.

As NASA and other agencies prepare for long journeys to the Moon and Mars, these results raise urgent questions. If stem cells lose their strength in orbit, astronauts could face weaker immune systems, higher infection risk, and even a greater chance of blood cancers on extended missions. Protecting the smallest building blocks of human health may prove as critical as shielding spacecraft from radiation.

But beyond spaceflight, this work also matters for people on Earth. The same patterns of stress and accelerated aging appear in cancer patients and in people with preleukemic disorders. Understanding how to slow or reverse these processes could lead to better treatments for blood cancers, age-related immune decline, and other diseases linked to stem cell exhaustion.

Note: The article above provided above by The Brighter Side of News.

Like these kind of feel good stories? Get The Brighter Side of News' newsletter.