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Stem cell experiments on the International Space Station show a curious contrast between the effects of microgravity on the adult human heart and on the initial formation of laboratory-grown cardiac structures.
Microgravity, known to cause changes in astronauts’ cardiovascular systems during space missions, may have a different effect when the goal is to produce human heart tissues from stem cells.
Researchers at Cedars-Sinai, in the United States, observed that cardiac organoids, three-dimensional structures that reproduce some initial characteristics of the heart, were produced more easily and in greater quantities aboard the International Space Station than in systems used in laboratories on Earth.
The finding exposes a contrast studied by space medicine.
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In the already formed human body, the almost total absence of gravity alters fluid distribution, reduces part of the heart’s workload, and can lead to loss of muscle conditioning.
In cells still developing, however, this same environment seems to favor stages of cellular organization, according to the researchers involved in the experiments.
The discovery was presented by Arun Sharma, director of the Cedars-Sinai Space Medicine Research Center, during the 46th annual meeting of the International Society for Heart and Lung Transplantation, held in Toronto, Canada.
For the scientist, the difference may lie in when the cells come into contact with microgravity.
“On the one hand, you have things that have already been produced before being exposed to low gravity and potentially deteriorating,” Sharma told Space.com. “On the other hand, you are actually creating these things from scratch in space. It is possible that the production process is facilitated by low gravity.”
Microgravity alters astronauts’ hearts
During space missions, the body stops responding to gravity as it does on Earth’s surface.
Without the physical reference of up and down, bodily fluids redistribute.
Part of the blood that would normally be more concentrated in the legs begins to shift to upper regions of the body, such as the head and chest.
This change also affects the heart.
Since the organ does not need to pump blood against gravity in the same way, its musculature can lose conditioning over time.
Research with astronauts and studies with heart cells sent into space have already pointed to changes in the contraction, metabolism, and shape of the heart under microgravity conditions.
This process does not mean that the heart stops functioning, but it indicates that the cardiovascular system needs to adapt to the space environment.
For this reason, astronauts’ heart health is monitored before, during, and after missions.
In the experiments led by Sharma, the question analyzed was different.
Instead of observing only already formed tissues, the team investigated the behavior of developing human cells when exposed to microgravity.
Human mini-hearts are cardiac organoids
The mini-hearts mentioned in the research are not complete organs and do not have the capacity to replace a human heart.
These are cardiac organoids, small cellular clusters organized in three dimensions, used to reproduce basic aspects of developing heart tissue.
These structures are created from stem cells.
In many current studies, scientists use adult cells, such as those taken from skin or blood, and reprogram them in the laboratory to return to a stem cell-like state.
Then, through specific chemical signals, these cells are directed to transform into cardiac cells.
As the process advances, they cluster, begin to organize, and can exhibit behaviors associated with developing heart tissue, including contractions.
This type of model is used in disease research, drug testing, and studies on organ formation.
On Earth, one of the strategies to produce organoids on a larger scale involves suspension bioreactors.
These devices keep cells floating, a condition considered useful for three-dimensional growth.
To do this, however, the systems need to continuously agitate or rotate them.
Sharma stated that cells benefit from this suspension growth but can react to forced movement.
“Cells love to be cultured this way. But to force them into suspension, you typically need to spin them and introduce some kind of force, which the cells can perceive. And they don’t like being constantly agitated like that,” she said.
International Space Station allows cells in suspension
On the International Space Station, suspension occurs without the same need for mechanical rotation.
In microgravity, cells remain floating with less interference from external forces used in terrestrial equipment.
This environment can contribute to more uniform organoid growth, according to researchers.
Sharma reported that the experiments showed “a very significant increase” in the production of these structures, but did not provide exact numbers because the results have not yet been published in a peer-reviewed scientific article.
So far, the report points to a possible application of microgravity in regenerative medicine research.
The proposal does not involve immediate clinical use, but rather the investigation of conditions that allow the production of human tissues with characteristics different from those obtained in a laboratory on Earth.
Cedars-Sinai was already investigating how microgravity influences human cells and organoids.
In August 2025, the institution reported that researchers were working with heart and brain organoids derived from stem cells, with the aim of studying diseases and evaluating whether production in microgravity could offer advantages over conventional methods.
Heart tissues made in space are still in the experimental phase
Despite the results described by the team, the mini-hearts cultivated in space have not yet been used in humans.
Nor are there any clinical trials announced to apply this type of tissue directly to patients in the short term.
One of the fronts being studied in the area involves cardiac patches produced with stem cells.
These patches are being researched as a possible resource for people with heart muscle injuries, as human heart tissue has a limited capacity for regeneration after significant damage.
According to Sharma, microgravity may allow the production of thicker and more resistant patches, with a lower risk of collapsing under their own structure when compared to those cultivated on Earth.
This possibility, however, still depends on experimental validation, safety tests, and evaluation by regulatory bodies before any medical use.
At a stage closer to current scientific practice, cardiac organoids produced in space can serve for drug research.
As these structures reproduce part of the behavior of human tissue, they can help observe responses to drugs, investigate mechanisms of heart diseases, and compare the behavior of cells with different genetic profiles.
This use occurs in a context of a high global burden of cardiovascular diseases.
As these diseases are among the leading causes of death worldwide, experimental models closer to human tissue are being studied to reduce differences between results obtained in the laboratory and responses observed in patients.
Heart cell research to have new shipments to space
Sharma’s team plans to send new heart cell experiments to the International Space Station on a NASA resupply mission operated by SpaceX, CRS-35, scheduled for no earlier than August.
The goal is to broaden the understanding of human cell development in microgravity and assess under what conditions this environment can be used to manufacture biological tissues in a controlled manner.
The cost of transporting materials to space remains among the obstacles for this type of research.
Furthermore, any medical application will require standardization of methods, independent repetition of results, and regulatory approval.
Still, low Earth orbit offers a physical condition that is not identically reproduced in terrestrial laboratories.
For scientists in the field, this allows observing cellular processes from a different perspective than what is available in a normal gravity environment.
The discovery does not indicate that heart transplants made with space-grown tissues are imminent, nor that artificial mini-hearts are ready for clinical use.
The result presented so far is more delimited: an environment associated with the loss of adult heart conditioning can, under certain conditions, favor the initial formation of human cardiac structures in the laboratory.
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