Credit: Michigan State University

Though an estimated 60 million people around the world have atrial fibrillation, or A-fib, a type of irregular and often fast heartbeat, it’s been at least 30 years since any new treatments have been developed. This is because researchers haven’t had accurate models of the human heart to study. Thanks to new developments from Michigan State University scientists, that is no longer the case.

In 2020, MSU researcher Aitor Aguirre and his team started creating and perfecting tiny working models of the human heart known as organoids. Now, these organoids can be modified to replicate atrial fibrillation, or A-fib.

Roughly the size of a lentil, the three-dimensional heart organoids are so accurate that researchers can study heart development, diseases and drug responses in ways that were previously impossible. The organoids’ rhythmic beating is so strong that it can be seen without a microscope.

How the organoids are created

Led by Aguirre, associate professor of biomedical engineering and chief of the division of developmental and stem cell biology in MSU’s Institute for Quantitative Health Science and Engineering, the team uses donated human stem cells, which can develop into many different types of cells and are crucial for growth and tissue repair throughout life, to develop the heart organoids. These organoids are truly mini hearts, complete with chamber-like structures and vascular networks including arteries, veins and capillaries.

This schematic summarizes the key breakthroughs and findings from the recent paper published in Cell Stem Cell. Graphic courtesy of the Aguirre lab at Michigan State University. Credit: Michigan State University

The latest milestone from the Aguirre lab comes from MSU osteopathic medicine physician-scientist student Colin O’Hern, who added immune cells to the organoids. In developing human hearts, these immune cells, or macrophages, help ensure proper growth and formation.

The researchers were able to initiate inflammation in the organoids to cause an irregular heartbeat, which mimics A-fib. The findings are published in Cell Stem Cell.

Testing new therapies for A-fib

"Our new model allows us to study living human heart tissue directly, something that hasn’t been possible before," O’Hern said. "When we added inflammatory molecules, the heart cells began beating irregularly. Then we introduced an anti-inflammatory drug, and the rhythm partially normalized. It was incredible to see that happen."

No new drugs have been developed for A-fib in more than 30 years. This condition remains poorly treated because current therapies tend to target symptoms rather than underlying mechanisms. Therapeutic drug development for A-fib has struggled because there are no reliable animal models that resemble the disease.

"This new model can replicate a condition that is at the core of many people’s medical problems," Aguirre added. "It’s going to enable a lot of medical advances so patients can expect to see accelerated therapeutic developments, more drugs moving into the market, safer drugs and cheaper drugs, too, because companies are going to be able to develop more options."

Understanding heart development and disease

In the study, O’Hern and his fellow researchers showed that long-lived, innate immune cells that reside in specific organs help guide the heart’s development and rhythm. This information also helps researchers understand the origins of congenital heart disorders, the most common birth defects in humans.

The researchers then took things one step further. They developed a system to age the organoids to resemble adult hearts by exposing them to the kind of inflammation that leads to A-fib.

To demonstrate how the new model can be used to test therapies for inflammation-driven heart conditions, the team introduced an anti-inflammatory drug that, based on the team’s findings, was predicted to treat A-fib. This restored the heart’s normal rhythm.

Aguirre explained that the addition of immune cells makes the models more physiologically accurate than ever before.

"We’re now seeing how the heart’s own immune system contributes to both health and disease," he said. "This gives us an unprecedented view of how inflammation can drive arrhythmias and how drugs might stop that process."

Implications for future research and treatment

The lack of physiologically accurate human models and the inability to test on human hearts have limited the discovery of new therapies and drugs to treat arrhythmias like A-fib.

"Our new human heart organoid model is poised to end this 30-year drought without any new drugs or therapies," said Aguirre.

Aguirre’s human-based organoid technologies directly support the National Institutes of Health’s New Approach Methodologies mission to modernize translational research and improve the predictivity of preclinical testing in the U.S.

MSU researchers now are collaborating with pharmaceutical and biotech partners to screen compounds to ensure they don’t lead to heart damage while preventing arrhythmia.

Aguirre’s team has published multiple studies, establishing Michigan State University as a global leader in human heart organoid research, and Aguirre says more advancements are coming soon.

"Our longer-term vision is to develop personalized heart models derived from patient cells for precision medicine and to generate transplant-ready heart tissues one day," Aguirre said.

Other significant contributors to this research include Christopher Contag, Nureddin Ashammakhi and Sangbum Park from the MSU Institute for Quantitative Health Science and Engineering; Nagib Chalfoun from Corewell Health; and Chao Zhou from Washington University.

More information

Colin O’Hern et al, Human heart-macrophage assembloids mimic immune-cardiac interactions and enable arrhythmia disease modeling, Cell Stem Cell (2025). DOI: 10.1016/j.stem.2025.09.011

Journal information: Cell Stem Cell

Citation: Scientists develop mini human heart organoid that mimics atrial fibrillation (2025, December 14) retrieved 14 December 2025 from https://medicalxpress.com/news/2025-12-scientists-mini-human-heart-organoid.html

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