Although an estimated 60 million people worldwide live with atrial fibrillation (A-fib)—a common condition marked by an irregular and often rapid heartbeat—no fundamentally new treatments have emerged in more than three decades. Researchers have long struggled to develop effective therapies largely because of the absence of accurate human heart models. Scientists at Michigan State University (MSU) have now overcome this barrier with a major breakthrough.
Mini Human Hearts Replicate A-fib
In 2020, Dr. Aitor Aguirre and his team at MSU began developing tiny, functional models of the human heart known as organoids. Over time, they refined these structures to the point where they could successfully replicate atrial fibrillation in the laboratory.
Roughly the size of a lentil, these three-dimensional heart organoids are remarkably lifelike. Not only do they mirror human heart development and disease, but they also beat rhythmically with enough force to be seen with the naked eye. As a result, researchers can now study cardiac function, disease mechanisms, and drug responses in ways that were previously impossible.
How Scientists Build Living Heart Organoids
Led by Dr. Aguirre, Associate Professor of Biomedical Engineering and Chief of the Division of Developmental and Stem Cell Biology at MSU’s Institute for Quantitative Health Science and Engineering, the team uses donated human stem cells to create the organoids.
These stem cells, which can develop into multiple cell types, enable the formation of miniature hearts complete with chamber-like structures and complex vascular networks, including arteries, veins, and capillaries. Consequently, the organoids closely resemble real human hearts at both structural and functional levels.
Immune Cells Trigger Irregular Heartbeats
In a key advancement, Colin O’Hern, an MSU osteopathic medicine physician-scientist trainee, introduced immune cells called macrophages into the heart organoids. During natural human heart development, these cells play a critical role in regulating growth and function.
By inducing inflammation within the organoids, the researchers successfully triggered irregular heart rhythms, closely mimicking A-fib. They published these findings in Cell Stem Cell, marking a significant milestone in cardiac research.
Testing Anti-Inflammatory Treatments in Real Time
Importantly, the team used the new model to test potential therapies. When researchers added inflammatory molecules, the heart organoids began beating irregularly. However, after introducing an anti-inflammatory drug, the heart rhythm partially normalized.
“This was the first time we could directly observe human heart tissue respond to both disease and treatment,” O’Hern said. “Seeing the rhythm improve was extraordinary.”
Because current A-fib treatments mainly address symptoms rather than underlying causes, this model opens new pathways for mechanism-based drug development.
A New Era for A-fib Drug Discovery
According to Dr. Aguirre, the absence of reliable human models has stalled A-fib drug development for decades. Existing animal models fail to accurately represent the disease, making therapeutic testing difficult.
“This model recreates a condition that lies at the heart of many serious medical problems,” Aguirre explained. “It will accelerate drug development, improve safety, and reduce costs—ultimately giving patients better options.”
Insights into Heart Development and Congenital Disorders
Beyond A-fib, the study also revealed that long-lived immune cells help regulate heart rhythm and development. This insight sheds light on the origins of congenital heart defects, the most common birth defects worldwide.
To further enhance realism, the researchers developed a method to age the organoids, exposing them to inflammation similar to what occurs in adult hearts. This allowed the team to model inflammation-driven arrhythmias with even greater accuracy.
More Realistic Models Through Immune Integration
By incorporating immune cells, the heart organoids now better reflect real human physiology. “We’re finally seeing how the heart’s own immune system contributes to both health and disease,” Aguirre said. “This gives us an unprecedented view of how inflammation drives arrhythmias—and how we might stop it.”
Transforming Future Research and Treatment
As reported by medicalxpress, the lack of physiologically accurate human heart models has long hindered arrhythmia research. With this innovation, MSU scientists believe they can end the 30-year stagnation in A-fib therapy development.
The technology aligns closely with the National Institutes of Health’s New Approach Methodologies (NAMs) initiative, which aims to modernize translational research and improve the predictive power of preclinical testing.
Collaborations and the Road Ahead
MSU researchers are already collaborating with pharmaceutical and biotechnology companies to screen drug candidates for both safety and anti-arrhythmic potential. Through multiple high-impact publications, Aguirre’s team has positioned MSU as a global leader in human heart organoid research.
Looking ahead, the team envisions creating personalized heart models from patient-derived cells for precision medicine and, eventually, generating transplant-ready heart tissue.
“Our goal is to transform how we study, treat, and ultimately prevent heart disease,” Aguirre said.
The study also involved key contributions from researchers at MSU, Corewell Health, and Washington University, underscoring the collaborative nature of this landmark achievement.