Adult human hearts appear to mount a previously unconfirmed cellular response after a heart attack: some surviving heart-muscle cells show increased signs of division. The finding, reported in Circulation Research, could give cardiac-regeneration researchers a new biological process to investigate, but it does not show that damaged hearts regrow enough muscle to restore function.
Researchers affiliated with the University of Sydney, the Baird Institute and Royal Prince Alfred Hospital examined tissue collected from consenting patients undergoing bypass surgery. They found increased mitosis and cytokinesis in adult cardiomyocytes in areas affected by ischemia after myocardial infarction, according to the study account published by the Baird Institute. Mitosis is the process in which a cell divides its genetic material; cytokinesis is the subsequent physical separation into daughter cells. Evidence of both is more informative than evidence of cell-cycle activity alone, because it points toward actual cell division.
Evidence from living human heart tissue
The work is significant because much of what is known about attempts to regenerate heart muscle has come from laboratory animals, cultured cells or tissue obtained after death. The investigators used samples from living patients, comparing diseased and non-diseased regions of the heart. The University of Sydney described the approach as the first demonstration in humans of increased cardiomyocyte mitosis after a heart attack, a claim that should be understood as the researchers’ assessment of the existing literature.
The analysis combined immunostaining, which can identify proteins associated with cellular processes, with bulk RNA sequencing, proteomics, metabolomics and single-nucleus RNA sequencing. Together, those approaches examine different layers of biology: gene activity, proteins, metabolic products and the behavior of individual cell nuclei. The design can strengthen a biological observation when signals converge across methods, but it remains an observational study of sampled tissue, not a test of a treatment.
In the material publicly available from the institutions, the researchers do not report the number of patients, the size of the observed increase, the timing of tissue collection relative to infarction, or measures of pumping function and longer-term clinical outcomes. Those missing details limit what can be concluded about the magnitude and practical importance of the response. The study is identified as Human Hearts Intrinsically Increase Cardiomyocyte Mitosis After Myocardial Infarction in the institutional accounts, while the journal citation is listed as DOI 10.1161/CIRCRESAHA.125.327486.
Why cell division is not the same as heart regeneration
Cardiomyocytes are the contracting cells that enable the heart to pump blood. A heart attack deprives part of the heart of oxygen, killing cardiomyocytes and leaving scar tissue. In adult people, replacement of those cells has generally been considered too limited to offset the loss after a major infarction.
The distinction between detecting cell division and demonstrating effective regeneration is substantial. For regeneration to improve a damaged heart, newly produced cardiomyocytes would need to persist in adequate numbers and contribute meaningfully to working heart muscle. The available accounts of this study establish neither replacement of all lost tissue nor recovery of heart function. They instead describe an intrinsic response that is insufficient to prevent the damaging consequences of a heart attack.
That conclusion is consistent with the broader scientific backdrop. A peer-reviewed review of cardiac regeneration found that adult cardiomyocyte renewal under ordinary conditions is minimal and clinically negligible, even though experimental research has pursued ways to stimulate repair. The new human-tissue finding does not overturn that assessment. It suggests that the adult heart retains some capacity for cardiomyocyte division after injury, while leaving open whether that capacity can ever be increased safely and usefully.
A starting point for therapy research, not a current therapy
The research may help explain why efforts to activate repair pathways after heart injury have been so challenging. Rather than beginning with the premise that adult human cardiomyocytes never divide, future work can examine what distinguishes the cells and conditions in which division was observed. Researchers would still need to determine whether the response is durable, whether it produces functional muscle, and whether deliberately increasing it benefits patients.
No such intervention was evaluated in this study. It was not a randomized clinical trial, did not test a drug or procedure intended to regenerate myocardium, and did not show prevention or reversal of heart failure. Patients who have had a heart attack should not interpret the result as evidence that the heart will naturally replace the muscle lost during an infarction.
The public description from the University of Sydney makes the same practical qualification: the natural response is not enough to replace the muscle destroyed by a heart attack. Its value is as a human biological lead. If subsequent studies can define how often the response occurs, how large it is and what becomes of the dividing cells, the observation could help narrow the path toward future regenerative-medicine research.