:INFO One Billion Cells. One Third Lost. Zero Replacements. Until Now. A single heart attack can kill up to a third of the cardiomyocytes inside the human heart. For most of medical history, that loss was considered permanent. The adult heart, unlike the liver or skin, was thought to have surrendered its capacity to divide at birth. Scar tissue filled the gap. Heart failure followed. On January 16, 2026, a paper published in Circulation Research quietly overturned that assumption. Researchers at the University of Sydney, the Baird Institute, and Royal Prince Alfred Hospital showed, for the first time in humans, that cardiomyocytes do increase cell division after a heart attack. The heart tries to heal itself. It just does not try hard enough. :LINK https://www.ahajournals.org/doi/full/10.1161/CIRCRESAHA.125.327486 Hume et al., "Human Hearts Intrinsically Increase Cardiomyocyte Mitosis \ :INFO A World First Built on Bypass Surgeries The difficulty with studying human heart regeneration had always been tissue. You cannot biopsy a heart the way you biopsy a kidney. What Dr. Robert Hume and Professor Sean Lal's team developed was a method of collecting pre-mortem samples from consenting patients undergoing coronary artery bypass surgery at Royal Prince Alfred Hospital. From each patient, they took tissue from both the infarct border zone and a remote zone of healthy tissue. The samples were subjected to immunostaining, bulk RNA sequencing, proteomics, metabolomics, and single-nucleus RNA sequencing, and cross-checked against the largest published human post-infarction single-nucleus dataset available. Both lines of evidence pointed to the same conclusion: adult human cardiomyocytes re-enter the cell cycle after ischemia. Some complete cytokinesis, the physical step of splitting into two daughter cells. This had been seen in rodents for 25 years. No one had shown it in humans until this paper. :IMAGE :INFO Credit: Dr. Robert Hume, University of Sydney. Pink areas show heart muscle \ cells replicating in tissue collected during bypass surgery from a post-infarction patient. This type of image had previously only been captured in mouse models. :INFO Why the Repair Fails: An Incomplete Signal The discovery does not mean the heart can fix itself. The increase in cardiomyocyte mitosis is real, but modest. A heart attack destroys hundreds of millions of cells in hours. The regenerative response, while genuine, replaces a tiny fraction of what was lost. And the process is incomplete: many cells that re-enter the cell cycle stall before cytokinesis, dividing their DNA but not their bodies, which can leave enlarged, dysfunctional cells rather than two healthy ones. The team identified multiple proteins involved in this partial response, including several already known from rodent cardiac regeneration studies. That overlap is important: it means the molecular machinery for regeneration is conserved between species. The heart has not lost the program. It has down-regulated it. That distinction matters enormously for therapy. A pathway that no longer exists cannot be reactivated. A pathway that is suppressed can, in principle, be unlocked. :QUOTE [quotetype:plain, subtitle:Dr. Robert Hume at University of Sydney (lead author)] Until now, we thought that because heart cells die after a heart attack, those areas were irreparably damaged. Our research shows that while the heart is left scarred, it produces new muscle cells, which opens up new possibilities. :STATS [icon:FIRE] Cardiomyocytes lost in a typical heart attack | up to 1 in 3 [icon:CLOCK] Years mouse studies showed post-MI mitosis before humans confirmed | 25 [icon:USERS] Australians living with heart failure | approximately 144,000 [icon:HEART] Heart transplants performed annually in Australia | approximately 115 [icon:CHART] Cardiovascular disease share of all deaths in Australia | 24% [icon:STAR] Journal | Circulation Research, Vol. 138, Issue 2, January 2026 :LINK https://www.sydney.edu.au/news-opinion/news/2026/01/20/human-heart-regrows-muscle-cells-after-heart-attack-world-first-study-shows.html University of Sydney: "Human heart regrows muscle cells after a heart attack, \ :INFO The Gap Between Signal and Treatment Is Large. The Direction Is Right. What the team now has is a laboratory model built from human tissue. Using it, they plan to test whether the identified proteins can be targeted to amplify the heart's own mitotic signal. If a drug could push the regenerative response from marginal to meaningful, a patient who today progresses from heart attack to heart failure might instead recover real cardiac function. That outcome is years away. No therapy has been shown in humans to regenerate cardiac muscle in clinically significant quantities. The path from "the heart tries" to "the heart recovers" requires understanding why the cell cycle stalls and whether the window for intervention is hours or days after the event. But the foundation is now built on human biology rather than mouse biology. That shift matters: cardiac regeneration approaches that worked in rodents have repeatedly failed in human trials because the underlying biology was assumed rather than confirmed. Hume and Lal's team has now confirmed it. :QUOTE [quotetype:plain, subtitle:Professor Sean Lal at University of Sydney (senior author)] Ultimately, the goal is to use this discovery to make new heart cells that can reverse heart failure.