Adult human heart-muscle cells are permanently post-mitotic and are never replaced.

For most of the twentieth century, adult heart-muscle cells were thought to be permanent. Once development was complete, cardiomyocytes were expected to last for life rather than divide and be replaced.

That seemed consistent with what happens after a major heart attack. Dead heart muscle is largely replaced by scar tissue instead of being rebuilt as new working muscle.

The problem was that extremely slow cell renewal is difficult to measure in a living human heart. Researchers found an unusual clock in atmospheric carbon-14. Above-ground nuclear tests during the Cold War caused a sharp, well-dated rise in carbon-14 in the atmosphere. When a cell makes new DNA, it incorporates carbon from that time.

By measuring carbon-14 in the DNA of heart-muscle cells and comparing it with the atmospheric record, researchers could estimate when those cells were formed. In 2009, this showed that some adult cardiomyocytes were younger than the people carrying them.

The estimated renewal rate is low—about 1% per year in younger adults and less with age—and exact estimates vary. That is enough to overturn the claim of no renewal, but nowhere near enough to regenerate the muscle lost in a major heart attack.