Until now scientists had believed that the human heart was unable to repair itself, but now researchers at Karolinska Institutet in Sweden have solved the mystery by measuring the radioactive isotope carbon-14 stored in cells, that heart cells are replaced throughout a person's life.

There is an urban myth of sorts that says that all cells in our body are replaced every seventh year. Science has shown, however, that some types of cell are renewed each week, others never, for many cell types, the rate of replacement is still a mystery.

For instance, it has long been mooted whether heart cells remain the same throughout life or whether they are replaced, a debate that is of immense significance to how cardiac diseases can be treated.

The results, which were presented in the journal Science, make the replacement of cells lost during myocardial infarction a real possibility.

Researchers at Karolinska Institutet have now shown that human heart cells undergo continual, slow replacement. In a 20-year old, one per cent of heart cells are renewed every year; this rate then gradually declines over the years, reaching 0.5 per cent in a 75-year old.

The slow rate of turnover means that the majority of heart cells remain unreplaced during a normal human lifespan, leaving the heart a patchwork of cells that have been there from birth and cells that have been formed later in life.

"The loss of heart cells after, say, a heart attack often leads to impaired cardiac function," says study leader Professor Jonas Frisén. "This new finding that heart cells can be replaced motivates further research into ways of stimulating the renewal mechanism to replace the cells that have been lost."

"A lot of people suffer from chronic heart failure," noted co-author Dr. Ratan Bhardwaj, also from the Karolinska Institute. "Chronic heart failure arises from heart cells dying," he said.

With this finding, scientists are "opening the door to potential therapies to having ourselves heal ourselves," Bhardwaj said. "Maybe one could devise a pharmaceutical agent that would make heart cells make new and more cells to overcome the problem they are facing."

But roadblocks remain. According to Bhardwaj, scientists do not yet know how to increase heart cell production to a rate that would replace cells faster than they are dying off, especially in older patients with heart failure.

In addition, the number of new cells the heart produces was estimated using healthy hearts -- whether the rate of cell turnover in diseased hearts is the same remains unknown.

The results were obtained using a unique method of cellular age determination developed by the research group itself.

The method exploits the fact that the nuclear tests of the Cold War produced a sharp increase in atmospheric concentrations of radioactive carbon-14. Carbon-14 is stored in the body's cells, and since levels have varied over the past decades, they serve as an indicator of when the cells were formed.