The theory that heart attacks cause irreversible damage to muscle tissue was challenged yesterday by scientists who said that cells were capable of large-scale regeneration.

Specialists from New York Medical College studied the hearts of 13 patients who had suffered cardiac arrest and found that muscle cells in two regions of their hearts had replicated themselves.

Dr Claude Lenfant, director of the US National Heart, Lung and Blood Institute, said the findings provided "the most dramatic and clear-cut demonstration to date of heart cell regeneration after cardiac injury". He added: "We have a new understanding of the heart that opens up the possibility of repairing heart muscle damage after a heart attack."

Cardiologists who detected the repair mechanism studied the division of myocytes (heart muscle cells) near the site of the heart attack, and at a more distant site. Compared with normal hearts, the number of myocytes multiplying in diseased hearts was found to be 70 times higher in the border zone and 24 times higher in the remote myocardium.

Dr Piero Anversa, who led the research team, said the dividing cells could be a sub-population of known heart cells or primitive stem cells which targeted damaged regions of the heart. "If we can prove the existence of cardiac stem cells and make them migrate to the region of tissue damage, we could improve the repair of damaged heart muscle and reduce heart failure," he said.