A team of American and Italian researchers has reported that in heart transplant patients, primitive cells from the recipient migrate to the new heart and aid in the growth of new muscle and blood vessels -- striking new evidence both of chimerism and the human heart's ability to repair itself.

The researchers made their discovery by studying men who received transplanted hearts from women, and finding male cells in the donated female hearts -- y-chromosomes that only could have come from their own bodies.

The researchers from New York Medical College and the University of Parma in Italy found that heart muscle and blood vessels grew rapidly in the new hearts after transplant. They calculated that as much as one-fifth of the donor heart had been rebuilt by the recipient's own cells.

Doctors have long assumed that damage from a heart attack or other ailment is irreversible and that the heart cannot regenerate tissue the way other organs can.

"Clearly this shows that the heart has the ability to regenerate," said Dr. Roberto Bolli of the University of Louisville, who wrote an editorial that accompanied the report in the New England Journal of Medicine. "It could be a milestone discovery if we learn how to exploit this phenomenon for therapeutic purposes to regenerate heart muscle in patients with heart failure."

"There have been hints from animal studies that the cells could migrate before, but this is the first demonstration in a human that it is actually possible," said John Fakunding of the National Heart, Lung and Blood Institute.