New research indicates that humans are not limited to the heart cells they have at birth. Rather, new heart cells, or "cardiomyocytes," are formed during people's lifetime.

In light of this finding, Dr. Jonas Frisen, from the Karolinska Institute, Stockholm and colleagues believe that it would be worthwhile to develop therapies for heart attack and other heart diseases that can stimulate renewal of heart cells.

According to the report in Friday's issue of Science, the researchers estimated the age of cardiomyocytes by measuring carbon-14, an isotope that was incorporated into DNA at relatively high levels after above-ground nuclear bomb testing in the 1950s. Since above-ground nuclear bomb testing was outlawed, DNA levels of carbon-14 have gradually decreased.

"Because DNA is stable after a cell has gone through its last cell division, the concentration of carbon-14 in DNA serves as a date mark for when the cell was born and can be used to retrospectively birth date cells in humans," the researchers explain.

They found that cardiomyocytes do, in fact, renew, albeit at very slow rates. Moreover, the rates decrease with age; at 25 years, 1 percent turn over annual, whereas by 75 years, just 0.45 percent renew annually.

During a normal lifespan, fewer than 50 percent of cardiomyocytes are renewed, Frisen and colleagues note.

The development of pharmacological strategies to stimulate heart cell renewal "may be a rational alternative or complement to" cell transplantation strategies for heart cell replacement, they conclude.