Muscle cells in damaged hearts may divide.

Crooners who lament that broken hearts never mend may need to find another tune. New evidence suggests that, contrary to scientific consensus, heart muscle cells do divide and the number of cells can increase.

The vast majority of heart muscle cells, or myocytes, had been thought to stop dividing by the time a person reaches the age of 9. These cells then pump blood for the rest of a healthy person's life. In people stricken by a heart attack, the cells die and are replaced by scar tissue.

The first report of a human myocyte caught in the act of dividing appears in the July 21 Proceedings of the National Academy of Sciences. The myocyte came from a left ventricle, the most powerful chamber of the heart and the one most afflicted by heart attacks. Piero Anversa of New York Medical College in Valhalla, N.Y., and his colleagues used a powerful optical microscope to look for dividing cells in tissue damaged by heart attacks. They detected cell nuclei that were splitting.

"When we saw that, we really jumped," Anversa says. Other scientists searching for dividing heart cells had not tried that technique, he adds.

The finding strengthens the possibility that scientists can develop medical treatments to enhance cell division and restore healthy heart muscle, says Anversa. It is too soon, however, to know to what extent such treatments might repair damaged hearts, such as those suffering congenital heart disease (see p. 62).

Anversa and his colleagues at the University of Udine in Italy, studied hearts removed from 27 people who had received transplants. The researchers stained slices of heart tissue half a micrometer thick and used a confocal microscope to count the individual cell nuclei caught in the process of division. Judging from these samples, he and his colleagues estimate that 131 to 152 myocytes in every million were dividing.

That proportion suggests that the cells of a damaged ventricle could be replaced within a year, the researchers calculate. However, many heart-attack victims do not recover their previous pumping efficiency nearly that quickly.

Anversa speculates that if new cells merely enlarge the heart, the weakened organ may suffer further damaging strain. If the cells grow in a way that thickens the heart wall, however, they might make the organ stronger. Scientists don't yet know how to influence cell division to regenerate healthy tissue.

The extent of cell proliferation that Anversa observed remains controversial. Myocytes can contain multiple nuclei, so dividing nuclei do not necessarily provide the best measure of cell proliferation, comments Kenneth R. Chien of the University of California, San Diego. However, Anversa says, the proportion of myocytes with multiple nuclei does not increase after heart attacks.

If the findings allow scientists to modify heart cell division, "it will be tremendously important," says Radovan Zak of the University of Chicago.