In experiments with pigs, scientists have for the first time used human embryonic stem (HES) cells to repair damaged hearts. A team headed by Lior Gepstein at the Israel Institute of Technology (Technion) in Haifa has shown that the stem cells can be grown into heart muscle cells that not only pulse in a dish, but integrate into living hearts and fix damaged tissue.

The team selected cells with cardiac-specific markers from cultures of HES cells. To see if they would indeed behave like heart cells, they mixed some of these in with cultures of heart cells from newborn rats. The new cells started pulsing in unison with the rat cells, indicating that they'd formed successful electrical connections.

The real test, though, was in living animals. The researchers performed a "heart block" on 13 pigs by disabling the nerve bundle that conducts electrical impulses between the atria and ventricles. This greatly slowed the heartbeat and created the kind of condition that might require a pacemaker in a human patient. Then they injected 100 or so tiny beating clusters of HES cells into the wall of the ventricle. After a few days, they report online on 26 September in Nature Biotechnology, the heart rhythms improved in 11 animals--and the improvement was regular and sustained in six. In contrast, the injection of noncardiac stem cells did nothing.

Stem cell researcher J–rgen Hescheler of the University of Cologne in Germany says that HES-derived heart muscle cells may be the only kind of tissue that could treat heart attacks. Researchers have been experimenting with several types of muscle and bone marrow stem cells, which are easier to obtain and sidestep the ethical debate over using HES cells. But Hescheler says his recent work proves that bone marrow cells will not differentiate into cardiac cells after transplantation.

The Israeli team believes heart cell grafts may one day replace mechanical pacemakers. But first, notes Gepstein, research will have to show that the transplanted cells can keep working over many years.

--CONSTANCE HOLDEN