Rebuilding tissue in the heart.

• Position: Stem Cells - Brief article

Injecting specialized cardiac stem cells into a patient's heart rebuilds healthy tissue after a heart attack, but where do the new cells come from and how are they transformed into functional muscle?

Researchers at Cedars-Sinai Heart Institute, Los Angeles, Calif., whose clinical trial results in 2012 demonstrated that stem cell therapy reduces scarring and regenerates healthy tissue after a heart attack, now have found that the stem cell technique boosts production of existing adult heart cells (cardiomyocytes) and spurs recruitment of existing stem cells that mature into heart cells.

"We're finding that effect of stem cell therapy is indirect. It stimulates proliferation of dormant surviving host heart tissue, and it attracts stem cells already in the heart. The resultant new heart muscle is functional and durable, but the transplanted stem cells themselves do not last long," explains Eduardo Marban, director.

Consistent with other studies, the researchers found that the heart's native stem cells are not responsible for the normal replenishment of lost...