Key to the heart? Scientists have identified what they say are the heart's "master" stem cells.

Lei Bu et al., Nature 460, 113 (2009)

Scientists Find Heart Stem Cells

Scientists have identified a cardiac stem cell that gives rise to all of the major cell types in the human heart. The find opens the way to using patients' own cells to heal their damaged hearts.

The cells in question express a protein, called Islet 1, which is present in the early stages of fetal heart formation. In recent years, scientists have identified the cells in embryonic mouse hearts. And now, a team in the laboratory of Kenneth Chien, director of the Cardiovascular Research Center at Massachusetts General Hospital in Boston, has found the same cell type in human fetal hearts.

Once the group pinpointed the cells, it took the next important step: generating new cardiac stem cells from human embryonic stem cells. Using fluorescent tags to identify the ones containing Islet 1, the researchers obtained a purified population. They then proved that the Islet 1 cells are what Chien calls "master stem cells" by showing that single cells could be made to grow into any of the heart's major cell types: heart muscle (cardiomyocytes), smooth muscle, and blood vessel lining (endothelium). The team reports its work today in Nature.

Chien cautions that these primordial stem cells, which are found only in fetuses, could not be used for therapy because they could develop into undesired cell types. Instead, he says, researchers need to isolate "intermediate" cells that are already heading for a particular fate. In the meantime, the primordial cells could be used for disease modeling and drug screening. They may also help shed light on congenital heart malformations. In the fetal heart, Islet 1 cells are clustered in areas that are "hot spots" for congenital heart defects, says Chien: "Congenital heart disease may be a stem cell disease."

Ultimately, researchers may be able to use the cells to grow human "heart parts" such as strips of heart muscle or a valve on scaffolds that could be inserted into patients, Chien says.

"The findings are very important if they can be reproduced," says cardiologist Richard Schatz of Scripps Clinic in San Diego, California. But Eduardo Marbán, director of the Cedars-Sinai Heart Institute in Los Angeles, says he's doubtful that the identification of Islet 1 cells will hasten new therapies. Marbán is currently heading a trial that involves removing a tiny chunk of heart tissue from a patient, cultivating cells from it, and reinjecting them into the patient's heart. He says Islet 1 cells "do appear to be important in development" but that "normal heart tissue can and does form in the complete absence" of the protein.

Chien has a different view, saying that there's little or no evidence that scientists can obtain stem cells by "grinding up hearts and culturing cells from them." It's important to identify authentic progenitor cells, he says, in order to identify cells that will help repair damaged hearts.