A CSP-derived cell (green) settles into injured heart tissue.

Before scientists can harness heart stem cells to repair damage to the organ, they have to pin down the elusive cells. On page 329, Oyama et al. bolster the case for a group of cells known as side population cells, showing for the first time that they home in on injured heart muscle and differentiate into all the types of cardiac cells in vivo.

Previous research has turned up several candidates for cardiac stem cells, including the side population cells. In the bone marrow, side population cells function as stem cells. The situation in the heart wasn't clear, however. If they are grown with heart muscle cells in vitro, cardiac side population (CSP) cells can specialize into heart muscle. However, researchers didn't know what stimulates the cells to differentiate or what they're capable of in vivo.

Oyama et al. offered cultured CSP cells a variety of growth factors, but found that only two—including the hormone oxytocin—coaxed the cells to differentiate. Whether these molecules control differentiation in vivo isn't clear, the scientists say, but their discovery might help researchers devise a recipe for nurturing the cells.

To study CSP cell behavior in vivo, the researchers injected tagged cells into rats. CSP cells ended up in several organs, but mainly in the heart. More of them found their way to the organ if it was damaged, indicating that they respond to signals from injured cardiac tissue. These new arrivals specialized into heart muscle cells, but also into blood vessel and connective tissue cells. That finding demonstrates that CSP cells can spawn all cell types in the heart, the researchers note. They conclude that CSP cells are serving as stem cells in the heart.