Adult stem cells, found in the liver, bone marrow and elsewhere, are the biological workhorses that repair injuries and form new tissue. Researchers hope to use them to cure Alzheimer's disease, say, or grow new livers; but the large numbers of cells such treatments would require can't reasonably be harvested from human donors-and many types of adult stem cells are difficult to mass-produce in culture. MIT bioengineer James Sherley may have found a way around that difficulty, potentially overcoming a huge barrier to putting adult stem cells to medical use.

Sherley's group has found a chemical that allows adult stem cells-at least those from rat livers-to multiply in culture indefinitely until it is removed. If Sherley's approach pans out for human adult stem cells, researchers and doctors could finally have a way of generating enough cells to repair damaged organs and tissues. "If you can grow these things," says Yale University stem cell biologist Diane Krause, "maybe we'll be able to inject them and get them to work [inside the body]."