We’ve heard a lot of talk about using stem cells to produce replacement organs. Many have dismissed it as science fiction. But that's no longer true. Researchers at the University of Minnesota have been quietly perfecting techniques to do precisely that. And they are using not embryonic stem cells—but the much more prosaic bone marrow cells. They have been able to strip a heart of all of its cells except for the gelatin-like extracellular matrix, and repopulate the matrix with fresh, healthy cells. It works in rats, and some of the techniques involved in this work are already in clinical trials—not yet to replace hearts, but to improve function in ailing hearts.