Until Fred Gage came along, brain scientists accepted as a matter of faith that the neurons, or brain cells, you were born with were all the brain cells you would ever have. Then, two years ago, this 49-year-old neurobiologist at the Salk Institute for Biological Studies in La Jolla, Calif., showed in a groundbreaking experiment that neurons are constantly being born, particularly in the learning and memory centers. Gage's discovery forced scientists to rethink some of their most basic ideas about how the brain works.

Even more exciting was the fact that the source of these new cells was neural-stem cells, master cells with the ability to morph into any type of brain cell, depending on the chemical signals they receive as they grow. Early studies hint that they may even belong to a more primitive population of stem cells that can form anything from skin to blood to liver. Gage showed that a part of the hippocampus contains actively developing neural-stem cells; he further speculated that this regeneration may eventually be controlled by the timely addition or subtraction of a few key growth factors in the brain's chemical soup.