The stem-cell menagerie

The numbers, types and locations of stem cells in the nervous system have been the subject of much discussion. This review summarizes data on the types of stem cell present at different stages of development and in the adult brain, and the markers suggested to distinguish between the various possibilities that have been reported. We present evidence that more than one class of stem cell is present in the developing and adult nervous systems, and that it might be possible to distinguish between stem-cell populations and to localize the cell of origin of a particular neurosphere, based on markers that persist in culture and by using universal stem-cell markers prospectively to identify stem cells in vivo.