Stem cells and the origin and propagation of brain tumors

In recent years there has been a flood of interest in the relationship between brain tumors and stem cells. Some investigators have focused on the sensitivity of normal stem cells to transformation, others have described phenotypic or functional similarities between tumor cells and stem cells, and still others have suggested that tumors contain a subpopulation of ;;cancer stem cells'' that is crucial for tumor maintenance or propagation. Although all these concepts are interesting and provide insight into the origins and properties of brain tumors, the use of similar terms to describe them has led to confusion. The goal of this review is to sort out some of that confusion and highlight what we know and what we have yet to learn.