Cancer stem cells and brain tumors: uprooting the bad seeds.

The cancer stem cell (CSC) hypothesis is predicated on the idea that not all cells have equal proliferative potential and that, in brain tumors, the cells with the greatest ability to proliferate and contribute to tumorigenesis have phenotypic and functional properties similar to normal neural stem cells (NSCs). Over the past few years, multiple investigators have shown that CSCs isolated from human brain tumors (glioma and medulloblastoma) undergo self-renewal and multilineage cell differentiation, similar to normal NSCs. In addition, CSCs from these tumors, when implanted into rodent brains, generate tumors histologically identical to the parental tumors, suggesting that progenitor/stem cells can fully recapitulate the neoplastic phenotype in vivo. While these seminal studies clearly highlight the central role of stem cells in brain tumors, they also evoke important questions regarding the importance of these unique cells to tumor initiation, maintenance and treatment.