The stem cell niche in health and malignancy.

Somatic stem cells play a well-defined and important role in tissue renewal. Their malignant counterparts, cancer stem cells, are thought to be responsible for tumor initiation and possibly chemotherapy resistance, although controversy remains regarding both the origin and characterization of these cells. Both somatic and cancer stem cells appear to occupy specialized microenvironments in many organs. These niches are important for both maintenance of quiescence and control of cellular survival and proliferation. Targeting cancer stem cells and their microenvironments may provide new therapies to eradicate tumors. The efficacy of several drugs in current use is mediated at least in part via effects on the microenvironment, and new drugs that target the niche are currently in clinical trials.