Fig. 1. Stem cells and their niches. (A) Supporting cells (green), in a cellular niche, provide a protective niche to stem cells (pink). The niche is composed of differentiated cell types that provide cell-cell contact and secreted factors (orange arrow) that maintain stem cells in a quiescent state. (B) Non-cellular niches have recently been identified; for example, the non-cellular niche of intestinal stem cells (ISCs) in the Drosophila ovary. In these niches, stem cells (pink) reside in a basement membrane and the signals that promote self-renewal come from the extracellular matrix (ECM; yellow). (C) Cancers arise from cancer stem cells (CSCs; red) - a rare population of self-renewing, multi-potent, tumor-initiating cells. It is not yet certain how CSCs arise; however, they may derive from normal stem cells that have acquired mutations. These mutations may confer on CSCs the ability to escape niche (blue) regulation. Alternatively, changes in the signals that emanate from the niche may be responsible.