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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.
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