Stem cells need the right microenvironment to remain multipotent and to regulate the timing and pattern of their divisions. In addition to intrinsic regulatory signals—such as the presence of certain transcription factors and other regulatory proteins—stem cells rely on extrinsic hormonal and other regulatory signals from surrounding cells to maintain their status as stem cells. The location where a stem-cell fate can be maintained is called a stem-cell niche, by analogy to an ecological niche—a location that supports the existence and competitive advantage of a particular organism. The right combination of intrinsic and extrinsic regulation, imparted by a niche, will create and sustain a population of stem cells.

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In order to investigate or use stem cells, we must find them and characterize them. It is often difficult to identify stem cells precisely; they are very rare among cells and generally lack distinctive shapes. Most stem cells divide rarely, if at all, until stimulated by signals that convey the need for new cells. For example, inadequate oxygen supplies can stimulate blood stem cells to divide, and injury to the skin can stimulate regenerative cell division starting with the activation of stem cells. Some stem cells, including those that form the continuously shed epithelium of the intestine, are continuously dividing, usually at a slow rate. In the rest of this section, we focus on four types of stem cells in plants and animals that are well characterized; in the coming years, other types of stem cells will also be understood in great detail.