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stem cells with the potential of replacing cells that are lost during the lifetime of the organism. It appears that stem cells reside within distinct
microenvironments, called niches, which provide the environmental signals that maintain stem cells throughout life and control the balance
between their self-renewal and differentiation. Stem cells are rare in adult
mammalian tissues, however, so the precise identification of stem cells and
their niches represents a major challenge in the field of stem cell biology.
For example, although the role of stem cells in maintenance of the intestinal
epithelium has long been recognized, the intestinal stem cells at the base of
the crypt (see Figure 17.19) were only recently identified by studies of Hans
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CELL DEATH AND CELL RENEWAL (A) (B) 21 Nucleus of muscle fiber Satellite
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