Stem cells have also been found in many other adult

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Unformatted text preview: contain 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 This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. © 2009 Sinauer Associates, Inc. UNCORRECTED PAGE PROOFS CELL DEATH AND CELL RENEWAL (A) (B) 21 Nucleus of muscle fiber Satellite cell On...
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