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the hematopoietic system, intestinal epithelium, and skin. Skeletal muscle is
composed of large multinucleated cells (muscle fibers) formed by cell
fusion during development (see Figure 12.21). Although skeletal muscle is
normally a stable tissue with little cell turnover, it is able to regenerate rapidly in response to injury or exercise. This regeneration is mediated by proliferation of satellite cells, which are the stem cells of adult muscle. Satellite
cells are located beneath the basal lamina of muscle fibers (Figure 17.21).
They are normally quiescent, arrested in the G0 phase of the cell cycle, but
are activated to proliferate in response to injury or exercise. Once activated,
the satellite cells give rise to progeny that undergo several divisions and
then differentiate and fuse to form new muscle fibers. The continuing
capacity of skeletal muscle to regenerate throughout life is due to selfrenewal of the satellite stem cell population.
Stem cells have also been found in many other adult tissues, including
the brain and heart, and it is possible that most—if not all—tissues...
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