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Unformatted text preview: f intestinal crypts (Figure 17.19). The stem cells give rise to a population of transit-amplifying cells, which divide rapidly and occupy about two-thirds of the crypt. The transit-amplifying cells proliferate for three to four cell divisions and then differentiate into the three cell types of the colon surface epithelium: absorptive epithelial cells and two types of secretory cells, called goblet cells and enteroendocrine cells. The small intestine also contains a fourth cell type, Paneth cells, which secrete antibacterial agents. Each crypt contains approximately six self-renewing stem cells, which can give rise to all of the different types of cells in the intestinal epithelium. Stem cells are also responsible for continuous renewal of the skin and hair. Like the lining of the intestine, the skin and hair are exposed to a harsh external environment--including ultraviolet radiation from sunlight--and are continuously renewed throughout life. The skin consists of three major cell lineages: the epidermis, hair follicles, and sebaceous glands, which
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CELL DEATH AND CELL RENEWAL 19 (A) Surface epithelium Cell shedding Absorptive epithelial cells Goblet Enterocell endocrine cell Crypt Transitamplifying cells Stem cell Transitamplifying cells (B) Surface epithelium Stem cell Crypt FIGURE 17.19 Renewal of the intestinal epithelium (A) Colon epithelial cells are renewed by division of stem cells located at the bottom of the intestinal crypt. The stem cell gives rise to a population of transit-amplifying cells, which occupy about two-thirds of the crypt and undergo three to four divisions before differentiating into the three cell types of the surface epithelium (absorptive epithelial cells, goblet cells, and enteroendocrine cells). The surface epithelial cells continually undergo apoptosis and are shed into the intestinal lumen. (B) Micrograph of a colon crypt and surface epithelium. Proliferating cells are stained with antibody against a cell cycle protein (brown nuclei). (From F. Radtke and H. Clevers, 2005. Science 307: 1904.) release oils that lubricate the skin surface. Each of these three cell populations is maintained by their own stem cells (Figure 17.20). The epidermis is a multilayered epithelium, which is undergoing continual cell renewal. In humans, the epidermis turns over every two weeks, with cells being sloughed from the surface. These cells are replaced by epidermal stem cells, which reside in a single basal layer. The epidermal stem cells give rise to transit-amplifying cells, which undergo three to six divisions before differentiating and moving outward to the surface of the skin. The stem cells responsible for producing hair reside in a region of the hair follicle called the bulge. The bulge stem cells give rise to transit-amplifying matrix cells, 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
CHAPTER 17 Epidermis Cell loss Hair follicle Hair shaft Sebaceous gland stem cells Differentiation Sebaceous gland Bulge (hair follicle stem cells) Epidermal stem cell Transitamplifying cells FIGURE 17.20 Stem cells of the skin The epidermis consists of multiple layers of epithelial cells. Cells from the surface are continually lost and replaced by epidermal stem cells in the basal layer. The stem cells give rise to transit-amplifying cells, which undergo several divisions in the basal layer before differentiating and moving to the surface of the skin. Stem cells of hair follicles reside in a region beneath the sebaceous gland called the bulge, and distinct stem cells of the sebaceous gland reside at its base. which proliferate and differentiate to form the hair shaft. Finally, a distinct population of stem cells resides at the base of the sebaceous gland. It is notable that, if the skin is injured, stem cells of the bulge can also give rise to epidermis and sebaceous glands, demonstrating their activity as multipotent stem cells from which both skin and hair can be derived. Skeletal muscle provides an example of the role of stem cells in the repair of damaged tissue, in contrast to the continual cell renewal just described in 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 normal...
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