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If these cells are lost they are replaced by the

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Unformatted text preview: derived from self-renewing stem cells, as discussed in the following section. Other types of differentiated cells, however, retain the ability to proliferate as needed to repair damaged tissue throughout the life of the organism. These cells enter the G 0 stage of the cell cycle but resume proliferation as needed to replace cells that have been injured or died. Cells of this type include fibroblasts, which are dispersed in connective tissues where they secrete collagen (Figure 17.13). Skin fibroblasts are normally arrested in G0 but rapidly proliferate if needed to repair damage resulting from a cut or wound. Blood clotting at the site of a wound leads to the release of platelet-derived growth factor (PDGF) from blood platelets. As discussed in Chapter 15, PDGF activates a receptor protein-tyrosine kinase, stimulating both the proliferation of fibroblasts and their migration into the wound where their proliferation and secretion of collagen contributes to repair and regrowth of the damaged tissue. The endothelial cells that line blood vessels (Figure 17.14) are another type of fully differ...
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