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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
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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