celldeath terpse

However cells are lost either due to injury or

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Unformatted text preview: adult tissues and organs, cell death must be balanced by cell proliferation. 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 14 CHAPTER 17 FIGURE 17.13 Skin fibroblasts Scanning electron micrograph of a fibroblast surrounded by collagen fibrils. ( CMEABG-UCBL/Photo Researchers, Inc.) In order to maintain this balance, most tissues contain cells that are able to proliferate as required to replace cells that have died. Moreover, in some tissues a subpopulation of cells divide continuously throughout life to replace cells that have a high rate of turnover in adult animals. Cell death and cell renewal are thus carefully balanced to maintain properly sized and functioning adult tissues and organs. Proliferation of Differentiated Cells Most types of differentiated cells in adult animals are no longer capable of proliferation. If these cells are lost, they are replaced by the proliferation of less differentiated cells 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 differentiated cell that remains capable of proliferation. Proliferation of endothelial cells allows them to form new blood vessels as needed for repair and regrowth of damaged tissue. Endothelial cell proliferation and the resulting formation of blood capillaries is triggered by a growth factor (vascular endothelial growth factor or VEGF) produced by 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 15 Capillary lumen FIGURE 17.14 Endothelial cells Electron micrograph of a capillary. The capillary is lined by a single endothelial cell surrounded by a thin basal lamina. ( Dr. Don W. Fawcett/Visuals Unlimited.) Endothelial cell cells of the tissue that the new capillaries will invade. The production of VEGF is in turn triggered by a lack of oxygen, so the result is a regulatory system in which tissues that have a low oxygen supply resulting from insufficient circulation stimulate endothelial cell proliferation and recruit new capillaries (Figure 17.15). Smooth muscle cells, which form the walls of larger blood vessels (e.g., arteries) as well as the contractile portions of the digestive and respiratory tracts and other internal organs, are also capable of resuming proliferation in response to growth factor stimulation. In contrast, differentiated skeletal and cardiac muscle cells are no longer able to divide. Tissue deprived of oxygen VEGF Endothelial cells proliferate in response to VEGF Capillary FIGURE 17.15 Proliferation of endothelial cells Endothelial cells are stimulated to proliferate by vascular endothelial growth factor (VEGF). VEGF is secreted by cells deprived of oxygen, leading to the outgrowth of new capillaries into tissues lacking an adequate blood supply. 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 16 CHAPTER 17 Liver FIGURE 17.16 Liver regeneration Liver cells are normally arrested in G0 but resume proliferation to replace damaged tissue. If two-thirds of the liver of a rat is surgically removed, the remaining cells proliferate to regenerate the entire liver in a few days. Removal of two-thirds of liver The epithelial cells of some internal organs are also able to proliferate to replace damaged tissue. A striking example is provided by liver cells, which are normally arrested in the G0 phase of the cell cycle. However, if large numbers of liver cells are lost (e.g., by surgical removal of part of the liver), the remaining cells are stimulated to proliferate to replace the missing tissue ( Figure 17.16 ). For example, surgical removal of two-thirds of the liver of a rat is followed by rapid proliferation of the remaining cells, leading to regeneration of the...
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This note was uploaded on 08/25/2009 for the course BIO 315 taught by Professor Steiner during the Spring '08 term at Kentucky.

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