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Cooper5e_Ch17 - UNCORRECTED PAGE PROOFS 17 Programmed Cell...

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Cell death and cell proliferation are balanced throughout the life of multi- cellular organisms. Animal development begins with the rapid prolifera- tion of embryonic cells, which then differentiate to produce the many spe- cialized types of cells that make up adult tissues and organs. Whereas the nematode C. elegans consists of only 959 somatic cells, humans possess a total of approximately 10 14 cells, consisting of more than 200 differentiated cell types. Starting from only a single cell—the fertilized egg—all the diverse cell types of the body are produced and organized into tissues and organs. This complex process of development involves not only cell prolif- eration and differentiation but also cell death. Although cells can die as a result of unpredictable traumatic events, such as exposure to toxic chemi- cals, most cell deaths in multicellular organisms occur by a normal physi- ological process of programmed cell death, which plays a key role both in embryonic development and in adult tissues. In adult organisms, cell death must be balanced by cell renewal, and most tissues contain stem cells that are able to replace cells that have been lost. Abnormalities of cell death are associated with a wide variety of ill- nesses, including cancer, autoimmune disease, and neurodegenerative disorders, such as Parkinson’s and Alzheimer’s disease. Conversely, the ability of stem cells to proliferate and differentiate into a wide variety of cell types has generated enormous interest in the possible use of these cells, particularly embryonic stem cells, to replace damaged tissues. The mechanisms and regulation of cell death and cell renewal have therefore become areas of research at the forefront of biology and medicine. Programmed Cell Death Programmed cell death is carefully regulated so that the fate of individual cells meets the needs of the organism as a whole. In adults, programmed cell death is responsible for balancing cell proliferation and maintaining constant cell numbers in tissues undergoing cell turnover. For example, about 5 × 10 11 blood cells are eliminated daily in humans by programmed cell death, balancing their continual production in the bone marrow. In addition, programmed cell death provides a defense mechanism by which damaged and potentially dangerous cells can be eliminated for the good of the organism as a whole. Virus-infected cells frequently undergo pro- UNCORRECTED PAGE PROOFS This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. © 2009 Sinauer Associates, Inc. Cell Death and Cell Renewal Programmed Cell Death 00 Stem Cells and the Maintenance of Adult Tissues 00 Embryonic Stem Cells and Therapeutic Cloning 00 KEY EXPERIMENT Identification of Genes Required for Programmed Cell Death 000 KEY EXPERIMENT Culture of Embryonic Stem Cells 000 17
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grammed cell death, thereby preventing the production of new virus parti- cles and limiting spread of the virus through the host organism. Other types
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