celldeath terpse

Celldeath terpse

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Unformatted text preview: UNCORRECTED PAGE PROOFS 17 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 Cell Death and Cell Renewal Cell death and cell proliferation are balanced throughout the life of multicellular organisms. Animal development begins with the rapid proliferation of embryonic cells, which then differentiate to produce the many specialized 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 1014 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 proliferation and differentiation but also cell death. Although cells can die as a result of unpredictable traumatic events, such as exposure to toxic chemicals, most cell deaths in multicellular organisms occur by a normal physiological 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 illnesses, 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. KEY EXPERIMENT Culture of Embryonic Stem Cells 000 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 1011 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 proThis 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 2 CHAPTER 17 grammed cell death, thereby preventing the production of new virus particles and limiting spread of the virus through the host organism. Other types of cellular insults, such as DNA damage, also induce programmed cell death. In the case of DNA damage, programmed cell death may eliminate cells carrying potentially harmful mutations, including cells with mutations that might lead to the development of cancer. During development, programmed cell death plays a key role by eliminating unwanted cells from a variety of tissues. For example, programmed cell death is responsible for the elimination of larval tissues during amphibian and insect metamorphosis, as well as for the elimination of tissue between the digits during the formation of fingers and toes. Another wellcharacterized example of programmed cell death is provided by development of the mammalian nervous system. Neurons are produced in excess, and up to 50% of developing neurons are eliminated by programmed cell death. Those that survive are selected for having made the correct connections with their target cells, which secrete growth factors that signal cell survival by blocking the neuronal cell death program. The survival of many other types of cells in animals is similarly dependent on growth factors or contacts with neighboring cells or the extracellular matrix, so programmed cell death is thought to play an important role in regulating the associations between cells in tissues. The Events of Apoptosis In contrast to the accidental death of cells that results from an acute injury (necrosis), programmed cell death is an active process, which usually proceeds by a distinct series of cellular changes known as apoptosis, first described in 1972 (Figure 17.1). During apoptosis, chromosomal DNA is (A) (C) Hours after induction of apoptosis 0 1 2 3 (B) DNA fragmentation Chromatin condensation Normal Fragmentation of nucleus Fragmentation of the cell Apoptotic FIGURE 17.1 Apoptosis (A) Diagrammatic representation of the events of apoptosis. (B) Light micrographs of normal and apoptotic human leukemia cells illustrating chromatin condensation and nuclear fragmentation. (C) Gel electr...
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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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