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Autor     Andreas Gewies
Titel    An Introduction to Apoptosis
Jahr    2003
URL    http://www.celldeath.de/encyclo/aporev/apointro.pdf [23.1.2013]
Webcite    http://www.celldeath.de/encyclo/aporev/aporev.htm
Fragmente    3


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1. Introduction

1. Apoptosis

1.1. The development of the term apoptosis

[...] Apoptosis is of Greek origin, having the meaning "falling off or dropping off", in analogy to leaves falling off trees or petals dropping off flowers. This analogy emphasizes that the death of living matter is an integral and necessary part in the life cycle of organisms.

Since the mid-nineteenth century, many observations have indicated that cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis and metamorphosis (Lockshin et al, 2001). The term programmed cell death was introduced in 1964, proposing that cell death during development is not of accidential nature but follows a sequence of controlled steps leading to locally and temporally defined self-destruction (Lockshin et al, 2001).

Eventually, the term apoptosis had been coined in order to describe the morphological processes leading to controlled cellular self-destruction and was first introduced in a publication by Kerr, Wyllie and Currie (Kerr et al, 1972). The apoptotic mode of cell death is an active and defined process which plays an important role in the development of multicellular organisms and in the regulation and maintenance of the cell populations in tissues upon physiological and pathological conditions. It should be stressed that apoptosis is a well-defined and possibly the most frequent form of programmed cell death, but that other, non-apoptotic types of cell death, e.g. necrosis also are of biological significance (Leist et al, 2001).

1.2. The significance of apoptosis

The development and maintenance of multicellular biological systems depends on a sophisticated interplay between the cells forming the organism. It sometimes even seems to involve an altruistic behavior of individual cells in favor of the organism as a whole. During development many cells are produced in excess, which eventually undergo [programmed cell death and thereby contribute to sculpturing many organs and tissues (Meier et al, 2000).]

Introduction to Apoptosis

1. The development of the term apoptosis

Already since the mid-nineteenth century, many observations have indicated that cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis and metamorphosis [Gluecksmann, 1951; Lockshin, 2001]. The term programmed cell death was introduced in 1964, proposing that cell death during development is not of accidential nature but follows a sequence of controlled steps leading to locally and temporally defined self-destruction [Lockshin, 1964].

Eventually, the term apoptosis had been coined in order to describe the morphological processes leading to controlled cellular self-destruction and was first introduced in a publication by Kerr, Wyllie and Currie [Kerr, 1972]. Apoptosis is of greek origin, having the meaning "falling off or dropping off", in analogy to leaves falling off trees or petals dropping off flowers. This analogy emphasizes that the death of living matter is an integral and necessary part of the life cycle of organisms. The apoptotic mode of cell death is an active and defined process which plays an important role in the development of multicellular organisms and in the regulation and maintenance of the cell populations in tissues upon physiological and pathological conditions. It should be stressed that apoptosis is a well-defined and possibly the most frequent form of programmed cell death, but that other, non-apoptotic types of cell death also might be of biological significance [Leist, 2001].

2. The significance of apoptosis

The development and maintenance of multicellular biological systems depends on a sophisticated interplay between the cells forming the organism, it sometimes even seems to involve an altruistic behaviour of individual cells in favour of the organism as a whole. During development many cells are produced in excess which eventually undergo programmed cell death and thereby contribute to sculpturing many organs and tissues [Meier, 2000].

Anmerkungen

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[During development many cells are produced in excess, which eventually undergo] programmed cell death and thereby contribute to sculpturing many organs and tissues (Meier et al, 2000).

A particularly instructive example for the implication of programmed cell death in animal development is the formation of free and independent digits by massive cell death in the interdigital mesenchymal tissue (Zuzarte-Luis et al, 2002). Other examples are the development of the brain, during which half of the neurons that are initially created will die in later stages when the adult brain is formed (Hutchins et al, 1998), as well as the development of the reproductive organs (Meier et al, 2000). Also cells of an adult organism constantly undergo physiological cell death, which must be balanced with proliferation in order to maintain homeostasis in terms of constant cell numbers. The majority of the developing lymphocytes die either during genetic rearrangement events during the formation of the antigen receptor, during negative selection or in the periphery, thereby tightly controlling the pool of highly efficient and functional but not self-reactive immune cells, and at the same time keeping lymphocyte numbers relatively constant (Rathmell et al, 2002). Taken together, apoptotic processes are of widespread biological significance; being involved in e.g. development, differentiation, proliferation/homoeostasis, regulation and function of the immune system and in the removal of defect and therefore harmful cells. Thus, dysfunction or dysregulation of the apoptotic program is implicated in a variety of pathological conditions. Defects in apoptosis can result in cancer, autoimmune diseases and spreading of viral infections, neurodegenerative disorders and AIDS (Fadeel et al, 1999).

Due to its importance in such various biological processes, programmed cell death is a widespread phenomenon, occurring in all kinds of metazoans (Tittel et al, 2000) such as in mammals, insects (Richardson et al, 2002), nematodes (Liu et al, 1999), and cnidaria (Cikala et al, 1999). Moreover, programmed cell death also might play a role in plant biology (Solomon et al, 1999), and apoptosis-like cell death mechanisms even have been observed and used as a model system in yeast (Fröhlich et al, 2000; Skulachev et al, 2002). Fascinating insights into the origin and evolution of programmed cell death might possibly be given by the fact, that programmed cell death is also an integral part of the life cycle of other unicellular eukaryotes (such as the kinetoplastid parasite Trypanosoma brucei, the ciliate Tetrahymena thermophila, and the slime mold Dictyostelium [discoideum), and that even prokaryotes (such as Bacillus subtilis, Streptomyces and Myxobacteria) sometimes undergo regulated cell death (Ameisen et al, 2002).]

[page 2]

During development many cells are produced in excess which eventually undergo programmed cell death and thereby contribute to sculpturing many organs and tissues [Meier, 2000].

[page 3]

A particularly instructive example for the implication of programmed cell death in animal development is the formation of free and independent digits by massive cell death in the interdigital mesenchymal tissue [Zuzarte-Luis, 2002]. Other examples are the development of the brain, during which half of the neurons that are initially created will die in later stages when the adult brain is formed [Hutchins, 1998] and the development of the reproductive organs [Meier, 2000]. Also cells of an adult organism constantly undergo physiological cell death which must be balanced with proliferation in order to maintain homeostasis in terms of constant cell numbers. The majority of the developing lymphocytes die either during genetic rearrangement events in the formation of the antigen receptor, during negative selection or in the periphery, thereby tightly controlling the pool of highly efficient and functional but not self-reactive immune cells and at the same time keeping lymphocyte numbers relatively constant [Rathmell, 2002].

Taken together, apoptotic processes are of widespread biological significance, being involved in e.g. development, differentiation, proliferation/homoeostasis, regulation and function of the immune system and in the removal of defect and therefore harmful cells. Thus, dysfunction or dysregulation of the apoptotic program is implicated in a variety of pathological conditions. Defects in apoptosis can result in cancer, autoimmune diseases and spreading of viral infections, while neurodegenerative disorders, AIDS and ischaemic diseases are caused or enhanced by excessive apoptosis [Fadeel, 1999a].

[page 4]

Due to its importance in such various biological processes, programmed cell death is a widespread phenomenon, occuring in all kinds of metazoans [Tittel, 2000] such as in mammals, insects [Richardson, 2002], nematodes [Liu, 1999], and cnidaria [Cikala, 1999]. Moreover, programmed cell death also might play a role in plant biology [Solomon, 1999], and apoptosis-like cell death mechanisms even have been observed and used as a model system in yeast [Frohlich, 2000; Skulachev, 2002]. Fascinating insights into the origin and evolution of programmed cell death might possibly be given by the fact, that programmed cell death is also an integral part of the life cycle of other unicellular eukaryotes (such as the kinetoplastid parasite Trypanosoma brucei brucei, the ciliate Tetrahymena thermophila, and the slime mold Dictyostelium discoideum) and that even prokaryotes (such as Bacillus subtilis, Streptomyces and Myxobacteria) sometimes undergo regulated cell death [Ameisen, 2002].

Anmerkungen

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[Fascinating insights into the origin and evolution of programmed cell death might possibly be given by the fact, that programmed cell death is also an integral part of the life cycle of other unicellular eukaryotes (such as the kinetoplastid parasite Trypanosoma brucei, the ciliate Tetrahymena thermophila, and the slime mold Dictyostelium] discoideum), and that even prokaryotes (such as Bacillus subtilis, Streptomyces and Myxobacteria) sometimes undergo regulated cell death (Ameisen et al, 2002).

1.3. Morphological features of apoptosis

Apoptotic cells can be recognized by stereotypical morphological changes: the cell shrinks, shows deformation and loses contact to its neighbouring cells. Its chromatin condenses and marginates at the nuclear membrane, the plasma membrane is blebbing or budding, and finally the cell is fragmented into compact membrane-enclosed structures, called 'apoptotic bodies' which contain cytosol, the condensed chromatin, and organelles. The apoptotic bodies are engulfed by macrophages and thus are removed from the tissue without causing an inflammatory response. Those morphological changes are a consequence of characteristic molecular and biochemical events occurring in an apoptotic cell, most notably the activation of proteolytic enzymes, which mediate the cleavage of DNA into oligonucleosomal fragments as well as the cleavage of a multitude of specific protein substrates which usually determine the integrity and shape of the cytoplasm or organelles (Saraste et al, 2000). Apoptosis can be distinguesh from necrotic mode of cell-death in which the cells suffer a major insult, resulting in a loss of membrane integrity, swelling and disrupture of the cells. During necrosis, the cellular contents are released uncontrolled into the cells environment which results in damage of surrounding cells and a strong inflammatory response in the corresponding tissue (Leist et al, 2001).

Fascinating insights into the origin and evolution of programmed cell death might possibly be given by the fact, that programmed cell death is also an integral part of the life cycle of other unicellular eukaryotes (such as the kinetoplastid parasite Trypanosoma brucei brucei, the ciliate Tetrahymena thermophila, and the slime mold Dictyostelium discoideum) and that even prokaryotes (such as Bacillus subtilis, Streptomyces and Myxobacteria) sometimes undergo regulated cell death [Ameisen, 2002].

3. Morphological features of apoptosis

Apoptotic cells can be recognized by stereotypical morphological changes: the cell shrinks, shows deformation and looses contact to its neighbouring cells. Its chromatin condenses and marginates at the nuclear membrane, the plasma membrane is blebbing or budding, and finally the cell is fragmented into compact membrane-enclosed structures, called 'apoptotic bodies' which contain cytosol, the condensed chromatin, and organelles (Fig. 2). The apoptotic bodies are engulfed by macrophages and thus are removed from the tissue without causing an inflammatory response. Those morphological changes are a consequence of characteristic molecular and biochemical events occurring in an apoptotic cell, most notably the activation of proteolytic enzymes which eventually mediate the cleavage of DNA into oligonucleosomal fragments as well as the cleavage of a multitude of specific protein substrates which usually determine the integrity and shape of the cytoplasm or organelles [Saraste, 2000]. Apoptosis is in contrast to the necrotic mode of cell-death in which case the cells suffer a major insult, resulting in a loss of membrane integrity, swelling and disrupture of the cells. During necrosis, the cellular contents are released uncontrolled into the cell's environment which results in damage of surrounding cells and a strong inflammatory response in the corresponding tissue [Leist, 2001].

Anmerkungen

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