Download Programmed Cell Death(Apoptosis)

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28/12/2015
Biology
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Programmed Cell Death(Apoptosis)
Programmed cell death is a normal physiological form of cell death
that plays a key role both in the maintenance of adult tissues and in
embryonic development.
In adult programmed cell death is responsible for balancing cell
proliferation and maintaining constant cell number in tissues
undergoing cell turnover. For example, many thousand blood cells
are eliminated by programmed cell death daily in human, balancing
their continual production in the bone marrow. In addition,
programmed cell death provides a defense mechanism by which
damaged potentially dangerous cells can be eliminated for the good
of the organism as a whole.
Virus-infected cells frequently undergo programmed cell death,
thereby:
1- Preventing the production of new virus particles.
2- limiting spread of virus through the host organism.
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. Apoptosis is
responsible for the elimination of larval tissues during amphibian and
insect metamorphosis. During apoptosis, the cell is digested by a
class of protases. More than 10 caspases been identified. Some of
them (e.g., caspase 8 and 10) are involved in the initiation of
apoptosis, other (caspase 3,6, and 7) execute the death order by
destroying essential proteins in the cell.
(1)
In apoptosis, the cell and its nucleus become compact decreasing in
size. At this stage the apoptotic cell shows a dark-stained nucleus
(pyknotic nucleus),easily identified with the light microscope. Next,
the chromatin is cut into pieces by DNA endonucleases.
During apoptosis the cell shows cytoplasmic large vesicles (biebs) that
detach from the cell surface. Finally, the cell itself shrinks and breaks
up into membrane-enclosed fragments called apoptotic bodies. Such
apoptotic cells and cell fragments are readily recognized and
phagocytosed by both macrophages and neighboring cells, so cells
that die by apoptosis are efficiently removed from tissues.
The accidental death of cells, a pathologic process , is called necrosis,
necrosis can be caused by microorganisms, viruses, chemicals, and
other harmful agents. Necrotic cells swell, their organelles increase in
volume, and finally burst, releasing their contents into the
extracellular space macrophages engulf the debris of necrotic cells by
phagocytosis and secrete molecules that active other
immunodefensive cells to promote inflammation.
(2)
(3)