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. Programmed Cell Death (Apoptosis) Cell Biology Lec.10 Dr: Buthaina Al-Sabawi

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. 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 as a result of acute injury swell and lyes, releasing their content into the extracellular space and causing inflammation.

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.

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