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The pathological or accidental death occur when the cell is deprived from its vital processes by physical or chemical lesions, induced by external factors, such as high temperature, radiation, trauma, toxins and deprivation of oxygen (as in infarct of myocardium).
The lesions can be also caused by biological factors, as in infections caused by bacterias and viruses. This type of cellular death is called "necrosis".
The necrosis is clearly visible by electron microscopy. The cell is swelled and the organelles - like the mitochondria - are damaged. However, the nucleus does not suffer significant changes. These lesions prevent the internal balance control (eg. water and some ions, which would normally pumped out from the cell, now flows freely toward its interior). This is responsible for the swelling and the lysis of the cell.
This rupture release the cellular contents, rich in proteases and others enzymes, to the neighboring tissue. Besides the direct toxicity for the neighboring cells, the release produces substances that attracts cells of the immunological system, inducing an intense inflammatory reaction. Some kinds of immunological cells, like neutrophils and macrophages, converges to the damaged tissue and swallow the dead cells. The inflammation - typical of necrosis - is important to limit the infection and to remove remaining cells. The activity and the secretion of substances by leukocytes, may damage normal neighboring tissues.
Physiological death is totally different from necrosis. At first, the cell don´t swell. On the contrary, detachment from the neighboring cells starts the zeiosis process - bubble formation on its surface. The membrane and organells keep its structure unbroken and there are no evident alterations in cytoplasm. The nucleus, however, undergoes great changes: the cromatin usually dispersed forms one or more agglomerations in the internal surfaces of the nuclear membrane. Most cells in apoptosis have its genetic material, the DNA, destructed: before the cellular death the DNA is cleavaged by endonucleases in specific regions between the nucleossomes. In others instances the formation of apoptotics bodies - structures that contain portions of the nucleus and cytoplasm - can be observed.
Apoptosis is a undisturbed process where there´s no aggressive physiological reaction, such as inflammation. The cells that had suffered zeiosis and initiated the cleavage process of genetic material (or had formed apoptotics bodies) are phagocyted by leukocytes before they are disintegrated. This prevent the spread of cellular content and therefore inflammation and tissues´ injury as observed in necrosis process.
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