Necrosis is the death of a part of a living organism. It is the death of a part as distinguished from the death of the entire body (somatic death). The causes of necrosis are (I) local injury, (2) vascular obstruction, and (3) trophic disturbances.

Under the local injuries are included those that are mechanical, chemical, thermal, and bacterial.

Mechanical injuries may cause destruction of the cells directly or by interference with the blood-supply. Pressure of foreign bodies will often bring about necrosis.

Chemical substances such as the acids and alkalis may cause destruction of the tissues.

Thermal injuries, those from extreme heat or cold, will more or less quickly destroy the vitality of the cells.

Bacterial products acting as toxic agents will frequently cause necrosis and gangrene.

If vascular obstruction take place suddenly, the nutrition will be shut off and necrosis result.

Trophic Disturbances

Trophic Disturbances will lessen the resisting power of the tissues with subsequent necrosis. This is seen in decubitus or bed-sore that occurs in various forms of spinal disease. The perforating ulcer of the foot is another example.

The cells in the necrosed areas will show different stages of disintegration. The cell wall may remain, but the cytoplasm will not stain. There may be complete destruction and breaking down of the cell. The granules in the protoplasm disappear, and it in turn becomes cloudy, gradually breaks up, and vacuoles form. The nucleus may lose its staining power or may undergo destruction in one of two ways: By karyorrhexis, a breaking down of the chromatin into granules, or by karyolysis, a liquefaction of the nuclear constituents.

Necrosis may be of different varieties.

Coagulation Necrosis

Coagulation Necrosis is a form of death of those tissues freely supplied with lymph, accompanied by a consolidation of the protein contents. It is a change similar to the coagulation of the blood. The fibrin ferment present acts upon the fibrin factors and fibrin is formed.

It is found in thrombi, blood-clots, and interstitial hemorrhages.

Occurs in various inflammatory exudates, particularly in croupous pneumonia and diphtheria, and in infarcts.

The seat of the necrosis is firmer and paler than normal, and dry. Later on it may become softer and discolored as a result of disintegration of blood.

Caseous Necrosis

Caseous Necrosis is a condition in which the tissues have been transformed into a cheese-like substance.

It is found only as a sequel to pre-existing coagulation necrosis. Is found most commonly in tuberculosis, but occurs in tumors and in syphilis.

Surrounding the area of caseation there is generally a zone of coagulation.

Liquefaction Or Colliquation Necrosis

Liquefaction Or Colliquation Necrosis is the death of the tissues with liquefaction. It occurs in those tissues that contain little protein substance, especially in anemic infarcts of the brain. The nervous tissue undergoes a softening, becomes semi-fluid, and eventually liquid, remaining as a colliquation cyst.