Necrosis is defined as focal death along with degradation of tissue by hydrolytic enzymes which are liberated by the cells. Necrosis may be caused by various agents such as hypoxia, physical and chemical factors, microbial agents and immunological injury.
Irreversible cell injury can be caused by –
1. Cell digestion by lytic enzymes
2. Denaturation of proteins
These changes can be particularly seen by changes in the nuclear and cytoplasmic characteristics. The cytoplasm appears homogenous and eosinophilic. The nuclear changes may be condensation of nuclear chromatin(pyknosis), may undergo dissolution(karyolysis) or fragmentation(karyorrhexis)
Morphologically there are 5 types of necrosis seen in humans, which include:
1. Coagulative necrosis – The most common type of necrosis with irreversible focal injury, mostly caused by sudden cessation of blood, generally Ischaemia. Less commonly by bacterial and chemical agents. The organs which are very commonly affected by coagulative necrosis are heart, spleen and kidney.
2. Liquefaction necrosis – Also called as Colliquative necrosis, is commonly due to ischaemic injury and bacterial or fungal infections. Thsi occurs due to degradation of tissues by the action of powerful hydrolytic enzymes. The places where liquefaction necrosis is commonly seen are Infarct brain and abscess cavity.
3. Caseous necrosis – This type of necrosis is found in the center of foci of the tuberculous infections. This necrosis has the combined features of both coagulative and liquefaction necrosis.
4. Fat necrosis – Fat necrosis is a special form of cell death that is caused at two anatomically different locations, and morphologically similar lesions. These lesions are –
Acute pancreatic necrosis and Traumatic fat necrosis. And this is commmonly seen in breasts.
In case of pancreas, there is liberation of pancreatic lipases, from the inflammed or injured tissues that results in necrosis of the pancreas and also on the fat deposits throughout the peritoneal cavity. This sometimes also affects the extra-abdominal adipose tissue.
5. Fibrinoid necrosis – Fibrinoid degeneration is characterised by deposition of fibrin-like material which has the staining properties of fibrin. This can be seen in various cases like immunological tissue injury, hypertension of arterioles, peptic ulcer etc.
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