This post will take a look at the various forms of nerosis that may occur in the body.
If you remember from earlier, necrosis is localised cell death that is characterised by cellular swelling. It is more of an accidental and passive cell death than apoptosis which is intentional and requires energy. You may also remember that pyknosis refers to the condensation of chromatin in the nucleus, karyorrhexis is the fragmentation of the nucleus which is followed by the distribution of chromatin fragments in the nucleus, and karyolysis is degradation of the chromatin in the cell.
This type of necrosis is usually seen in ischaemia and infarction and is very common in the kidneys, liver and muscles. When coagulative necrosis occurs the basic architecture of the cell is preserved as well as ‘ghostly’ outlines of the cells. The detail within the cell is lost because the proteins (which include the proteolytic enzymes) become denatured and this makes them more resistant to proteolytic degradation.
Histologically, cells that have undergone coagulative necrosis have very eosinophilic pink staining nuclei and much of the basophilic staining from nuclei and ribosomes is lost. This is because of karyolysis where the chromatin is degraded, pyknosis and karyorrhexis is also seen.
Liquefactive necrosis occurs when the tissue is digested by enzymes which results in the loss of the cell’s overall architecture. A thick liquid is left behind. This kind of necrosis is characteristic of bacterial abscessation and is due to the presence of both bacterial and neutrophilic enzymes at the same time. Liquefactive necrosis may also occur in the brain when glial cells and neurons are killed.
Caseous necrosis is like a combination of coagulative and liquefactive necrosis. Much of the cellular architecture is lost and the cell outlines are lost but the cell is not totally destroyed. Grossly, this type of necrosis appears as pale/cream-coloured with a cheesy (‘caseous’ is Greek for ‘cheese’) appearance and texture. Classically, this is seen in tuberculosis.
Fat necrosis occurs when adipose tissue is destroyed. Lipases break down triglycerides and this produces fatty acids and glycerol. The fatty acids bind with calcium and form insoluble calcium soaps which appear as chalky white deposits grossly. This is often seen in pancreatitis where pancreatic enzymes are released during inflammation of the pancreas.
Gangrene refers to the local cell death within a living body that is often associated with loss of blood supply in which there is bacterial invasion and putrefaction. Usually the extremities are affected by gangrene and gangrenous tissues appear dark and have an unpleasant odour. Often there is accompanying fever and pain too. There are three main types of gangrene:
o Dry Gangrene: this is predominantly coagulative in nature and is due to a gradual reduction in blood supply with little bacterial decomposition.
o Wet Gangrene: this is mostly liquefactive and more often is a result of the sudden stoppage of blood from heat, acid, cold, thrombosis or tourniquet. Toxins are produced in the affected tissue and these are absorbed which causes cell death.
o Gas Gangrene: This is tissue death which has been complicated by infection with gas-producing bacteria. Much pain and swelling as well as a serosanguinous exudate accompanies this form of gangrene. The gas may be visible grossly in the cut surfaces of tissues and will have a bubbly appearance.