Types of Necrosis

Necrosis may be coagulative, liquifactive, caseous, fat necrosis, gummatous necrosis or fibrinoid necrosis.

Coagulative Necrosis

Coagulative necrosis is the commonest type and is ischemic. It may occur in heart, kidney, or adrenal glands and is firm in texture.
In coagulative necrosis, architecture of dead tissue is preserved for some days. It may occur due to denaturation of proteins including enzymes.
Ischemia results in coagulative necrosis, except in brain. An infarct is localized area of coagulative necrosis.

Morphology

Gross features:

The necrosis area is swollen, firm and pale.

Light Microscopy:

Cell detail is lost, but architecture preserved. The dead cells retain their outline but only indistinctly.
This type of necrosis is frequently caused by lack of blood supply and is exemplified well in infarcts of solid organs, e. g. heart, spleen, kidney.

Liquefactive Necrosis

Necrosis of big tissue with super added putrefaction, with black, foul-smelling appearance is known as liquefactive necrosis (black or green color is due to breakdown of haemoglobin).

In liquefactive necrosis, digestion of dead cells leads to liquid mass (infections & hypoxic death in CNS). Autolysis predominates resulting in liquified mass. Examples include cerebral infarction and abscess. Brain cells secrete increased hydrolases. These make neural tissue soft & liquid. Abscess hyrolases from neutrophils liquefy tissue.

Morphology

Gross:

Soft and liquid

Microscopy:

Enzymes digest the cell and convert it to a formless proteinaceous mass. Ultimately, discharge of the contents forms a cystic space

Gangrenous Necrosis

It is the clinical term for ischemic necrosis of lower limb involving multiple tissue planes with super added bacterial infections.
Necrosis and putrifaction by saprophytes takes place.

Types

Wet gangrene

Coagulative necrosis by ischemia + liquifactive necrosis by superimposed infection.
Conditions: Both arterial and venous obstruction; wet in environment;
Character: wet, swollen, foul-smelling, black or green.
Common in small intestine, appendix, lung, and uterus, also in limbs.

Dry gangrene

Drying of dead tissue associated with peripheral vascular diseases. Necrosis is separated from viable tissue by line of demarcation.
Conditions: only occurs on the skin surface following arterial obstruction. It is particularly liable to affect the limbs, especially the toes.
Character: mummification

Gas gangrene

Gas is produced in necrotic tissue by anaerobic bacteria, clostridium perfringes.
Conditions: deep contaminated wounds in which there is considerable muscle damage by gas forming bacteria.
Character: swollen obviously, gas bubbles formation. The infection rapidly spreads and there is associated severe toxaemia.
Only occasionally in civilian practice but is a serious complication of war wounds.

Caseous Necrosis  

It is cheese-like, as in tuberculosis.

Histology
Granuloma is formed with central cheesy material rimmed by epitheloid cells & giant cells (foreign body giant cells/Langhan giant cells). Tuberculosis coagulative necrosis is modified by capsule of lipopolysaccharide of Tb bacilli.

Morphology

Gross features: soft, granular, and friable as cream -cheesy appearance.
Light Microscopy: Granular and eosinophilic. Architecture is completely destroyed.
Examples: Tuberculosis, syphilis, some fungal infections.

Fat necrosis

In fat necrosis, there is focal area of fat destruction (pancreatic lipase digest cell membrane & form fatty acid + calcium white deposits).

Morphology

Gross: Opaque and chalky
Light Microscopy: outline of necrotic fat cells filled with amorphous basophilic material (calcium soaps). i.e. digestion of peritoneal fat by pancreatic enzymes in pancreatic inflammation.

Types

a.    Truamatic fat necrosis –Foreign body giant cell + foamy histiocytes form calcifications producing hard lump.
b.    Acute pancreatits -released enzymes digest fat
c.    Adipose tissue -TG + FFA lead to saponification + calcification.

Fibrinoid necrosis

This is not a true degeneration but a strongly eosinophilic stain like fibrin.
Location: interstitial collagen and blood vessels (small artery and arteriole)
Nature: one kind of necrosis.

Examples include:
a.     in allergic reactive diseases: active rheumatism, polyarteritis nodose
b.    in non-allergic reactive diseases: malignant hypertension.
Deposition of immune complexes & fibrin in arterial wall results in fibrinoid necrosis. Arterial wall shows amorphous pink circumferential necrosis with inflammation.

Haemorrhagic Necrosis

Ischemic tissue necrosis in organs with dual blood supply like, portal & systemic. One patent vascular channel results in haemorrhagic morphology. Examples include:
•    Liver
•    Spleen
•    Intestine
•    Lung is supplied by bronchial & pulmonary artery