Types of necrosis

Depending on the type of tissue involved and the nature of the injurious agents there are different types of necrosis could be identified.

There are two main types of necrosis, coagulative necrosis and liquefactive necrosis. Also there are four other special types of necrosis which includes, fat necrosis, caseous necrosis, fibrinoid necrosis and gangrene.

Coagulative necrosis
Coagulative necrosis is a form of necrosis in which the components of the cells are necrosed but the cells retain its cellular outline or architecture often for several days. But the nucleus disappears. Under light microscope coagulative necrosis is seen as eosinophilic cell shadows without nuclei. In some cells pyknotic and karyorrhectic nuclei may be seen.Coagulative necrosis is commonly seen in solid organs like heart, kidney, liver and adrenals and the affected tissues become firm in texture. Typically the injurious agent is hypoxia. For example myocardial infarction, renal infarction. Other causes of coagulative necrosis are coagulative necrosis of the liver in viral hepatitis,coagulative necrosis of skin in burns.

Mechanism of coagulative necrosis
Injurious agents like hypoxia(due to increased intracellular acidity) and heat denature the intracellular proteins, including the lysosomal enzymes. This prevents cell being autolysed and the structural outline persist. Later the enzymatic activity of inflammatory cells digests these cells. This process Is called heterolysis.

Macroscopy of coagulative necrosis
Initial several hours the necrotic areas may appear normal. Then the area show a mottled appearance due to seepage of blood into the necrotic area from damaged blood vessels. Later blood gets cleared out and the necrotic area appears pale. A few days later necrosis area become firm and usually pale. This is the typical appearance of coagulative necrosis. However, sometimes the necrotic focus could look like hemorrhagic. For example necrosis occurring in already congested tissues like in testis undergone torsion and in tissues with dual blood supply like in lung infarction.

Microscopy of coagulative necrosis
Microscopic appearance depends on the time since onset of cell death. Nuclear changes described above are typically seen in coagulative necrosis. After 1-2 days cellular outlines with eosinophilic homogenous appearance is seen without a nucleus. There will be associated inflammation.
Caseous necrosis

Caseous necrosis is a type of coagulative necrosis characterized by yellowish crumbling nature of necrotic tissues and it is Like cottage cheese. The caseous necrosis typically occurs in infection by Mycobacterium tuberculosis and sometimes in fungal infections.

Liquefactive necrosis
In liquefactive necrosis, necrotic cells undergo rapid lysis or liquefaction due to the lysosomal enzymes released from necrotic cells and this is called autolysis. The typical example in liquefactive necrosis of brain following ischemia (brain infection). This may be due to high content of proteolytic enzymes in brain cells. Another example of liquefactive necrosis is suppuration. Here the liquefaction is due to proteolytic enzymes released by the neutrophils and this is called heretolysis.

Fibrinoid necrosis
Fibrinoid necrosis occur in connective tissues characterized by deposition of homogeneous bright pink (hyaline) material in necrotic foci. This material contains various amounts of immunoglobulins, compliments and breakdown products of fibrin and collagen.

Situations where fibrinoid necrosis is seen:

• Walls of arteries in vasculitis and malignant hypertension (especially in arterioles).
• In rheumatic carditis in Aschoff bodies.

Clinical manifestations:
In small vessel vasculitis, fibrinoid necrosis of vessel wall leads to red cell extravasation. (Eg. Palpable purpura in skin). In rheumatic carditis, fibrinoid necrosis manifest as Aschoff bodies. They heal by fibrosis. Fibrous distortion of the cardiac valve cusps leads to valvular abnormalities like stenosis and incompetent. Glomeruli may affect in some types of glomerulonephritis.