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Necrosis
Necrosis (Gr. Nekrosis-Deadness) is defined as death of cells in a living vascularised tissue or local death of cells in a living animal. Grossly, necrotic tissue is pale, grayish white, dull and depressed surrounded by hyperaemic zone. Microscopically, nuclear changes are characteristic. These are
Depending on the gross and microscopic features necrosis is divided into following four types.
Grossly, the necrotic tissue is dry, white or grayish white and homogeneous and slightly depressed from the surrounding healthy tissue.
Microscopically, the architectural details of the area is maintained and cellular details are lost. This is due to blockage of proteolysis with denaturation of proteins including enzymatic proteins of the cell. The cellular shape is preserved and nuclear details are lost (Nuclei show pyknosis, karyorrhexis and karyolysis or absence). The cytoplasm appears homogeneous and eosinophilic due to coagulation of protein. It takes long time for the removal of dead materials because the autolytic enzymes are destroyed and no leukocytic responses. This type of necrosis is characteristically found in parenchymatous organs like kidney, liver and muscle except the brain.
Caseous necrosis (L. Caseous - Cheese)
Grossly, the necrotic tissue is converted into a homogeneous, soft, friable, grayish white cheesy granular mass. The dead tissue attracts calcium deposits and is enclosed within a connective tissue capsule.
Microscopically, structure less, amorphous necrotic area surrounded by epithelioid cells, giant cells, lymphocytes and plasma cells with central area of dystrophic calcification
Liquefactive necrosis Necrotic tissue is liquid in consistency. It is especially seen in the central nervous system (malacia) and any infection with pyogenic bacteria leading to pus formation (Abscess). The former is due to severe hypoxic or toxic injury with focal dissolution of the neuropil. The later is due to autolysis or heterolysis from enzymes of neutrophils leading to collection of pus containing necrotic tissue, microorganisms and dead neutrophils (Suppuration). The pus becomes caseous and insipid if stands for longer time.
Microscopically, the pus or the purulent area shows dark, contracted and agranular neutrophils with varying amounts of tissue debris, fibrin and plasma proteins. An abscess is a localised collection of pus (Liqefactive necrosis) caused by suppuration, deep in tissues. The process is designed to contain the pathogenic organisms and sequestering necrotic tissue from spreading in the animal. The pyogenic organisms cause localized necrosis and attract neutrophils to the necrotic areas. This is a part of inflammatory response.
Fat necrosis It is death of adipose tissue in a living animal. There are different types of fat necrosis Enzymatic fat necrosis It is commonly found in steatitis (Inflammation of fat) and other inflammatory lesions affecting adipose tissue, e.g. Pancreatic fat
Traumatic fat necrosis It results from mechanical injury to adipose tissue.
Mesenteric, omental and retroperitoneal fat show necrosis containing large masses. Stenosis of intestine may occur in extreme cases. Nutritional fat necrosis This is the result of necrotic alteration in fat associated with extreme emaciation. e.g. Tuberculosis and Johne’s disease in cattle and sheep.
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