General Veterinary Pathology

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

• Pyknosis: Shrinkage or condensation of nucleus which takes up deep blue colour. The chromatin condenses to a structureless mass
• Karyorrhexis (Gr. Karyo-Nucleus; rhexis-Fragmentation): Fragmentation of nucleus.
• Karyolysis (Gr. Karyo-Nucleus; Lysis-Dissolution): Dissolution or disappearance of nucleus.
• Cytoplasm is swollen, homogeneous and stained intensely pink due to decreased basophilia with loss of ribosomes.

Depending on the gross and microscopic features necrosis is divided into following four types.

• Coagulative necrosis
• Caseation necrosis
• Liquefactive/suppurative necrosis
• Fat necrosis

Coagulative necrosis

• “While architectural details of tissue are retained, the structural details are lost due to necrosis”
• This type of necrosis occurs due to ischaemia, whit muscle disease in vitamin E and selenium deficiency, necrosis of liver in Fusobacterium sphaerophorus infection, mercuric toxicity in renal tubular epithelial cells and cutaneous or mucosal epithelium in contact poison with phenol.

Grossly, the necrotic tissue is dry, white or grayish white and homogeneous and slightly depressed from the surrounding healthy tissue.

Coagulative necrosis

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)

• The architectural and cellular details are lost. This is more chronic type of lesion often associated with poorly degraded lipid materials of bacterial origin. This type of necrosis occurs in Mycobacterium tuberculosis infection, oesophagostomosis and caseous lymphadenitis in sheep and tularaemia in primates.

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.

Caseous necrosis - Tuberclosis - Lung

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. 

 

Liquefactive necrosis - Pyometra

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

• Pathogenesis - In acute pancreatic necrosis and pancreatitis, the lipase released from acinar cells gets activated and saponification occurs by digestion of triglycerides into glycerol and fatty acids. Glycerol being water soluble, is absorbed. The released fatty acids when combine with calcium results in the presence of chalky white flakes.
• Grossly, hard, white, opaque masses resembling that of soap flakes are seen. The fat loses yellow translucent nature.
• Microscopically, necrotic adipocytes may show eosinophilic shadow outlines, become basophilic due to dystrophic calcification and surrounded by inflammatory reactions along the area due to acute to chronic injury. Fat solvents do not remove necrotic fat.

Traumatic fat necrosis

It results from mechanical injury to adipose tissue.

• Causes : working, biting, parturition (perivaginal fat in cattle, subcutaneous and intramuscular fat in recumbent cattle)
• Grossly, firm, opaque, chalky masses found in the area with acute to chronic inflammatory reaction.
• Example: Surgical injury to subcutaneous fat, injury to vagina during dystocia and abdominal fat necrosis in cattle

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.

• Grossly, necrotic fat is opaque, foamy and chalky white and may be calcified.
• Microscopically, necrotic adipocytes are pale pink (eosinophilic) and show numerous clubs (fatty acids) and crystals. The derivatives of fat, glycerol dissolves in body fluids, and fatty acid crystals dissolve in fat solvents leaving clefts. Calcified area is basophilic, surrounded by chronic inflammatory cells.
• Differential diagnosis: Inflammatory reaction and calcification are lacking in autolytic fat.