Necrosis

Necrosis is the death of most or all of the cells in an organ or tissue due to disease, injury, or failure of the blood supply.

Necrotic changes:

Karolysis

  • It is the process of nuclear fading.
  • Chromatin dissolution due to action of DNAases and RNAases.

Pyknosis

  • It is the process of nuclear shrinkage
  • DNA condenses into shrunken basophilic mass

Karyorrhexis

  • It is the process of cell fragmentation
  • Pyknotic nuclei membrane ruptures and nucleus undergo fragmentation
Three significant necrotic changes

Types of necrosis

Liquefactive necrosis

  • Occurs when a lytic (dissolving) enzyme liquefies necrotic cells
  • It is generally associated with abscess formation.
  • Typically, liquefication necrosis occurs in brain infarcts and pancreatic necrosis.
  • Liquefaction by leukocytic enzymes is called suppuration and the resultant fluid is called pus.
  • Liquefaction or colliquative necrosis occurs commonly due to ischemic injury and bacterial or fungal infections.
  • It occurs due to degradation of tissue by the action of powerful hydrolytic enzymes.
  • The common examples are infarct brain and abscess cavity

Caseous necrosis

  • The necrotic cells disintegrate but the cellular pieces remain undigested for months or years.
  • This type of necrotic tissue gets its name from its crumbly, cheese-   like (caseous) appearance.
  • It commonly occurs in lung tuberculosis.
  • Accumulation of amorphous (no structure) debris within an area of necrosis.
  • Tissue architecture is abolished, and viable cells are no longer recognizable.
  • Characteristically associated with the granulomatous inflammation of tuberculosis. Also seen in some fungal infection.

Fat necrosis

  • In fat necrosis enzymes called lipases break down intracellular triglycerides into free fatty acids.
  • These free fatty acids combine with sodium, magnesium, or calcium ions to form soaps.
  • The tissue becomes opaque and chalky white.
  • It is characteristic of tissues adjacent to pancreatic necrosis.
  • Grossly, fat necrosis appears as yellowish-white and firm deposits
  • Microscopically, the necrosed fat cells have cloudy appearance and are surrounded by an inflammatory reaction.

Coagulative necrosis

  • It is the most common form of necrosis in cells without large number of lysosomes.
  • Commonly occur when the blood supply to any organ (except the brain is interrupted
  • It typically affects the kidney, heart and adrenal glands.
  • Lytic (lysosomal) enzyme activity in the cells is inhibited, so that the necrotic cells maintain their shape, at least temporarily.
  • Grossly, foci of coagulative necrosis in the early stage are pale, firm, and slightly swollen.
  • Microscopically, the hall mark of coagulative necrosis is the conversion of normal cells into their ‘tombstones’.
  • The outlines of the cells are retained so that the cell types can still be recognized but their cytoplasmic and nuclear details are lost.
Coagulative necrosis of the kidney

Fibrinoid necrosis

  • It is due to deposition of fibrin like material.
  • It is seen in immunologic cell injury, hypertension, peptic ulcer.
  • Microscopically, fibrinoid necrosis is identifies by brightly eosinophilic, hyaline like deposition in the vessel wall

Gangrene

It is a form of coagulative necrosis, typically results from lack of blood flow and is complicated by an overgrowth and invasion of bacteria. It commonly occurs in the lower legs as a result of arteriosclerosis or in the gastrointestinal tract.

Gangrene can occur in three forms,

  • Dry gangrene
  • Moist or wet gangrene
  • Gas gangrene

Dry gangrene

  • Occurs when bacterial invasion is minimal.
  • It is marked by dry, wrinkled, dark brown or blackened tissue on an extremity.
Dry gangrene of the foot

Wet gangrene

  • Develops with liquefactive necrosis that includes extensive lytic activity from bacteria and white blood cells to produce a liquid center in area of tissue.
  • It occurs in the internal organs as well as the extremities.

Gas gangrene

  • It develops when anaerobic bacteria of the genus clostridium infect tissue.
  • Its more likely to occur with severe trauma and may be fatal.
  • The bacteria release toxins that kill nearby cells and the gas gangrene rapidly spreads.
  • Release of gas bubbles from affected muscle cells indicates that gas gangrene is present.