necrosis

• Necrosis is a spectrum of morphological changes that occur after cell death in a living tissue.

  these changes could be seen microscopically[ light or electron]
• Morphologic appearance of a necrotic cell may be as a result of protein denaturation or enzyme digestion
• Necrotic cells have lost their membrane integrity and intracellular substances can leak out to cause inflammation
• Enzymes that digest the cell are from the lysosomes of the cell itself or from the leukocytes due to inflammation
• Morphology of necrotic cell by light microscope:

  • Increased eosinophilia due to loss of RNA
  • Glassy homogenous appearance due to loss of glycogen
  • Vacuolated and moth eaten cytoplasm
  • Presence of myelin figures from damaged cell membrane
• Morphologic changes seen by electron microscope:

  • Presence of discontinuities in the nuclear membrane, organelle membrane and plasma membrane
  • Swelling of the mitochondria with amorphous densities
  • Myelin figures in the cytoplasm
  • Amorphous debris
  • Aggregates of fluffy materials which may be denatured proteins
• Nuclear changes are seen as:

  • Karyolysis, fading away of the Basophilia of the chromatin
  • Pyknosis, shrinking and condensation of the nucleus
  • Karyorrhexis, fragmentation of the nucleus
• Pattern of Tissue necrosis:

  • Coagulative necrosis
  • There is preservation of the structure of the cell for few days following cell death
  • Affected tissue is firm due to denaturation of proteins and enzymes capable of carrying out proteolysis
  • Eosinophilic cells without nuclei persist for days
  • Eventually, necrotic cells are removed by phagocytic leukocytes and Lysosomal enzymes
• Ischaemia is a cause of coagulative necrosis

  • A localized area of coagulative necrosis is called an infarct
• Liquefactive necrosis:

  • Enzymatic digestion of dead cells
  • The tissue eventually becomes a liquid mass
  • Characterized by focal bacterial or occasional fungal infection
  • Hypoxic death of cells in the CNS often end in liquefactive necrosis
  • If there are leukocytes in the liquid mass, the color changes from white to yellow (pus)
• Gangrenous necrosis:

  • Term used in clinical practice
  • Applicable to a limb that has lost its blood supply
  • Dry gangrene if there is no bacterial infection
  • Wet gangrene once there is bacterial infection
  • A form of coagulative necrosis
• Caseous necrosis:

  • Encountered mainly in foci of tuberculosis
  • Distinctive form of coagulative necrosis
  • Grossly appears as a cheesy white area
  • Under the light microscope, necrotic area appears as a structureless collection of fragmented coagulated and lysed cells and amorphous granular debris within a clear inflammatory border called a granuloma
• Fat necrosis:

  • Descriptive of focal areas of fat destruction
  • Result of the action of released pancreatic lipases into the pancreas and peritoneal cavity
  • Seen in acute pancreatitis
  • Released fatty acids combine with calcium to form chalky white areas known as fat saponification
• Fibrinoid necrosis:

  • Seen in immune reaction involving the blood vessels
  • Occurs when complexes of antigen and antibody are deposited in the walls of blood vessels(arteries)
  • Appears as bright pink deposits on H&E and is a mixture of fibrin protein and the immune complex
• The process of necrosis can be divided into three stages: coagulation, lysis, and phagocytosis.
• Coagulative Necrosis - the most common type of necrosis characterized by swelling of cells due to loss of potassium ions from the cytoplasm, followed by shrinkage (coagulation) as water moves out of the cell.