Definition- Necrosis refers to spectrum of morphologic changes that follow cell death in living tissue, largely resulting from the progressive degradative action of enzymes on lethally injured cell
The morphologic appearance of necrosis is the result of denaturation of intracellular proteins & enzymatic digestion of the cell
The enzymes are derived from either lysosomes of the dead cells themselves (Autolysis) or from the lysosomes of the immigrant leukocytes (heterolysis )
Morphology of necrotic cell
Microscopic appearance
Cytoplasmic changes
Increased eosinophilia – Due to denatured proteins and loss of cytoplasmic RNA
Glassy homogenous appearance – loss of glycogen particles
Moth eaten appearance of cytoplasm – enzymatic digestion of cytoplasmic organelles
There is preservation of basic outline of the cell
The increasing intracellular acidosis denatures not only structural proteins but also the enzymes & so blocks the proteolysis of the cell
E.g. Myocardial infarct
This is characteristic of hypoxic death of cells in all tissues except the brain
LIQUEFACTIVE NECROSIS
Characterized by digestion of dead cells resulting in transformation of the tissue into a liquid viscous mass
Is characteristic of focal bacterial , or occasional fungal infections because microbes stimulate the accumulation of inflammatory cells
E.g. hypoxic death in central nervous system
If the process is initiated by acute inflammation, the material is creamy yellow because of the presence of dead white cells called pus
GANGRENOUS NECROSIS
The term is applied to a limb , usually lower leg that has lost its blood supply & has undergone coagulation necrosis.
When bacterial infection is superimposed , coagulative necrosis is modified by the liquefactive action of the bacteria & the attracted leukocytes (wet gangrene )
Types of the gangrenes
Dry gangrene – dead necrotised tissue remains uninfected.
Wet gangrene-necrotic tissue becomes infected
Gas gangrene – Wet gangrene infected by one of the gas forming Clostridia
Dry gangrene
Gangrene begins in distal part of limb due to ischemia
Causes-
Atherosclerosis of anteries supplying limb
Thromboangitis obliterans
Raynauds disease
Trauma
Ergot poisoning
Due to ischemia tissues undergoes infarction and line of separation is formed between the necrotic tissue and healthy tissue. At the margins of necrotic tissue, granulation tissue develops
Morphology
Grossly the affected part is dry shrunken and dark in colour resembling the mummified foot
Black colour is due to release of haemoglobin from the haemolysed RBC’s which are acted upon by the hydrogen disulfide produced by bacteria which results in the formation of black iron sulphide
Gradually the gangrenous tissue falls from the healthy tissue at the line of separation
Because of the risk of infection it should be surgically separated
Wet gangrene
Wet gangrene typically occurs in the organs with venous blockage or both venous and arterial blockage
Diabetic foot –High glucose content in the necrosed tissue favours bacterial growth
Bed sores –In bed ridden patients at the pressure on the sites like sacrum, buttocks and heel
Also in organs like bowel (due to strangulated hernia, volvulus or intusscesception ) lung, mouth, cervix and vulva
Gas gangrene
It is wet gangrene infected by one of the gas forming Clostridia (Gram positive anaerobic bacteria)
Gas forming Clostridia enters the tissue through a contaminated wound or as a complication of operation on colon which normally contains Clostridia
Clostridia rapidly spreads through tissues especially in muscles and cause extensive necrosis and massive edema
Morphology
Gross – the affected part is edematous, swollen and has crepitations on palpation due to accumulation of gas in tissues
Microscopically
Muscle fibres undergo coagulative necrosis with liquefaction
At the periphery, a zone of leukocytic infiltration, oedema and congestion are found
CASEOUS NECROSIS
Distinctive form of coagulative necrosis most often encountered in tuberculous infection
The term caseous is derived from cheesy white gross appearance of area of necrosis which is friable and white
Microscopic appearance
Necrotic focus – collection of fragmented or lysed cells and amorphous granular debris enclosed with in distinctive inflammatory border – granulomatous reaction
FAT NECROSIS
It is descriptive term for focal areas of fat destruction, typically occurring as a result of release of activated pancreatic lipases into the substance of pancreas & the peritoneal cavity
The fatty acids released combine with calcium to produce grossly visible chalky white areas (fat saponification)
Microscopic appearance
Necrosis take the form of shadowy outlines of necrotic fat cells, with basophilic calcium deposits surrounded by an inflammatory reaction
FIBRINOID NECROSIS
Special form of necrosis seen in immune reactions involving blood vessels
Occurs when complexes of antigens and antibodies are deposited in the walls of arteries
Along with these complexes fibrin which has leaked out of vessels give bright pink amorphous appearance called fibrinoid (Fibrin like)
Reference
Vinay kumar, Abul K.Abbas, Jon C. Aster. Cellular responses to stress and Toxic insults: Adaptations, injury and Death. In: Robbins and Cotran Pathologic basis of disease 9th edition Vol 1.31-68.