Explain necrosis and it's types

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Necrosis and Its Types

Necrosis is a form of pathologic, unregulated cell death that occurs as a consequence of severe injury. It is distinct from apoptosis (programmed cell death) in that it is accompanied by membrane disruption, leakage of cellular contents, and a local inflammatory reaction.

Definition and Basic Mechanism

Necrosis results when cells suffer irreparable damage. The main causes include:

Ischemia (loss of oxygen supply)
Microbial toxins and infections
Burns and physical/chemical injury
Active proteases leaking from cells (e.g., in pancreatitis)

These triggers cause denaturation of cellular proteins and damage to plasma and organellar membranes. Lysosomal enzymes leak into the cytoplasm and digest the cell from within. Cellular contents spill into the extracellular space and act as damage-associated molecular patterns (DAMPs) - including ATP and uric acid - which are recognized by macrophage receptors, triggering phagocytosis and cytokine-mediated inflammation.

This is a key distinction: necrosis always elicits inflammation; apoptosis does not.

Morphologic Features of Necrosis

Cytoplasmic Changes

Increased eosinophilia on H&E staining - due to denatured cytoplasmic proteins binding eosin, plus loss of basophilic RNA
Glassy, homogeneous appearance - from loss of glycogen particles
Vacuolated, "moth-eaten" cytoplasm - when organelles are digested by enzymes
On electron microscopy: discontinuities in membranes, dilated mitochondria with amorphous densities, disrupted lysosomes

Nuclear Changes (3 patterns)

Pyknosis - nuclear shrinkage and increased basophilia; DNA condenses into a dark mass
Karyorrhexis - fragmentation of the pyknotic nucleus
Karyolysis - fading of basophilia due to DNA digestion by DNase; nucleus dissolves over 1-2 days

Types of Necrosis

There are 6 morphologically distinct patterns of tissue necrosis:

1. Coagulative Necrosis

Most common type. The underlying tissue architecture is preserved for several days after the injury.

The injury denatures both structural proteins and enzymes, halting proteolysis
Dead cells appear as eosinophilic, anucleate "ghost cells" that persist for days to weeks
Eventually digested by leukocyte lysosomal enzymes and cleared by phagocytosis
Affected tissue takes on a firm texture

Characteristic of: Infarcts in all solid organs except the brain (heart, kidney, spleen, liver)

Coagulative necrosis - wedge-shaped kidney infarct (yellow area) with distinct margins

FIG. 1.6 - Coagulative necrosis: a wedge-shaped kidney infarct (yellow) with preserved outline but loss of nuclei.

2. Liquefactive Necrosis

The dead cells are completely digested, transforming tissue into a viscous liquid that is eventually removed by phagocytes.

Occurs when microbial infections (bacterial or fungal) stimulate accumulation of inflammatory cells, whose enzymes "liquefy" the tissue
Also occurs in hypoxic death of CNS cells (for unclear reasons - the brain is the exception to coagulative necrosis)
When initiated by acute inflammation (e.g., bacteria), the liquid material is creamy yellow = pus
A localized collection of pus = abscess

Characteristic of: Brain infarcts; bacterial/fungal infections (abscesses)

Liquefactive necrosis - brain infarct showing dissolution of tissue

FIG. 1.7 - Liquefactive necrosis: brain infarct showing complete tissue dissolution.

3. Caseous Necrosis

Named from the Latin for "cheese" - the tissue has a friable, yellow-white, cheese-like gross appearance.

On microscopy: amorphous granular pink (eosinophilic) debris; tissue architecture is completely obliterated (unlike coagulative necrosis)
Cellular outlines cannot be discerned
Typically surrounded by a rim of macrophages and other inflammatory cells = granuloma

Characteristic of: Tuberculous infections (most classic), some fungal infections

Caseous necrosis - tuberculosis of the lung with cheesy yellow-white debris

FIG. 1.8 - Caseous necrosis: tuberculosis of the lung with a large area of cheesy necrotic debris.

4. Fat Necrosis

Focal areas of fat destruction. Two main mechanisms:

Traumatic fat necrosis - direct physical trauma to fatty tissue (e.g., breast)
Enzymatic fat necrosis - in acute pancreatitis, lipases leak from damaged pancreatic acinar cells and ducts and digest peritoneal fat cells, releasing fatty acids which combine with calcium to form chalky white deposits (saponification)

On histology: shadowy outlines of necrotic fat cells surrounded by granular basophilic calcium deposits and inflammation

Characteristic of: Acute pancreatitis (mesentery/omentum), breast trauma

Fat necrosis - white chalky calcium soap deposits in mesentery in acute pancreatitis

FIG. 1.9 - Fat necrosis in acute pancreatitis: white chalky deposits = saponification.

5. Gangrenous Necrosis

Not a histologically distinct pattern - rather a clinical term referring to necrosis of a limb (usually lower leg) that has lost its blood supply.

Dry gangrene = coagulative necrosis without bacterial superinfection; tissue is dry, dark, and mummified
Wet gangrene = when bacterial infection is superimposed, the appearance becomes liquefactive due to destruction by bacterial enzymes and leukocytes; tissue is moist and malodorous
Gas gangrene = caused by Clostridium species; gas bubbles form in the tissue due to fermentation

Characteristic of: Diabetic foot, peripheral vascular disease, frostbite

6. Fibrinoid Necrosis

Detected only by microscopy (no gross appearance). A special form seen in vessel walls.

Caused by deposition of antigen-antibody complexes (immune complexes) or plasma proteins that have leaked into vessel walls, often in severe hypertension
Appears as a bright pink, amorphous material in the vessel wall on H&E - resembling fibrin ("fibrin-like" = fibrinoid)

Characteristic of: Certain forms of vasculitis, malignant hypertension, transplant rejection

Fibrinoid necrosis in an artery - bright pink amorphous material in vessel wall

FIG. 1.10 - Fibrinoid necrosis in an artery.

Summary Table

Type	Mechanism	Gross Appearance	Location/Cause
Coagulative	Protein denaturation preserves architecture	Firm, pale, "ghost cells"	Infarcts - heart, kidney, spleen (not brain)
Liquefactive	Enzymatic digestion liquefies tissue	Fluid-filled cavity / pus	Brain infarcts, bacterial abscesses
Caseous	Combination of coagulation + liquefaction	Friable, yellow-white, cheese-like	Tuberculosis, some fungi
Fat	Lipase digestion of fat	White chalky deposits (saponification)	Pancreatitis, breast trauma
Gangrenous	Ischemia ± bacterial infection	Dry/mummified (dry) or wet/malodorous (wet)	Diabetic foot, PVD
Fibrinoid	Immune complex deposition in vessels	Bright pink, amorphous (microscopy only)	Vasculitis, malignant hypertension

Clinical Relevance: Biomarkers of Necrosis

Necrosis-associated leakage of intracellular proteins into the circulation forms the basis for clinical blood tests:

Cardiac troponins - detect myocardial necrosis (elevated as early as 2 hours after MI); central to MI diagnosis
Transaminases (ALT, AST) - detect hepatocyte necrosis
Alkaline phosphatase - bile duct epithelial necrosis

Sources: Robbins & Kumar Basic Pathology, 11th ed., pp. 21-23; Robbins, Cotran & Kumar Pathologic Basis of Disease, pp. 1377-1388

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