Cell injury and necrosis
Cell injury, necrosis, apoptosis, accumulations and calcification (FCPS Part 1)
Reversible and irreversible cell injury, types of necrosis, apoptosis and Bcl-2, pigments and dystrophic versus metastatic calcification. Each change is linked to the mechanism that produces it, so exam answers follow logically.
Causes and core mechanisms
- Causes: hypoxia (commonest: ischaemia, anaemia, carbon monoxide, respiratory failure), toxins and drugs, infection, immune reactions, physical agents, genetic defects, nutritional imbalance.
- ATP depletion is the mechanism common to ischaemic, toxic and burn injury. Its effects explain the early changes:
| ATP falls, so... | Result |
|---|---|
| Na+/K+ ATPase fails | Sodium and water enter: cell swelling, the first visible change (hydropic change) |
| Anaerobic glycolysis rises | Glycogen used up, lactic acid lowers pH, chromatin clumps |
| Ribosomes detach from rough ER | Protein synthesis falls; fat is not exported, giving fatty change |
| Calcium pumps fail | Cytosolic calcium rises and activates phospholipases, proteases, endonucleases and ATPases |
- Mitochondrial damage releases cytochrome c (triggering apoptosis) and stops ATP production. Free radicals (reperfusion, radiation, paracetamol, carbon tetrachloride) cause lipid peroxidation of membranes.
- Membrane damage, of the plasma membrane and lysosomes, is the decisive step that makes injury irreversible.
Reversible versus irreversible injury
| Reversible | Irreversible |
|---|---|
| Cell swelling, fatty change, blebs, dilated ER, detached ribosomes, myelin figures | Severe mitochondrial damage (amorphous densities), plasma membrane rupture, lysosomal rupture, nuclear change: pyknosis (shrinks), karyorrhexis (fragments), karyolysis (fades) |
- Enzymes leak only through broken membranes: troponin and CK (heart), ALT and AST (liver; raised after hepatotoxic remedies), so their rise marks irreversible damage.
Types of necrosis
| Type | Mechanism and look | Classic examples |
|---|---|---|
| Coagulative | Proteins (including enzymes) denatured, so outlines persist as ghost cells | Infarcts of heart, kidney, spleen |
| Liquefactive | Enzymes digest tissue to fluid | Brain infarct; abscess (for example after an infected finger cut) |
| Caseous | Cheesy, structureless debris inside a granuloma | Tuberculosis (look for acid-fast bacilli with Ziehl–Neelsen), histoplasmosis |
| Fat necrosis | Lipase frees fatty acids that bind calcium (saponification, chalky white) | Acute pancreatitis and pancreatic surgery (enzymatic); breast trauma (traumatic) |
| Fibrinoid | Immune complexes and fibrin in vessel walls, bright pink | Vasculitis (polyarteritis nodosa), malignant hypertension |
| Gangrenous | Coagulative necrosis of a limb (dry) with bacterial liquefaction (wet) | Diabetic foot, strangulated bowel; gas gangrene (*Clostridium perfringens*) |
- Ischaemic acute tubular necrosis: prolonged shock reduces renal perfusion; the straight proximal tubule and thick ascending limb die, cells slough and form muddy brown granular casts; oliguria and rising creatinine follow. Tubular cells can regenerate if the patient survives.
- Progressive bacterial synergistic gangrene (Meleney): about two weeks after surgery a slowly spreading painful ulcer around a suture, with a central ulcer, purple middle zone and outer red zone; caused by microaerophilic streptococci with *Staphylococcus aureus*. It spreads slowly, unlike gas gangrene or necrotising fasciitis.
Apoptosis
- Programmed, energy-dependent death of single cells without inflammation, for example embryonic remodelling, hormone withdrawal (endometrium), removal of self-reactive lymphocytes, viral hepatitis (Councilman bodies in yellow fever and viral hepatitis), and cells with damaged DNA.
| Pathway | Trigger | Key molecules |
|---|---|---|
| Intrinsic (mitochondrial) | DNA damage, growth factor withdrawal, misfolded proteins | p53 and BH3-only sensors activate Bax and Bak, which let cytochrome c leak out; it activates caspase-9. Bcl-2 and Bcl-xL inhibit this pathway by keeping the mitochondrial membrane intact |
| Extrinsic (death receptor) | Fas ligand, TNF | Fas (CD95), FADD, caspase-8 |
| Common end | Both | Executioner caspases 3 and 6; endonucleases cut DNA into nucleosome-sized pieces (DNA ladder) |
- Cancer and apoptosis: tumours escape apoptosis by over-expressing Bcl-2 (follicular lymphoma, t(14;18)) or losing p53; Bcl-2 is the classic anti-apoptotic factor.
| Feature | Necrosis | Apoptosis |
|---|---|---|
| Cell size | Swelling | Shrinkage |
| Chromatin | Pyknosis, karyorrhexis, karyolysis | Condensed against the nuclear membrane, then fragmented |
| Membrane | Ruptured | Intact, blebs, apoptotic bodies eaten by macrophages |
| Inflammation | Yes | No |
| Energy | Passive | ATP-dependent |
- Other forms: necroptosis, pyroptosis (inflammasome, caspase-1, IL-1β), ferroptosis.
Intracellular accumulations and pigments
| Substance | Settings |
|---|---|
| Fat (steatosis) | Liver: alcohol, NAFLD, kwashiorkor, toxins, Reye syndrome, pregnancy; heart "tigered" in hypoxia |
| Cholesterol | Atheroma, xanthoma, strawberry gallbladder |
| Protein | Russell bodies (plasma cell immunoglobulin), Mallory–Denk bodies (alcoholic hepatitis), alpha-1 antitrypsin globules (PAS-positive, diastase-resistant) |
| Lipofuscin | Wear-and-tear brown pigment in old heart, liver, neurones (brown atrophy); harmless |
| Haemosiderin | Bruises, haemochromatosis, transfusion; Perls Prussian blue stain |
| Melanin | Increased skin pigmentation in Addison disease (ACTH and MSH from POMC) |
| Anthracotic carbon | Black hilar lymph nodes in city dwellers and smokers, harmless |
- Amyloid is extracellular misfolded protein in beta-pleated sheets. It stains with Congo red and shows apple-green birefringence under polarised light.
Pathological calcification
| Feature | Dystrophic | Metastatic |
|---|---|---|
| Serum calcium | Normal | Raised (or high calcium-phosphate product) |
| Tissue | Dead or damaged tissue | Normal tissue, especially where alkaline: lung interstitium, kidney (nephrocalcinosis), gastric mucosa, vessels |
| Examples | Atheroma, calcific aortic stenosis, caseous TB, fat necrosis, psammoma bodies, dead parasites | Hyperparathyroidism (commonest cause in the kidney), vitamin D excess, chronic kidney disease, milk-alkali syndrome, sarcoidosis, myeloma, bone metastases |
- Psammoma bodies (laminated calcium in dying tumour cells, a dystrophic process): papillary thyroid carcinoma, serous ovarian tumours, meningioma, mesothelioma.
- Myositis ossificans: after a deep muscle bruise (thigh), a haematoma calcifies and then ossifies. At 2–3 weeks a firm, painful swelling shows calcification with a bony rim at the periphery (zonal maturation), the opposite of osteosarcoma, which is most mature in the centre. Rest; excise only once mature.
- Calcification stains with von Kossa and alizarin red.
Remember Swelling first, membrane rupture last: ATP failure is reversible, membrane damage is not.
Exam trap Brain infarcts liquefy; infarcts of other solid organs coagulate.
Exam trap Dystrophic calcification occurs with normal serum calcium; metastatic calcification needs high calcium or phosphate.
How FCPS asks about this
- First visible change when ATP falls: cell swelling as the Na+/K+ pump fails; membrane damage marks irreversibility.
- Protein family blocking the intrinsic apoptotic pathway: Bcl-2 and Bcl-xL; caspase-9 intrinsic, caspase-8 extrinsic.
- Feature of apoptosis rather than necrosis: cell shrinkage with condensed chromatin and no inflammation.
- Muddy brown granular casts after shock: ischaemic acute tubular necrosis.
- Calcified mass with a bony rim in a bruised thigh: myositis ossificans.
- Slowly spreading ulcer around a suture with purple and red zones: Meleney gangrene, microaerophilic streptococci with *S. aureus*.
- Amyloid stain: Congo red with apple-green birefringence; black hilar nodes are anthracosis.
Test yourself on this topic
5 of the 83 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
Trypsin cleaves peptide bonds on the carboxyl side of which amino acids?
Answer: Lysine and arginine
Trypsin is an endopeptidase that cuts after the basic amino acids lysine and arginine. Chymotrypsin cuts after large aromatic residues (phenylalanine, tyrosine, tryptophan) and elastase after small neutral ones such as alanine. Trypsin also activates the other pancreatic zymogens, which is why trypsinogen activation by enteropeptidase is the key first step.
Pancreatic proteases
- Trypsin: after lysine, arginine
- Chymotrypsin: after aromatic residues
- Elastase: small neutral residues
- Trypsin activates the others
Remember Trypsin likes the basic ones.
Exam trap Pepsin, not trypsin, is the stomach's protease.
Why the other options are wrong
- Phenylalanine and tyrosine Chymotrypsin's sites.
- Alanine and glycine Elastase's sites.
- Glutamate and aspartate Acidic residues; not trypsin.
- Proline and methionine Not trypsin's sites.
A man dies 2 days after a myocardial infarction. Which microscopic changes are expected in the infarct?
Answer: Coagulative necrosis, neutrophils
Infarct timeline: under 4 hours—usually no light microscopic change (wavy fibres at the edge; electron microscopy shows damage); 4–12 hours—early coagulative necrosis, oedema, haemorrhage; 12–24 hours—contraction band necrosis, early neutrophils; 1–3 days—coagulative necrosis with loss of nuclei and striations and a heavy neutrophil infiltrate (risk of fibrinous pericarditis); 3–7 days—macrophages remove dead cells, the wall is softest (rupture risk: free wall, septum, papillary muscle); 1–2 weeks—granulation tissue; 2 months onwards—dense collagen scar (aneurysm risk). Reperfusion injury causes contraction band necrosis and haemorrhage.
MI timeline
- <4 h: no change on light microscopy
- 1–3 days: coagulative necrosis + neutrophils (pericarditis)
- 3–7 days: macrophages, softest → rupture
- 1–2 weeks: granulation tissue
- >2 months: scar → aneurysm
Remember Neutrophils first, macrophages next, scar last.
Why the other options are wrong
- Granulation tissue, new vessels 1–2 weeks.
- Dense collagenous scar Weeks to months.
- Normal on light microscopy Under about 4 hours.
- Lipofuscin macrophages, fibrosis Not the picture at 2 days.
In a myocardial infarct, which cells dominate the dead area about 5 days after onset?
Answer: Macrophages
After MI: coagulative necrosis from about 4–12 hours; neutrophils peak at 1–3 days; macrophages remove dead muscle at about 3–7 days, when the wall is softest and free-wall rupture is most likely; granulation tissue forms from about 1–2 weeks; and a dense collagen scar is complete by about 2 months.
MI timeline
- 1–3 days: neutrophils
- 3–7 days: macrophages; rupture risk
- 1–2 weeks: granulation tissue
- ~2 months: scar
Remember Day 5: macrophages are clearing the debris.
Exam trap Free-wall rupture is most likely when macrophages have softened the infarct (3–7 days).
Why the other options are wrong
- Neutrophils Peak at 1–3 days.
- Lymphocytes Not the main cell in infarct healing.
- Plasma cells Not the main cell in infarct healing.
- Fibroblasts with dense collagen Dense collagen scar takes about 2 months.
Which staining property identifies amyloid in a tissue biopsy?
Answer: Congo red, apple-green
Amyloid is extracellular deposition of misfolded proteins forming insoluble fibrils with a β-pleated sheet structure. On H&E it appears as amorphous, pink, hyaline material; with Congo red it stains salmon-pink and shows apple-green birefringence under polarised light (diagnostic). Biopsy sites: abdominal fat pad aspirate, rectal or affected organ biopsy. Typing (immunohistochemistry or mass spectrometry) determines the precursor protein, which guides treatment. PAS stains glycogen and mucins; Prussian blue stains haemosiderin.
Amyloid
- Misfolded protein, β-pleated sheet
- Congo red: apple-green birefringence
- Biopsy: fat pad, rectum, organ
- Type the protein to guide treatment
Remember Congo red, apple green.
Why the other options are wrong
- PAS-positive glycogen Glycogen and mucin.
- Prussian blue iron Iron.
- Ziehl–Neelsen acid-fast Mycobacteria.
- Sudan black fat Lipid.
A 34-year-old woman has a massive postpartum haemorrhage with hours of hypotension. Over the next days her urine output falls and creatinine rises; urine shows muddy brown granular casts. Which tubular segment, with tall cuboidal cells bearing a dense brush border and abundant mitochondria, is most damaged?
Answer: Proximal convoluted tubule
The proximal tubule, lined by cuboidal cells with a microvillous brush border and many mitochondria for active reabsorption, has very high oxygen demand. Together with the medullary thick ascending limb, it is the segment most vulnerable to ischaemia. Prolonged hypotension causes ischaemic acute tubular necrosis: cells lose their brush border and slough into the lumen, forming granular casts and blocking flow.
Ischaemic ATN
- Most hit: proximal tubule (and thick ascending limb)
- Brush border, many mitochondria
- High oxygen demand
- Muddy brown casts
Remember The hardest-working cells die first.
Exam trap Bilateral renal cortical necrosis, not ATN, follows the most severe obstetric haemorrhage with DIC.
Why the other options are wrong
- Thin descending limb Thin, flat cells with low metabolic needs.
- Collecting duct Relatively resistant to ischaemia.
- Distal convoluted tubule Less vulnerable than the proximal tubule.
- Parietal Bowman capsule Not part of the tubule.
Where this sits in the CPSP syllabus
- Paper 1: General pathology, Cell injury, cell death (necrosis, apoptosis), intracellular accumulations, calcification
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