Quick Answer
Cell injury and death is a foundational NEET PG pathology cluster that recurs across INI-CET, AIIMS and JIPMER in single-best-answer form. Nail these six anchor points and the marks fall into place.
- Adaptations — atrophy (smaller), hypertrophy (bigger), hyperplasia (more), metaplasia (reversible type-swap eg Barrett), dysplasia (disordered, premalignant).
- Reversible injury — cellular swelling and fatty change. Irreversible — pyknosis, karyorrhexis, karyolysis, membrane rupture, mitochondrial permeability transition pore opening.
- Six necrosis patterns — coagulative (ischaemic MI, kidney, spleen), liquefactive (brain, abscess), caseous (TB), fat (pancreatitis), fibrinoid (malignant HTN, vasculitis), gangrenous (dry, wet, gas).
- Apoptosis — intrinsic (Bax/Bak, cytochrome c, caspase 9) and extrinsic (Fas/FasL, caspase 8) converge on caspase 3. Cell shrinkage, chromatin condensation, apoptotic bodies, no inflammation.
- Regulated non-apoptotic death — necroptosis (RIPK1/3, MLKL), pyroptosis (inflammasome, caspase 1, IL-1 beta), ferroptosis (iron, lipid peroxidation).
- Amyloidosis — Congo red, apple-green birefringence. AL (myeloma), AA (chronic inflammation, India high-yield), ATTR (cardiac), beta-2 microglobulin (dialysis), beta-amyloid (Alzheimer).
Cell injury, adaptation and death is the general pathology chapter examiners return to because every organ-specific chapter downstream builds on it. If you can classify a necrosis pattern from a five-line stem, name the caspase in an apoptosis pathway or explain why a Barrett oesophagus biopsy is metaplastic but a cervical CIN III biopsy is dysplastic, you unlock roughly 8 to 12 marks across the NEET PG paper.
This NEETPGAI deep dive covers the full spectrum — from the reversible swelling of the hepatocyte to the apple-green birefringence of amyloid — with the India-specific context (TB caseous necrosis, secondary AA amyloid, sickle-cell autosplenectomy) that examiners love. Pair it with the inflammation and wound healing guide for the acute and chronic response that follows every injury.
Cellular adaptations — atrophy, hypertrophy, hyperplasia, metaplasia, dysplasia
Cells respond to persistent stress by changing size, number or type. These adaptations are reversible if the stimulus is removed, and each has classical examples that appear verbatim in NEET PG stems.
| Adaptation | What changes | Classical example |
|---|
| Atrophy | Decreased cell size | Denervated muscle, disuse (limb in plaster), endocrine (post-menopausal endometrium), pressure (hydronephrosis) |
| Hypertrophy | Increased cell size, same cell number | Left ventricle in aortic stenosis, gravid uterus, skeletal muscle in athletes |
| Hyperplasia | Increased cell number | Endometrial hyperplasia from unopposed oestrogen, benign prostatic hyperplasia, liver after partial hepatectomy |
| Metaplasia | Reversible switch to another differentiated cell type | Squamous to columnar in Barrett oesophagus (GERD), columnar to squamous in smoker bronchus, connective tissue metaplasia in myositis ossificans |
| Dysplasia | Disordered growth with atypia | Cervical intraepithelial neoplasia (CIN), oral leukoplakia, ductal carcinoma in situ precursor lesions |
Metaplasia is fully reversible if the stimulus is removed but carries a small malignant risk because the metaplastic epithelium is more prone to further genetic hits — Barrett to oesophageal adenocarcinoma is the archetype. Dysplasia already carries architectural and nuclear atypia, and severe dysplasia is by convention indistinguishable from carcinoma in situ.
Reversible vs irreversible cell injury
Cells injured by hypoxia, chemical toxins or immune attack first pass through a reversible phase before crossing the point of no return.
Reversible injury features
- Cellular swelling (hydropic change) — sodium influx from Na-K-ATPase failure, water follows.
- Fatty change (steatosis) — hepatocytes accumulate triglycerides in alcoholic liver, kwashiorkor, drug toxicity.
- Ribosomal detachment from rough endoplasmic reticulum, reduced protein synthesis.
- Mitochondrial swelling without permeability transition pore opening.
Irreversible injury markers
- Nuclear changes — pyknosis (shrunken dense nucleus), karyorrhexis (fragmentation), karyolysis (fading).
- Plasma membrane rupture — leakage of cytoplasmic enzymes (troponin, CK, LDH, AST, ALT) into blood, the biochemical basis of infarct diagnosis.
- Mitochondrial permeability transition pore (MPTP) opening — the biochemical point of no return; cytochrome c leaks out, ATP collapses.
- Lysosomal rupture with intracellular acid hydrolase release.
- Massive calcium influx activating destructive enzymes (phospholipases, proteases, endonucleases).
Necrosis — six morphological patterns
Necrosis is unregulated cell death from severe injury; the resulting inflammation and enzyme release define the pattern.
- Coagulative necrosis — ischaemic infarct in solid organs (myocardium, kidney, spleen, adrenal). Tissue architecture is preserved for days as ghost outlines because lysosomal enzymes are also denatured. The classical microscopy is anucleate eosinophilic cells with preserved shapes.
- Liquefactive necrosis — hypoxic brain infarcts (neuronal lysosomes are abundant) and bacterial abscesses. The tissue liquefies into pus.
- Caseous necrosis — cheese-like, granular, amorphous debris surrounded by a granuloma of epithelioid histiocytes, Langhans giant cells and lymphocytes. Pathognomonic for tuberculosis; also seen in histoplasmosis and some fungi. Extraordinarily high yield in India given TB burden.
- Fat necrosis — enzymatic (acute pancreatitis, where released lipase splits fat into glycerol and fatty acids that combine with calcium as chalky white saponification) or traumatic (breast fat necrosis mimicking carcinoma clinically and mammographically).
- Fibrinoid necrosis — bright pink amorphous deposits in vessel walls where immune complexes and plasma proteins collect. Seen in malignant hypertension, polyarteritis nodosa, Henoch-Schonlein purpura and rheumatic fever Aschoff bodies.
- Gangrenous necrosis — a clinical term. Dry gangrene is coagulative necrosis of a limb from arterial insufficiency (diabetic foot). Wet gangrene adds bacterial liquefaction (bowel infarction). Gas gangrene is Clostridium perfringens myonecrosis with crepitus, foul-smelling exudate and haemolytic anaemia.
Apoptosis — programmed cell death without inflammation
Apoptosis is the physiological, energy-dependent, non-inflammatory suicide programme used in embryogenesis (interdigital web removal), hormone-dependent involution, thymic selection and elimination of virus-infected cells.
The two pathways
| Feature | Intrinsic (mitochondrial) | Extrinsic (death receptor) |
|---|
| Trigger | DNA damage, growth factor withdrawal, misfolded proteins, cytotoxic drugs | Fas ligand on cytotoxic T cells binding Fas (CD95); TNF binding TNFR1 |
| Regulators | Bax and Bak (pro-apoptotic) overwhelm Bcl-2 and Bcl-xL (anti-apoptotic) | Adapter proteins FADD, TRADD |
| Initiator caspase | Caspase 9 (via apoptosome + Apaf-1 + cytochrome c) | Caspase 8 (via DISC) |
| Executioner | Caspase 3, 6, 7 | Caspase 3, 6, 7 |
| Example disease | Follicular lymphoma (BCL2 translocation, apoptosis resistance) | Autoimmune lymphoproliferative syndrome (Fas mutation) |
Both pathways converge on caspase 3, which cleaves the inhibitor of caspase-activated DNase (ICAD) releasing CAD, which fragments DNA into 180 base pair ladders. Phosphatidylserine flips to the outer leaflet as an eat-me signal for macrophages, which is why apoptosis is inflammation-free.
Morphology on light microscopy
- Cell shrinkage with dense eosinophilic cytoplasm.
- Chromatin condensation against the nuclear envelope.
- Nuclear fragmentation.
- Apoptotic bodies (membrane-bound cell fragments) engulfed by neighbouring cells or macrophages.
- No adjacent inflammatory infiltrate.
Regulated non-apoptotic cell death — the newer additions
Post-2010 pathology has recognised several other programmed death forms — examiners have begun asking them in AIIMS and INI-CET stems.
- Necroptosis — programmed necrosis. When caspase 8 is blocked (viral infection, drug), RIPK1 and RIPK3 phosphorylate MLKL which oligomerises and punctures the plasma membrane. Releases DAMPs and drives sterile inflammation. Relevant in ischaemia-reperfusion injury and pancreatitis.
- Pyroptosis — inflammasome-driven death. NLRP3, AIM2 or NLRC4 inflammasomes recruit and activate caspase 1, which cleaves pro-IL-1 beta and pro-IL-18 to their active forms and cleaves gasdermin D. Gasdermin D N-terminal fragments oligomerise into plasma-membrane pores. Central to septic shock, gouty arthritis and familial Mediterranean fever.
- Ferroptosis — iron-dependent lipid peroxidation. Loss of glutathione peroxidase 4 (GPX4) or cystine import allows Fe2+ to catalyse peroxidation of polyunsaturated fatty acids in membranes. Cancer therapeutics increasingly exploit ferroptosis; also implicated in neurodegeneration and haemorrhagic stroke.
- Autophagy — a survival response in starvation (LC3 marks autophagosomes engulfing organelles for lysosomal degradation). When overwhelmed, it tips into autophagic cell death — implicated in neurodegeneration (Parkinson, Huntington) and studied as a cancer therapy target.
Free radicals, aging and cellular senescence
Reactive oxygen species (ROS) — superoxide anion, hydroxyl radical and hydrogen peroxide — arise from mitochondrial electron leak, inflammation (NADPH oxidase burst in neutrophils), radiation, drug metabolism (paracetamol, carbon tetrachloride) and reperfusion injury. They damage lipids (peroxidation of membranes), DNA (8-oxoguanine mutations) and proteins (carbonylation). The defence is the antioxidant system — glutathione (GSH), superoxide dismutase (SOD), catalase, and vitamins A, C and E.
Aging at the cellular level
- Telomere shortening — 50 to 200 base pairs lost per division; when telomeres reach a critical length, replicative senescence occurs (Hayflick limit, roughly 50 divisions).
- Progressive DNA damage — accumulated oxidative and replicative lesions overwhelm repair.
- Cellular senescence — permanent cell cycle arrest with a senescence-associated secretory phenotype (SASP) contributing to age-related inflammation ("inflammaging").
- Hutchinson-Gilford progeria — LMNA mutation producing progerin, a defective lamin A that disrupts the nuclear envelope; children age at roughly seven times the normal rate.
- Calorie restriction — extends lifespan in model organisms via sirtuin (SIRT1) and mTOR pathway modulation.
Amyloidosis — one topic examiners never skip
Amyloid is misfolded protein deposited extracellularly in a cross-beta-pleated sheet configuration. The gold-standard stain is Congo red, which shows apple-green birefringence under polarised light.
Classification by precursor protein
| Type | Precursor | Setting |
|---|
| AL (primary) | Immunoglobulin light chains (usually lambda) | Multiple myeloma, MGUS, plasma cell dyscrasias |
| AA (secondary) | Serum amyloid A (SAA), an acute-phase reactant | Chronic inflammation — rheumatoid arthritis, TB, bronchiectasis, IBD, familial Mediterranean fever |
| Beta-2 microglobulin | Beta-2 microglobulin | Long-term haemodialysis; carpal tunnel deposition |
| ATTR | Transthyretin (wild-type or mutant) | Familial amyloid cardiomyopathy and polyneuropathy; senile cardiac amyloid |
| Beta-amyloid | Amyloid precursor protein cleavage | Alzheimer disease plaques |
| IAPP | Islet amyloid polypeptide | Type 2 diabetes mellitus pancreatic islets |
| Calcitonin | Procalcitonin | Medullary thyroid carcinoma stromal amyloid |
Clinical manifestations by organ
- Kidney — nephrotic-range proteinuria, the most common presenting feature of AL and AA.
- Heart — restrictive cardiomyopathy with sparkling myocardium on echo; low-voltage ECG despite thick walls.
- Liver — hepatomegaly with waxy consistency, mildly deranged LFTs.
- Tongue — macroglossia (classic in AL).
- Skin — periorbital purpura ("pinch purpura") after minor trauma.
- Nerves — carpal tunnel syndrome (beta-2 microglobulin), autonomic and sensorimotor neuropathy (ATTR).
Diagnosis and treatment
- Subcutaneous fat pad aspirate or rectal biopsy for tissue diagnosis (Congo red positive).
- Serum free light chains for AL; SAP scintigraphy for organ mapping.
- Treatment — AL is treated as plasma cell dyscrasia (bortezomib-based regimens, autologous stem cell transplant). AA needs treatment of the underlying inflammation (colchicine cures FMF-associated AA). ATTR is treated with tafamidis (transthyretin stabiliser), patisiran (siRNA) or inotersen (antisense).
India-specific and NEET PG traps
- Caseous necrosis = TB until proven otherwise in Indian stems. Second-line answer is histoplasmosis.
- AA amyloid — remember chronic TB, bronchiectasis and rheumatoid arthritis are all major secondary causes in India.
- Sickle cell autosplenectomy — repeated infarctions produce coagulative necrosis then fibrous shrinkage of the spleen; high yield in tribal-belt haematology stems.
- Fatty change of liver — think alcohol, kwashiorkor, non-alcoholic steatohepatitis (rising in urban India), tetracycline in pregnancy, Reye syndrome.
- Wet gangrene of the bowel — mesenteric ischaemia; often SMA embolus from AF.
- Fibrinoid necrosis with onion-skin arterial thickening — malignant hypertension.
- Bcl-2 translocation t(14;18) — anti-apoptotic overexpression in follicular lymphoma.
- p53 loss — impaired intrinsic apoptosis after DNA damage; Li-Fraumeni syndrome; the most commonly mutated gene in human cancer.
- Fas/FasL defects — autoimmune lymphoproliferative syndrome (ALPS) with lymphadenopathy and double-negative T cells.
- Pyroptosis + IL-1 beta — the mechanism behind gout crystal disease and cryopyrin-associated periodic syndromes; canakinumab (anti-IL-1 beta) treatment.
- Ferroptosis — proposed mechanism in neuronal death after intracerebral haemorrhage; also the target of experimental cancer drugs.
- Congo red apple-green birefringence — the classic image stem for amyloidosis; do not confuse with periodic acid-Schiff (glycogen), Prussian blue (iron) or von Kossa (calcium).
- Periorbital purpura and macroglossia — AL amyloid until proven otherwise.
- Restrictive cardiomyopathy with low-voltage ECG plus thick walls on echo — cardiac amyloid.
- Progerin — accumulates in Hutchinson-Gilford progeria; LMNA mutation.
- Hayflick limit — approximately 50 divisions; underpins replicative senescence.
- Barrett oesophagus — squamous to columnar metaplasia with goblet cells; risk of adenocarcinoma.
- Metaplasia — reversible; dysplasia — premalignant; carcinoma in situ — full-thickness dysplasia without basement membrane invasion.
- Mitochondrial permeability transition pore — biochemical point of no return in cell injury.
- Reperfusion injury paradox — restored oxygen generates a burst of ROS that can extend infarct size; the basis for antioxidant and neuroprotection research after stroke and MI thrombolysis.
Frequently asked questions
What is the difference between hypertrophy, hyperplasia, metaplasia and dysplasia?
Hypertrophy is an increase in cell size with the same number of cells — classic examples are cardiac muscle in aortic stenosis and skeletal muscle in weight training. Hyperplasia is an increase in cell number, seen in endometrium under oestrogen and hepatocytes after partial hepatectomy. Metaplasia is a reversible change of one differentiated cell type to another — squamous to columnar in Barrett oesophagus, columnar to squamous in the bronchus of a smoker. Dysplasia is disordered cell growth with pleomorphism, hyperchromasia and mitoses — it is not yet cancer but is the premalignant step that can regress if the insult is removed or progress to carcinoma in situ.
How do you distinguish the six morphological types of necrosis?
Coagulative necrosis preserves tissue architecture and follows ischaemic infarcts of the heart, kidney and spleen — cells look like ghost outlines. Liquefactive necrosis fully digests tissue and is seen in the brain after infarction and in bacterial abscesses. Caseous necrosis is cheese-like amorphous debris inside granulomas, pathognomonic for tuberculosis. Fat necrosis (enzymatic) occurs in acute pancreatitis with saponification producing chalky calcium soaps, or traumatic in breast. Fibrinoid necrosis deposits pink fibrin-like material in vessel walls in malignant hypertension and vasculitis. Gangrenous necrosis is a clinical term — dry (coagulative + ischaemia) in diabetic toes, wet (superimposed bacterial liquefaction) in bowel, gas gangrene in Clostridium perfringens myonecrosis.
What is the difference between the intrinsic and extrinsic apoptosis pathways?
The intrinsic (mitochondrial) pathway is triggered by internal stressors — DNA damage, growth factor withdrawal, misfolded proteins. Pro-apoptotic Bax and Bak overwhelm anti-apoptotic Bcl-2 and Bcl-xL, permeabilising the mitochondrial outer membrane. Cytochrome c leaks into the cytosol, forms the apoptosome with Apaf-1 and activates initiator caspase 9. The extrinsic (death receptor) pathway starts with Fas ligand binding Fas (CD95) or TNF binding TNFR1, recruiting FADD and activating caspase 8. Both pathways converge on executioner caspase 3, which cleaves nuclear and cytoskeletal substrates. Morphology is identical in either case: cell shrinkage, chromatin condensation (pyknosis), fragmentation into membrane-bound apoptotic bodies and phagocytosis without inflammation.
What are the newer regulated cell death pathways beyond apoptosis?
Necroptosis is programmed necrosis using RIPK1, RIPK3 and MLKL to form membrane pores when caspase 8 is inhibited — it releases DAMPs and drives inflammation, relevant in ischaemia-reperfusion. Pyroptosis is inflammasome-driven death via caspase 1 cleaving gasdermin D, which forms plasma membrane pores and releases IL-1 beta and IL-18 — central to sepsis and NLRP3 inflammasome disease. Ferroptosis is iron-dependent lipid peroxidation driven by loss of GPX4 and glutathione, sensitising cancer cells and injured neurons. Autophagy is a survival response that can tip into death — lysosomes degrade organelles and misfolded proteins in starvation and neurodegeneration. Examiners now ask which cell death fits which trigger, so learn the trigger, executioner molecule and downstream inflammatory signature for each.
How is amyloidosis classified and what is the diagnostic gold standard?
Amyloidosis is deposition of misfolded proteins in a cross-beta-pleated sheet configuration. The gold standard is Congo red staining showing apple-green birefringence under polarised light. AL (primary) amyloidosis comes from monoclonal immunoglobulin light chains in multiple myeloma and plasma cell dyscrasias. AA (secondary) amyloidosis derives from serum amyloid A during chronic inflammation — rheumatoid arthritis, tuberculosis, bronchiectasis and familial Mediterranean fever, which is a major cause in India. Beta-2 microglobulin amyloid deposits at carpal tunnels in long-term haemodialysis patients. Transthyretin (ATTR) amyloid causes familial and senile cardiac amyloidosis. Beta-amyloid drives Alzheimer disease plaques, and islet amyloid polypeptide (IAPP) is seen in type 2 diabetes pancreas.
This content is for educational purposes for NEET PG exam preparation. It is not a substitute for professional medical advice, diagnosis, or treatment. Clinical information has been reviewed by qualified medical professionals.
Written by: NEETPGAI Editorial Team
Reviewed by: Pending SME Review
Last reviewed: September 2026
