Pigments & Pathological Calcification
Pathology · General Pathology · lean revision notes
Pigments & Pathological Calcification
Cellular accumulations of coloured substances (pigments) and abnormal tissue deposition of calcium salts are favourite NEET PG topics because they tie morphology to mechanism. This note covers exogenous and endogenous pigments, dystrophic versus metastatic calcification, and the ever-popular psammoma bodies.
Overview & classification
A pigment is a coloured substance, either endogenous (synthesised within the body) or exogenous (introduced from outside). Pathological calcification is the abnormal tissue deposition of calcium salts, together with smaller amounts of iron, magnesium and other minerals.
| Category | Examples |
|---|---|
| Exogenous pigments | Carbon (anthracosis), tattoo pigments, dust (silica, asbestos), carotenoids |
| Endogenous pigments | Lipofuscin, melanin, haemosiderin, bilirubin, lipochromes |
| Calcification | Dystrophic (normal calcium, damaged tissue), Metastatic (high calcium, normal tissue) |
High-yield: The single most repeated theme is dystrophic = normal serum calcium in dead/damaged tissue versus metastatic = raised serum calcium in normal tissue. Memorise this dichotomy first.
Exogenous pigments
Carbon / coal dust (anthracosis)
The commonest exogenous pigment. Inhaled carbon (from polluted air, smoke, mining) is phagocytosed by alveolar macrophages and transported to peribronchial and hilar lymph nodes, blackening them. In itself anthracosis is innocuous, but heavy occupational exposure produces coal worker's pneumoconiosis with fibrosis (progressive massive fibrosis).
Tattoo pigments
Inert metallic salts (e.g., mercuric sulphide, cobalt) injected into the dermis are engulfed by dermal macrophages and persist for life, usually without inflammation.
High-yield: Anthracotic pigment is non-refractile and not birefringent, distinguishing it from silica (birefringent under polarised light).
Endogenous pigments
1. Lipofuscin ("wear-and-tear" pigment)
- An insoluble yellow-brown, finely granular intracytoplasmic, often perinuclear pigment.
- Composed of polymers of lipids and phospholipids complexed with protein, derived from free-radical–catalysed peroxidation of polyunsaturated membrane lipids.
- It is not injurious to the cell; rather it is a marker of past free-radical injury and lysosomal "indigestible residue".
- Seen prominently in ageing, cachexia and severe malnutrition → when atrophy of organs (heart, liver) is accompanied by lipofuscin, the organ looks brown = brown atrophy (classically of the heart and liver).
- Stains: PAS-positive, acid-fast, autofluorescent; stains with Sudan black (lipid component).
High-yield: Lipofuscin = "wear and tear" / "lipochrome" pigment of ageing → brown atrophy of heart. It is a sign of injury but does NOT itself harm the cell.
2. Melanin
- The only endogenous brown-black pigment synthesised in melanocytes (neural crest origin) from tyrosine via tyrosinase within melanosomes.
- Tyrosinase deficiency → albinism. Localised loss → vitiligo (autoimmune melanocyte destruction).
- Accumulation: melasma, naevi, melanoma. Adrenal insufficiency (Addison's disease) → diffuse hyperpigmentation due to high ACTH/MSH.
- A related pigment, homogentisic acid in ochronosis (alkaptonuria) deposits black pigment in cartilage/connective tissue.
High-yield: Melanin synthesis: Tyrosine → DOPA → dopaquinone → melanin (rate-limiting enzyme tyrosinase, a copper-containing enzyme).
3. Haemosiderin
- A golden-yellow to brown, granular or crystalline iron-storage pigment; an aggregate of ferritin micelles.
- Forms wherever there is local or systemic excess of iron. Local example: a bruise changing colour (haemoglobin → biliverdin → bilirubin → haemosiderin).
- Demonstrated by the Prussian blue (Perls') reaction turning it blue.
- Haemosiderosis = generalised iron overload without organ damage. Haemochromatosis = iron overload WITH parenchymal damage (cirrhosis, "bronze diabetes", cardiomyopathy, pigmentation).
- "Heart failure cells" = haemosiderin-laden alveolar macrophages in chronic pulmonary congestion (e.g., mitral stenosis, LV failure).
High-yield: Prussian blue (Perls' stain) is the answer for "stain for haemosiderin/iron". Haemochromatosis triad = cirrhosis + diabetes + skin pigmentation = bronze diabetes.
4. Bilirubin
- The normal major pigment of bile, derived from haemoglobin breakdown but contains NO iron.
- Excess deposition produces jaundice (icterus); in the CNS of neonates with severe unconjugated hyperbilirubinaemia it causes kernicterus (basal ganglia staining).
| Pigment | Colour | Iron present? | Key stain | Classic association |
|---|---|---|---|---|
| Lipofuscin | Yellow-brown | No | PAS+, autofluorescent | Brown atrophy, ageing |
| Melanin | Brown-black | No | Masson-Fontana / DOPA | Albinism, melanoma, Addison's |
| Haemosiderin | Golden-brown | Yes | Prussian blue (Perls') | Haemochromatosis, bruise |
| Bilirubin | Green-yellow | No | Van den Bergh (serum) | Jaundice, kernicterus |
| Carbon | Black | No | None (refractile-negative) | Anthracosis |
High-yield: A classic single-best-answer trap: Lipofuscin and melanin both lack iron; haemosiderin contains iron; bilirubin (despite being a haem derivative) contains NO iron.
Pathological calcification
Abnormal deposition of calcium salts in tissues. Two distinct types must be sharply separated.
Dystrophic calcification
- Calcification in dead, dying or degenerated tissue with NORMAL serum calcium and normal calcium metabolism.
- The damaged/necrotic tissue acts as a nidus; mitochondria of injured cells accumulate calcium.
- Pathogenesis: initiation + propagation. Initiation occurs extracellularly in membrane-bound matrix vesicles and intracellularly in mitochondria of dying cells; propagation involves crystal (hydroxyapatite) formation.
Common sites / lesions (very high-yield):
- Atherosclerotic plaques (calcified intima)
- Aging or damaged heart valves → aortic stenosis (senile calcific aortic stenosis), calcific mitral disease
- Caseous necrosis of tuberculosis (calcified lymph nodes, Ghon focus → Ranke complex)
- Fat necrosis (e.g., acute pancreatitis → saponification)
- Old thrombi → phleboliths
- Dead parasites (cysticercosis, hydatid, Schistosoma eggs)
- Monckeberg medial sclerosis (medial calcification of muscular arteries)
- Psammoma bodies (see below)
- Damaged/atrophic tissues; dystrophic calcification of a leiomyoma ("womb stone").
High-yield: Whenever the stem says "normal serum calcium" + a damaged structure (TB node, atheroma, old valve, scar, dead parasite) → answer dystrophic calcification.
Metastatic calcification
- Deposition of calcium salts in otherwise NORMAL (vital) tissues, secondary to HYPERcalcaemia (deranged calcium metabolism).
Causes of hypercalcaemia (mnemonic — "PAM P. SCHMIDT" simplified):
- Hyperparathyroidism (primary — parathyroid adenoma; or secondary in chronic renal failure)
- Bony destruction — multiple myeloma, metastatic carcinoma, leukaemia, Paget's disease, immobilisation
- Vitamin D–related — vitamin D toxicity, sarcoidosis (macrophage 1-α-hydroxylase), idiopathic hypercalcaemia of infancy (Williams syndrome)
- Renal failure → secondary hyperparathyroidism + phosphate retention
- Milk-alkali syndrome
Preferred sites: tissues that lose acid / have an internal alkaline compartment (calcium salts precipitate in alkaline interstitium):
- Gastric mucosa (loses acid)
- Kidneys (nephrocalcinosis — loses acid in distal tubule)
- Lungs (alveolar septa — lose CO₂)
- Systemic arteries and pulmonary veins
High-yield: Metastatic calcification favours the "acid-losing" tissues: stomach, kidney, lungs, blood vessels. Mnemonic for sites: think of tissues that excrete acid → relatively alkaline interior.
Dystrophic vs metastatic — the table they love
| Feature | Dystrophic | Metastatic |
|---|---|---|
| Serum calcium | Normal | Raised (hypercalcaemia) |
| Calcium metabolism | Normal | Deranged |
| Tissue status | Dead / damaged / necrotic | Normal / viable |
| Reversible if cause removed | No | Sometimes |
| Examples | Atheroma, TB node, old valve, fat necrosis, psammoma body | Hyperparathyroidism, vit D toxicity, renal failure, milk-alkali |
| Preferred sites | Site of injury | Stomach, kidney, lung, vessels |
Stepwise approach to a calcification stem: Check serum calcium → if normal, look for dead/damaged tissue → dystrophic. If calcium is high, look for the cause (PTH, vit D, bone destruction, renal failure) → metastatic.
Histologically both appear as basophilic (blue), amorphous, granular, sometimes clumped deposits on H&E. Special stains: von Kossa (silver, stains phosphate/carbonate component black) and Alizarin red S (stains calcium directly red-orange).
High-yield: von Kossa stains the anion (phosphate), Alizarin red S stains calcium itself. Examiners love this distinction.
Psammoma bodies
- Psammoma = "sand" (Greek psammos). These are laminated, concentric, lamellated calcific (dystrophic) spherules, formed around a nidus, with onion-skin layering.
- They form by dystrophic calcification of necrotic single tumour cells or on a vascular core, and progressively calcify in concentric rings.
Classic "PSaMMoma" associations (mnemonic):
- P — Papillary thyroid carcinoma
- S — Serous papillary cystadenocarcinoma of ovary (serous ovarian tumours)
- M — Meningioma
- M — Mesothelioma
Also seen in: papillary renal cell carcinoma, prolactinoma, somatostatinoma, and occasionally papillary tumours elsewhere.
High-yield: "Concentric lamellated calcified bodies" → think papillary thyroid carcinoma, serous ovarian carcinoma, meningioma, mesothelioma. In thyroid FNAC, psammoma bodies strongly suggest papillary carcinoma.
A related concentric structure to distinguish: Corpora amylacea (prostate, brain, lung) are also laminated but are proteinaceous, not calcified (though they may calcify with age).
Clinical correlations & investigation of choice
- Brown atrophy of heart — seen at autopsy in elderly/cachectic patients; lipofuscin in perinuclear myocytes confirms.
- Haemochromatosis — investigation: raised serum ferritin and transferrin saturation (>45%); confirm with HFE gene (C282Y) testing and/or liver biopsy with Perls' stain; MRI shows hepatic iron. Treatment of choice: repeated phlebotomy (chelation with deferoxamine/deferasirox if anaemic or in secondary overload).
- Metastatic calcification — investigation: serum calcium, phosphate, PTH, vitamin D; imaging (nephrocalcinosis on USG/CT). Treat the underlying cause (parathyroidectomy, stop vit D, manage CKD-mineral bone disease).
- Psammomatous tumour on FNAC — investigate with histopathology/immunohistochemistry; for thyroid, BRAF V600E supports papillary carcinoma.
Complications & differentials
- Heavy anthracosis → progressive massive fibrosis, cor pulmonale.
- Untreated haemochromatosis → cirrhosis (and hepatocellular carcinoma risk), cardiomyopathy, diabetes, hypogonadism, arthropathy.
- Dystrophic valve calcification → calcific aortic stenosis with LV hypertrophy/heart failure.
- Metastatic calcification of kidney (nephrocalcinosis) → renal failure; of lung → restrictive defect.
- Vascular calcification (Monckeberg) → arterial stiffness (usually clinically silent, does not narrow lumen).
Key differentials to keep straight:
- Dystrophic vs metastatic calcification (serum calcium).
- Haemosiderosis (no damage) vs haemochromatosis (organ damage).
- Lipofuscin (no iron, brown atrophy) vs haemosiderin (iron, Prussian blue +).
- Psammoma bodies (calcified, laminated) vs corpora amylacea (proteinaceous) vs Schaumann/asteroid bodies (sarcoidosis).
- Hyaline change (pink, glassy, structureless) is NOT a pigment — don't confuse with these accumulations.
Recently asked / exam angle
- "Wear and tear pigment" → Lipofuscin (also asked: which pigment in brown atrophy of heart).
- "Pigment containing iron" → Haemosiderin; "stain for it" → Prussian blue / Perls'.
- "Dystrophic calcification occurs at normal serum calcium" — true/false and matching questions.
- Site of metastatic calcification — stomach, kidney, lung (favourite single-best-answer).
- Psammoma body associations — papillary thyroid, serous ovarian, meningioma, mesothelioma (also papillary RCC).
- Stain for calcium: von Kossa / Alizarin red S (which stains calcium directly? → Alizarin red).
- Rate-limiting enzyme of melanin synthesis → tyrosinase; albinism is its deficiency.
- Bronze diabetes = haemochromatosis; classic triad cirrhosis + diabetes + skin pigmentation.
- Bilirubin contains no iron despite being a haem product — a recurring trap.
- Monckeberg medial sclerosis is a form of dystrophic calcification of the media of muscular arteries.
Rapid revision
- Lipofuscin = wear-and-tear, lipid-peroxidation pigment of ageing → brown atrophy of heart/liver; harmless marker of past injury; no iron, autofluorescent, PAS+.
- Haemosiderin is the only common pigment containing iron; detected by Prussian blue (Perls').
- Bilirubin contains NO iron; excess → jaundice; in neonates → kernicterus.
- Melanin from tyrosine via tyrosinase; deficiency = albinism; autoimmune loss = vitiligo.
- Anthracosis = carbon in lung/lymph nodes; commonest exogenous pigment; non-birefringent.
- Dystrophic calcification = normal serum calcium in damaged/dead tissue (atheroma, TB caseation, old valves, fat necrosis, dead parasites).
- Metastatic calcification = raised serum calcium in normal tissue; causes = hyperparathyroidism, vit D toxicity, bone destruction, renal failure, milk-alkali.
- Metastatic calcification favours stomach, kidney, lung, blood vessels (acid-losing/alkaline tissues).
- Psammoma bodies = concentric laminated dystrophic calcifications → Papillary thyroid, Serous ovarian, Meningioma, Mesothelioma (+ papillary RCC).
- Calcium stains: von Kossa (phosphate), Alizarin red S (calcium directly).
- Haemosiderosis (no damage) vs haemochromatosis (organ damage = bronze diabetes); treat with phlebotomy.
- Both calcification types appear basophilic and granular on H&E; the discriminator is serum calcium + tissue viability.