Histochemical Stains
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Direct answer
Histochemical (special) stains exploit a specific chemical reaction to colour a tissue component that haematoxylin-eosin leaves invisible — polysaccharides, lipids, minerals, microorganisms or fibres. Congo red identifies amyloid by apple-green birefringence, periodic acid-Schiff lights up glycogen, basement membranes and fungal walls (diastase-resistant staining marks mucin and fungi), Perls' Prussian blue colours haemosiderin blue, and trichrome and reticulin methods map collagen and architectural framework. In Indian practice the infectious stains carry extra weight: Ziehl-Neelsen for tubercle bacilli, Fite-Faraco for the poorly staining Mycobacterium leprae, and Giemsa for Leishmania donovani bodies. Choosing the stain is the diagnosis half-made, which is why FMGE papers test the stain-disease pair rather than the chemistry.
What you must remember
- Carbohydrate pair: PAS demonstrates glycogen, basement membranes and fungal walls; diastase digestion beforehand removes glycogen, so PAS-diastase resistance identifies mucin and fungi.
- Amyloid: Congo red with apple-green birefringence in polarised light remains the practical standard.
- Iron and copper: Perls' Prussian blue for haemosiderin (transfusional iron overload, marrow iron stores); rhodanine for copper in Wilson disease; orcein for copper-associated protein and HBsAg ground-glass hepatocytes.
- Connective tissue set: Masson trichrome for collagen in fibrosis staging, reticulin for architectural framework (one- to two-cell-thick hepatocyte plates in nodules), Verhoeff-van Gieson for elastic laminae in vasculitis and dissection.
- Indian infection quartet: Ziehl-Neelsen for AFB, Fite-Faraco for M. leprae, Giemsa for Leishmania and malaria, and Warthin-Starry for Bartonella of cat-scratch disease and spirochaetes.
- Capsule and fungal stains: India ink (negative stain) and mucicarmine for the Cryptococcus capsule; Grocott methenamine silver for fungi including Pneumocystis jirovecii.
- Fat rule: Sudan and Oil Red O need frozen sections because paraffin processing dissolves lipid; Fontana-Masson blackens melanin.
Where students slip
Cryptococcus questions fail on the specimen: India ink is a negative stain on wet CSF sediment, while mucicarmine stains the capsule red on tissue sections — mixing the two loses the mark. Students also swap Perls and Fontana-Masson, staining iron where melanin was asked. Remember that leprosy bacilli lose their acid-fastness to standard strong acid decolourisation, hence the milder Fite method — a genuinely Indian exam point. Finally, diastase logic trips candidates: it is PAS-diastase-resistant material (mucin, fungi) that survives digestion, while glycogen vanishes and proves itself glycogen.
Frequently asked questions
Which stain confirms amyloid in tissue?
Congo red, with confirmation by apple-green birefringence under polarised light — the pair FMGE quotes as a single answer.
What does Perls' Prussian blue demonstrate?
Haemosiderin iron, appearing as blue granules — used for marrow iron stores, transfusional overload and sites of old haemorrhage.
Which stains identify fungal elements?
Periodic acid-Schiff and Grocott methenamine silver stain fungal walls; mucicarmine additionally highlights the capsule of Cryptococcus in tissue.
Why does Mycobacterium leprae need the Fite-Faraco stain?
Because lepra bacilli lose acid-fastness with the harsh decolourisation of standard Ziehl-Neelsen; the milder Fite-Faraco method preserves their staining.
Which stain shows Leishmania parasites in a splenic or marrow smear?
Giemsa, demonstrating the amastigote LD bodies with their characteristic nucleus and kinetoplast inside macrophages.