Overview
- Special histochemical stains for connective tissue and structural components remain indispensable in anatomical pathology, complementing haematoxylin and eosin ($H\&E$) by selectively highlighting specific extracellular matrix constituents, fibre types, and abnormal deposits.
- Four stains, Masson's trichrome, Gordon-Sweet reticulin, Elastic van Gieson (EVG), and Congo red, exploit distinct chemical interactions to render diagnostically ambiguous structures interpretable.
- Mastery of their chemistry, chromogenic patterns, organ-specific applications, and interpretive pitfalls is essential at Fellowship level.
Masson's Trichrome
Chemistry and Mechanism
- Masson's trichrome is a polychromatic stain employing sequential dye application.
- Tissue is first treated with an iron haematoxylin (nuclei and myofibrils), followed by an acid fuchsin/Ponceau mixture (cytoplasm and muscle), and finally a small-molecule anionic dye, typically light green or aniline blue, for collagen.
- Differential staining depends on tissue density and porosity: compact fibres (collagen) absorb the smaller dye molecules preferentially, while denser structures (muscle, cytoplasm) retain larger fuchsin complexes after phosphomolybdic/phosphotungstic acid differentiation.
Staining Results
| Structure | Colour |
|---|---|
| Collagen / fibrous tissue | Blue or green (dye-dependent) |
| Smooth and cardiac muscle | Red/pink |
| Nuclei | Black |
| Cytoplasm / keratin | Red/pink |
| Fibrin | Red |
| Erythrocytes | Red |
Key Diagnostic Applications
- Liver fibrosis staging: Masson's trichrome is the standard stain for hepatic fibrosis assessment in chronic liver disease, enabling semi-quantitative staging by Metavir, Ishak, or Knodell schemes (periportal F1-F2, bridging F3, cirrhotic F4).
- Perisinusoidal (pericellular/"chicken-wire") fibrosis in NAFLD and alcoholic liver disease, portal tract expansion, and fibrous septa are clearly delineated.
- In early cirrhosis, thin fibrous bridges visible on trichrome may be underappreciated on $H\&E$.
- Sirius/picrosirius red is an alternative connective-tissue stain amenable to morphometric quantification of fibrosis, though reproducibility differs between methods.
- Masson's trichrome and collagen III immunostaining correlate better with eGFR in morphometric studies than Sirius red.
- Renal pathology: Interstitial fibrosis and tubular atrophy (IFTA) are scored in transplant biopsies using Banff ci/ct criteria.
- The trichrome distinguishes expanded interstitial collagen (stains blue/green) from oedema, oedema separates but does not colour collagen fibres.
- Glomerulosclerosis and periglomerular fibrosis are well demonstrated.
- Importantly, Congo red-negative eosinophilic interstitial expansion on trichrome represents fibrosis, while Congo red-positive material represents amyloid, a diagnostically critical distinction in renal biopsy interpretation.
- Cardiac pathology: Replacement fibrosis and interstitial fibrosis in cardiomyopathy are confirmed.
- In endomyocardial biopsy, Masson's trichrome distinguishes pathological fibrosis from biopsy-site artefact and confirms myocyte damage.
- It also highlights the internal elastic membranes of intussuscepted ("telescoped") small arteries, a common biopsy artefact that can mimic luminal occlusion.
Bone marrow: A trichrome variant grades collagen fibrosis in myeloproliferative neoplasms, particularly at MF-2 or higher where collagenisation (coarse fibres not dissolved by reticulin staining alone) must be confirmed.
Pleura and lung: Trichrome staining highlights the distribution and extent of fibrosis in fibrosing lung diseases and identifies collagen in the submesothelial layer of pleura and peritoneum (where the main submesothelial constituents are elastic fibres, collagen, and glycosylated proteins).
Gordon-Sweet Reticulin (Silver Impregnation)
Chemistry and Mechanism
- The Gordon-Sweet method is a silver impregnation technique.
- Sections are sensitised with iron alum, oxidised with potassium permanganate, then treated with ammoniacal silver solution.
- Reticulin fibres (collagen type III) reduce silver ions to metallic silver, rendering them black.
- Gold toning counterstains background yellow/gold.
- Collagen type I (the main component of dense, birefringent connective tissue fibres) is found predominantly in portal tracts and hepatic vein walls; collagen type III (reticulin) runs along sinusoids and is demonstrated by silver impregnation.
Staining Results
| Structure | Colour |
|---|---|
| Reticulin fibres (collagen III) | Black |
| Coarser collagen type I fibres | Variable brown/gold |
| Background | Yellow/gold |
| Nuclei | Black |
Key Diagnostic Applications
- Liver architecture and fibrosis: Reticulin staining is essential in liver biopsy assessment.
- The normal hepatic lobule shows a delicate pericellular reticulin network supporting single-cell-thick plates.
- In hepatocellular carcinoma (HCC), this framework is disrupted, trabeculae are thickened (≥2 cells wide) and sinusoidal reticulin is lost or compressed.
- Regenerative nodules in cirrhosis show preserved but compressed reticulin.
- In massive hepatic necrosis, reticulin condensation (collapse of the scaffold) indicates zonal necrosis and distinguishes collapse from active fibrogenesis; trichrome comparison is essential.
- Early sinusoidal collagenisation (space of Disse fibrosis) in alcoholic and metabolic liver disease may be identified before bridging fibrosis is apparent on trichrome.
- Lymphoma pattern analysis: Reticulin highlights nodal architecture.
- In follicular lymphoma, thickened reticulin encases follicles.
- In diffuse large B-cell lymphoma (DLBCL), reticulin is coarse and disorganised.
- In nodular sclerosis classical Hodgkin lymphoma, reticulin fibres outline lacunar-cell compartments.
- Reticulin patterns also assist in distinguishing primary hepatic lymphoma from metastatic carcinoma.
Bone marrow fibrosis (WHO 2022 grading):
| WHO Grade | Reticulin Pattern |
|---|---|
| MF-0 | Scattered individual fibres; no intersections |
| MF-1 | Loose network with intersections, especially perivascular |
| MF-2 | Diffuse dense network with intersections; occasional coarse collagen fibres |
| MF-3 | Diffuse dense reticulin with coarse collagen; focal osteosclerosis |
Grades MF-2 and MF-3 should be confirmed with a collagen stain (trichrome).
- Glomerular pathology: Reticulin delineates mesangial matrix and assists in distinguishing mesangial expansion (IgA nephropathy, diabetic nephropathy) from hypercellularity.
- Immunostains for collagen type IV and reticulin staining of the lung highlight reticulin networks useful in research but are not generally applied in routine diagnostic surgical pulmonary pathology.
Elastic van Gieson (EVG)
Chemistry and Mechanism
- EVG combines Verhoeff's elastic stain (iron haematoxylin with iodine and ferric chloride, selectively staining elastic fibres black via a mordant complex with elastin) with van Gieson counterstain (picric acid/acid fuchsin, colouring collagen red and muscle yellow).
- Some laboratories substitute Weigert's or Miller's elastic stain.
- The Verhoeff-van Gieson (VVG) designation is used interchangeably in some institutions.
- Elastic fibres in the liver follow the distribution of collagen type I, as demonstrated by orcein, resorcin, or Victoria blue stains; these alternatives are used for hepatitis B surface antigen (HBsAg) and copper-binding protein in addition to elastic fibres.
Staining Results
| Structure | Colour |
|---|---|
| Elastic fibres | Black/dark blue |
| Collagen | Red |
| Smooth / cardiac muscle | Yellow |
| Nuclei | Black |
| Erythrocytes | Yellow |
Key Diagnostic Applications
Vascular disease: EVG is the premier stain for vascular pathology. It delineates internal and external elastic laminae, enabling:
- Grading of arteriosclerosis and intimal fibrosis
- Detection of medial hypertrophy in pulmonary hypertension (Heath-Edwards grading)
- Identification of vasculitis with elastic lamina destruction
- Distinction of arteries from veins, veins have an incomplete or absent internal elastic lamina; in chronic cardiac failure, pulmonary veins may acquire an arterialized appearance with internal and external elastic laminae, requiring EVG at multiple levels to avoid misclassification
- Detection of vascular invasion by neoplasms (bowel, lung, vascular tumours)
- Assessment of transplant arteriopathy (intimal thickening with collagen red against elastic black)
- Identification of intussuscepted ("telescoped") small arteries in endomyocardial biopsy artefact, EVG highlights the internal elastic membranes of both vessel segments
- Pulmonary vascular disease: EVG identifies concentric laminar intimal fibrosis, plexiform lesions, and medial hypertrophy.
- In severe long-standing cardiac failure, medial hypertrophy, intimal fibrosis, and hyalinisation are prominent venous abnormalities; elastic lamellae are seldom complete in these abnormal veins, and medial fibrosis is proportionally greater than in corresponding arteries.
- Elastic staining is more informative than trichrome alone for pulmonary vascular assessment.
- Mesothelioma vs reactive mesothelial proliferation: In pleural pathology, EVG highlights the submesothelial elastic lamina (the elastic lamina forms a well-defined layer in pleura and peritoneum; beneath it, collagen fibres are arranged in parallel bundles separated by loose connective tissue rich in elastic fibres).
- Invasion through the elastic lamina by malignant mesothelioma cells is a key criterion distinguishing malignant invasion from reactive mesothelial hyperplasia, reactive cells proliferate above the elastic lamina, whereas malignant cells penetrate it.
- Lung pathology: EVG assists in identifying vascular invasion in lung tumours, highlights pleural elastica for T-staging (visceral pleural invasion), and defines bronchiolar architecture in obliterative bronchiolitis.
- Elastic stains can confirm the bronchiolar or alveolar duct origin of small scars in complete airway obliteration and are generally more informative than trichrome in lung surgical pathology.
Congo Red (Amyloid Detection)
Chemistry and Mechanism
- Congo red is a linear diazo dye that intercalates non-covalently into the parallel $\beta$-pleated sheet configuration common to all amyloid fibrils, regardless of precursor protein type.
- This ordered binding creates a parallel alignment of dye molecules that rotates polarised light, producing the pathognomonic apple-green birefringence.
- On routine light microscopy, amyloid stains salmon-pink/red.
Staining Results
| Condition | Standard Light | Polarised Light |
|---|---|---|
| Amyloid | Salmon-pink/red | Apple-green birefringence |
| Collagen | Non-specific pink | White/yellow birefringence |
| Normal tissue | Pale/negative | No apple-green birefringence |
Key Diagnostic Applications
Systemic amyloidosis: Congo red is mandatory for amyloid diagnosis in any organ biopsy where amyloid is suspected:
- Renal biopsy: mesangial deposits initially cause subtle mesangial matrix thickening with uneven widening of glomerular capillary basement membranes; interstitial peritubular, arterial, and arteriolar deposits occur in advanced disease
- Cardiac biopsy: interstitial amyloid between myocytes (particularly ATTR in elderly patients)
- Liver biopsy: perisinusoidal (along sinusoids and portal vessel walls) deposits staining salmon-pink with Congo red
- Fat pad aspirate: screening for systemic amyloidosis
- Rectal biopsy: submucosal vessel and lamina propria deposits
Congo red-negative eosinophilic interstitial expansion on trichrome represents fibrosis; Congo red-positive material represents amyloid, this distinction is fundamental in renal and cardiac biopsy interpretation.
- Typing of amyloid: Congo red positivity confirms amyloid but does not distinguish type.
- Subtyping requires immunohistochemistry (anti-SAA for AA; anti-kappa/lambda light chains for AL; anti-transthyretin for ATTR; anti-beta-2-microglobulin for dialysis-associated).
- Mass spectrometry-based proteomics on laser-captured tissue is the current gold standard for accurate subtyping.
- Thioflavin T/S fluorescence staining is a sensitive complementary method.
Diagnostic Pitfalls by Stain
| Stain | Pitfall | Comment |
|---|---|---|
| Masson's trichrome | Fibrin stains red, not blue | Can mimic muscle in areas of thrombosis or haemorrhage |
| Masson's trichrome | Oedema does not stain collagen | Interstitial oedema separates but does not colour fibres |
| Masson's trichrome | In end-stage kidneys, interstitial fibrotic areas can acquire elastic staining properties | Correlate with EVG and clinical context |
| Gordon-Sweet reticulin | Over-impregnation causes non-specific background silver blackening | Obscures architecture; positive and negative controls essential |
| Gordon-Sweet reticulin | Reticulin condensation (collapse) vs active fibrosis in hepatic necrosis | Trichrome comparison and clinical correlation required |
| Gordon-Sweet reticulin | Basement membranes (collagen IV) may also stain | Not specific to collagen III alone |
| EVG | Veins with acquired internal and external elastic laminae (arterialized veins) may mimic arteries | Examine multiple levels; correlate with vessel calibre and location |
| EVG | Pleural elastic lamina may be focally discontinuous in normal tissue | Fat invasion is a more reliable criterion for pleural invasion in some guidelines |
| Congo red | Polarisation shadow: at any given stage orientation, only a portion of amyloid shows green birefringence | Rotation of stage is mandatory; small deposits can be missed without full rotation |
| Congo red | Excess retained dye causes false birefringence | Adequate differentiation and washing steps are critical |
| Congo red | Pre-amyloidotic deposits (IHC-positive for precursor protein, Congo red-negative) | Electron microscopy required, deposits are non-fibrillar |
| Congo red | Collagen shows white/yellow birefringence, not apple-green | A critical distinction from amyloid |
| Congo red | Strong transmitted light source required; examination in a darkened room improves sensitivity | Sensitivity depends directly on light-source intensity and pupil accommodation |
Pre-Analytical and Analytical Variables
| Variable | Effect |
|---|---|
| Fixation (formalin, duration) | Over-fixation reduces dye uptake in trichrome; under-fixation causes diffuse silver impregnation in reticulin |
| Section thickness | Optimal $4{-}6\ \mu\text{m}$ for Congo red; thick sections increase background and false birefringence; thin sections reduce reticulin network visibility |
| Tissue processing / decalcification | Acid decalcifiers reduce collagen staining intensity in trichrome; bone marrow trephines require modified protocols for optimal IHC and special stains |
| Positive tissue controls | Liver capsule or cirrhotic liver (trichrome/reticulin); aorta or elastic artery (EVG); spleen or kidney with known amyloid (Congo red), mandatory each staining run |
| Polarising microscope | Sensitivity for amyloid birefringence depends on transmitted light intensity; darkened room strongly recommended |
Integrated Use in Diagnostic Panels
In practice, stains are used in combination:
| Specimen | Minimum Special Stain Panel |
|---|---|
| Liver biopsy (medical) | $H\&E$, Masson's trichrome, Gordon-Sweet |