Histochemical demonstration of metals and minerals in tissue sections identifies pathological accumulations that cause or indicate disease. These methods exploit specific chemical reactions between metal ions and organic reagents, producing colored insoluble precipitates at the site of metal deposition.
Iron: Perls’ Prussian Blue
Perls’ Prussian Blue is the standard histochemical stain for ferric iron (Fe3+). The reaction uses hydrochloric acid to release ferric ions from proteins (ferritin, hemosiderin) and potassium ferrocyanide to form ferric ferrocyanide — an insoluble, intensely blue pigment (Prussian blue). The intensity of blue staining reflects the amount of iron present.
Iron accumulation occurs in hereditary hemochromatosis (hepatocyte iron, predominantly periportal), secondary iron overload (transfusion siderosis, hemolytic anemias — iron in Kupffer cells and sinusoidal lining cells), alcoholic liver disease (mixed hepatocyte and Kupffer cell iron), and localized iron deposition (hemorrhage sites, infarcts, tumors). In bone marrow, Perls’ stain grades storage iron from 0 (absent) to 6 (massively increased). In lung, hemosiderin-laden macrophages (“heart failure cells”) indicate pulmonary congestion or hemorrhage.
Perls’ with DAB enhancement uses diaminobenzidine to amplify the Prussian blue signal, converting blue to dark brown and increasing sensitivity 10-100 times for detecting small quantities of iron.
Copper: Rhodanine and Timm’s Stains
Rhodanine stain (p-dimethylaminobenzylidene rhodanine) detects copper in tissue sections. Copper ions bind to rhodanine, forming a red-orange precipitate. The stain is most sensitive for large copper aggregates in lysosomes. Timm’s sulfide-silver method is more sensitive, detecting both copper and zinc by precipitating heavy metal sulfides and amplifying them with physical silver development.
Copper accumulation defines Wilson disease (hepatolenticular degeneration) — copper deposits in hepatocytes (periportal, associated with steatosis, inflammation, and fibrosis), in the brain (putamen, globus pallidus), and in Descemet’s membrane of the cornea (Kayser-Fleischer rings). Hepatic copper >250 µg/g dry weight is diagnostic. Rhodanine staining is positive in advanced Wilson disease but may be negative in early stages — quantitative copper measurement by atomic absorption spectroscopy is the gold standard.
Heavy Metals
Lead — tissue lead is demonstrated by the dithizone method, which forms a red lead-dithizone complex. Lead deposits in bone (as lead lines in growing bone), kidney (intranuclear inclusions in proximal tubular cells), and brain (cerebellar capillaries and neurons). Lead inclusions are also visible on H&E as eosinophilic intranuclear inclusions.
Mercury — deposits are demonstrated by the sulfide-silver method (autometallography). Mercury accumulates in kidney (proximal tubules), brain (cerebellar granule cells), and sensory ganglia. Autometallography can detect mercury at extremely low concentrations.
Silver — autometallography demonstrates silver deposits in tissues of individuals with silver toxicity (argyria). Silver accumulates in skin (perieccrine sweat glands, basement membranes), kidney (glomerular basement membranes), and cornea. In histology laboratories, silver stains are used for reticulin, fungi, and spirochetes; residual silver in processing equipment can cause art factual staining.
Aluminum
Aluminum is detected by the aluminon stain (aurine tricarboxylic acid), which forms a red lake with aluminum ions. Aluminum deposits in bone (at the mineralization front — associated with dialysis-related osteomalacia), in brain (neurofibrillary tangles in Alzheimer disease — association not proven), and in spleen and liver. Aluminum toxicity occurs in chronic renal failure patients on dialysis with aluminum-contaminated water.
Calcium and Phosphates
Calcium is demonstrated by Von Kossa (silver substitution, black) and Alizarin Red S (orange-red complex). These methods are described in detail in Amyloid, Pigment and Mineral Stains. Calcium phosphate is the most common mineral deposit in soft tissues; calcium oxalate requires polarized light (birefringent crystals) or Alizarin Red S.
Quality Control
All metal histochemistry requires positive controls: iron-overloaded liver (Perls’), Wilson disease liver (rhodanine), calcified artery (Von Kossa). Reagent purity is critical — trace metal contamination in reagents causes false-positive staining. Deionized water must be used for all solutions. Metal stains are sensitive to fixation — formalin is suitable for iron and calcium but prolonged fixation may leach copper. Consult quality assurance guidelines for validation protocols when implementing these stains.
