The A to Z of Anatomical Histological and Medical terms
Perl’s iron stain is the classic method for demonstrating iron in
tissues. The section is treated with dilute hydrochloric acid to release
ferric ions from binding proteins. These ions then react with
potassium ferrocyanide to produce an insoluble blue compound (the
Prussian blue reaction).
Lipochrome (lipofuschin) pigments - Age spots
These are the breakdown products within cells from oxidation of
lipids and lipoproteins. They are the wear-and-tear pigments found
most commonly in heart, liver, CNS, and adrenal cortex (zona
reticularis). The less highly oxidized “ceroid” pigment of testis
interstitium and seminal vesicle is another form of lipochrome.
Lipochrome can be stained by Sudan black B, long Ziehl-Neelson
acid fast, and Schmorl’s methods. Lipochrome may also exhibit a
strong orange autofluorescence in formalin-fixed, unstained paraffin
sections.
Melanin stains
The Fontana-Masson stain, relies upon the melanin granules to
reduce silver nitrate (note, argentaffin, chromaffin, and some
lipochrome pigments also will stain black as well). Schmorl’s
method uses the reducing properties of melanin to stain granules
blue-green, but the most specific method is an enzyme
histochemical method, DOPA-oxidase. DOPA substrate is acted upon
by DOPA-oxidase in the melanin-producing cells to produce a
brownish black deposit.
Bleaching techniques remove melanin in order to get a good look at
cellular morphology. They make use of a strong oxidizing agent such
as potassium permanganate or hydrogen peroxide. Ocular melanin
takes hours to bleach, while skin melanin takes minutes.
Formaldehyde-induced fluorescence can be used to highlight
biogenic amines (chromaffin, argentaffin) and melanin in tissues.
Formalin fixation imparts a strong yellow autofluorescence to
unstained tissues with these substances.
Pseudomelanin of melanosis coli is PAS positive whereas true
melanin is not. Pseudomelanin pigment is also found in
macrophages.
Microorganisms - bacteria stains
Gram stain gram positive organisms stain purple and gram negative
organisms stain red poorly. In H&E all bacteria appear as blue rods or
cocci regardless of gram reaction. Colonies appear as fuzzy blue
© A. L. Neill 14
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