The Prussian Blue reaction is a typical example of a histochemical method. It relies on the production of a colored end product at the site of the structure of interest. In this case, the intent is to localize iron (the result of the breakdown of hemoglobin) and the chemical reactions that take place in the tissue produce a blue colored iron salt in situ. The image above is from a an area of minor hemorrhage that's being clear out by macrophages. The blue material is hemosiderin, a breakdown product of hemoglobin (see Exercise 3). Since hemoglobin contains iron, the Prussian Blue reaction product is quite obviously present in the macrophages; in fact, little else can be seen.
If you dig through the literature on stains like this one, they're almost always adaptations of commercial fabric dyes to biological specimens. The flourishing coal-tar dye industry in mid-19th Century Europe (especially in Germany) generated vast numbers of reactions in the search for new colors with which to dye cloth, as an alternative to the ancient vegetable dyes. Probably at one time or another almost all commercial fabric dyes have been tried on tissues, and some have worked so well they've become standards.
As a fabric dye, Prussian Blue was used in the manufacture of uniforms for the Prussian Army. The French, still smarting from the drubbing they got in the Franco-Prussian War of 1870-71, fastidiously call it "Paris Blue," instead; but it's the same product. It's also used as a commercial pigment in typewriter ribbons and carbon paper.
Corpus hemorrhagicum, dromedary camel; Prussian blue stain, paraffin section, 200x
My sincerest thanks to Dr. Amal Arafat Moukthar of the Suez Canal University for this image!
| H&E | PAS | Masson's CT Stain | Verhoeff-van Gieson | Verhoeff-Masson | Mallory's CT Stain | Golgi Stain|
| Cresyl Violet | Cresyl Violet-Luxol Fast Blue | Kluver-Barrera | Fontana-Masson | Prussian Blue | Toluidine Blue|
|Osmium Tetroxide | Oil Red O | Sudan Black | Fluorescent & Enzymatic Tagging |
