The Role of Iron in Histological Staining

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Abstract
Rapid methods for staining sections with iron sulphide are described which give results comparable with iron haematoxylin. Nucleic acids and proteins are mainly responsible for the uptake of iron. The staining capacity of different proteins runs parallel with the number of free carboxyl groups they contain. It is concluded that the iron forms non-ionizing complexes with carboxyl and phosphoric acid groups. The intensity of staining is a measure of the relative abundance of such groups in the proteins and nucleic acids present, and of the concentration density of these substances. In the staining of nuclei and chromosomes with iron the proteins are at least as important as the nucleic acids. The iron taken up is mostly in the ferric state but it is partially converted to ferrous iron by reducing substances (chiefly sulphydryl groups) present in many fixed tissues. Iron in the form of iron sulphide or undifferentiated iron haematoxylin therefore acts as a metachromatic stain: a blue colour indicates ferrous iron, a brown colour ferric iron.

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