On the nature of Romanowsky--Giemsa staining and its significance for cytochemistry and histochemistry: an overall view.

The chances of Romanowsky---Giemsa (RG) staining becoming a reliable and useful histochemical procedure are reviewed, based on the now proven fact that RG staining requires two dyes only, namely, cationic Azure B and anionic Eosin Y. These two dyes differ from otherwise similar dye combinations in that they give, on distinct biological substrates, one additional colour, purple, which cannot be obtained by the use of either dye alone. The purple colour characterizes the Romanowsky--Giemsa effect (RGE), which is the essential feature of RG staining. Consideration is given to the physico-chemical and morphological implications of RGE. Of primary importance is the nature of the biological substrates where RGE occurs, and also of those where it has never been observed. The way substrates react to RG stains largely depends on the kind of pretreatment they have received; for instance, alcoholic fixation preserves RGE but formaldehyde may inhibit it. Physico-chemical factors are considered which, by altering either the biological substrates or the composition of the staining solutions, may modify the RG staining pattern. This review also serves as an introduction for a series of experimental papers that will follow and which are intended to consolidate the basis of RG staining, a method which holds much promise as a useful histochemical tool.