Observations on the contrasting reaction of some electron dense stains applied on epoxy-embedded tissue sections

Epon sections from glutaraldehyde-fixed rat bone marrow were treated with aqueous solutions of the following electron contrasting agents: uranyl acetate, ruthenium red, potassium permanganate, potassium dichromate, stannous chloride, palladium (II) chloride, sodium molybdate, phosphomolybdic acid, molybdenum heteropolyblue, phosphotungstic acid, iron(II)-phenanthroline, aluminium-hematoxylin, mercurochrome, cuprolinic blue, and sirius light turquoise blue. At the ultrastructural level, a high degree of electron opacity was always observed in mast cell granules and the crystalline inclusion (internum) of eosinophil granules. The chromatin revealed a somewhat lower and variable contrasting reaction, while the matrix (externum) of eosinophil granules appeared with scarce or no contrast. This pattern of electron opacity showed no correlation with the type of agent used; therefore, it can be assumed that binding processes based on the own chemical reactivity of the compounds are rather of secondary importance. The differential epoxy resin embedding of cell structures and the variable access of aqueous reagents through the non-polar plastic could be the predominant factors which account for these contrasting reactions.