The influence of inorganic salts when staining with preformed metal complex dyes

Summary The influence of various inorganic salts on the staining of tissue sections by a variety of preformed metal complex dyes (MCDs) has been studied. The staining patterns resulting were found to be extremely complex, though certain generalisations are possible.Usually the staining of basophilic tissue components (e. g. cartilage matrix and cell nuclei) was reduced as the amount of salt in the dyebath was increased. On the other hand the staining of some acidophilic tissue components (especially elastic fibres) was increased as the salt concentration rose. Both these effects can be rationalised by use of the Donnan equilibrium.The limited occurrence of salt-induced increases in staining intensity may be attributed in part to the extremely low rates of staining observed with acidophilic substrates.The decreases and increases in staining seen were to some extent dependant on the particular salt used, and presented an extremely complex picture. One general effect seen was that salts of Al³⁺, Cr³⁺, and Mg²⁺ had a greater inhibitory effect on staining than salts of K⁺ and Na⁺. This is explicable in terms of coordinative binding of the polyvalent ions by tissue substrates.On the basis of the information obtained comments were possible on various topics of biological staining interest. Thus it was concluded that of the MCDs studied only the aluminon-chromium complex showed evidence of binding by a mordanting mechanism. Further, it was found that the Critical Electrolyte Concentration theory was commonly inapplicable when staining tissue sections with MCDs.