Light and electron microscopical demonstration of methylene blue accumulation sites in taste buds of fish and mouse after supravital dye injection.

Electron microscopical data regarding methylene blue staining of taste buds in the epithelia of the goldfish lip and the cirumvallate papilla of the mouse tongue after supravital dye application are presented for the first time. The ultrastructural details were compared with the corresponding light microscopical findings. The dye was applied in different concentrations by injection or in crystalline from directly to the surface of the tissues. Both methylene blue and tissue were simultaneously fixed by immersion in a paraformaldehyde-glutaraldehyde solution with the addition of phosphomolybdic acid. The ensuing dye precipitate was further stabilized by ammonium heptamolybdate. On the light microscopical level, the taste bud's receptive structures, i.e. the receptor area (fish) and the taste pit (mouse), exhibited the highest affinity for the dye. Additionally, the mucous material within the trenches around the circumvallate papillae in mice was intensely stained. On the electron microscopical level, the cationic phenothiazine dye bound to the receptor villi or to the mucus coating the receptive structures. In the case of higher dye concentrations, a staining of single taste bud cells took place starting apically and proceeding down to the base. Dye accumulations within the intercellular clefts between the epithelial cells or within other structures were observed only if the dye concentration was further increased. Since similar results were also obtained with the cationic phenazo dye Janus green, dye accumulation in the mucus covering the receptor villi may be representative of the general binding of organic cations, which are known to induce bitter taste sensations.