Fluorescence reactions of orcein: new applications for an old stain

The fluorescence properties of orcein are described in this paper. Under suitable exciting light, acidic solutions of orcein showed emission peaks at 587 nm (low concentration) and at 620 nm (high concentration). In fluorescence microscopy an intense orange-red emission was found in the cytoplasm and nucleoli from acetic orcein squashes of meristematic cells, as well as in the cytoplasm of Ehrlich tumour cells and in chicken erythrocyte nuclei. When semithin sections of Epon-embedded tissues were treated with orcein, a strong fluorescence reaction (orange-red) appeared in starch granules, cell walls, elastin, collagen, reticulin, keratohyalin, chromatin and mucin. Cytofluorometric measurements of orcein-stained chromatin revealed an emission peak at 585 nm with a shoulder at 620 nm. These spectroscopic and microscopical results suggest the possibility of employing orcein as a fluorochrome.