Alkanet (Alkanna tinctoria) is a plant native to and cultivated in parts of Europe, Asia and the Middle East. It has been used for thousands of years as a medicinal agent and as a colorant for textiles, food and cosmetics. An extract from the root of this plant has been used with a mordant to stain nuclei. We describe here the versatility of different extracts from this plant to stain lipids red and to counterstain certain other tissue elements blue. The color variation and selective differential staining is due to solvent polarity and pH. Extracts contain numerous chemical species, all of which are derivatives of the indicator dye, naphthazurin. Our red extract is nonionic below pH 7 and partitions from its somewhat polar solvent of 100% isopropanol to nonpolar lipids. The blue extract is dianionic at high pH in 70% isopropanol and binds ionically to cationic tissue structures such as collagen, muscle and cytoplasm of other cells.
Keywords: Alkanet; Alkanna tinctoria; counterstain; histology; lipid stain; naphthazurin; natural dyes.