The Picro-Indigo stock solution is predominantly used in histological practice to make cell structures visible under the microscope. Its special suitability lies in its ability to highlight cell nuclei and clearly distinguish certain structures, especially connective tissue and muscle.

The stock solution is composed of saturated picric acid and indigocarmine. The picric acid acts as a fixative and helps to preserve the cell structures, while the indigocarmine acts as a dye and gives the cells a characteristic coloration.

Before use, the stock solution is diluted with aqua destillata. This dilution produces a working solution that is applied to the tissue to be stained. The picric acid in the solution fixes the tissue, while the indigocarmine stains specific cell structures.