The term lysochrome is the technical name for what are more plainly called fat stains, dyes such as sudan III, oil red O, and sudan black. Most of these are azo dyes which because of their structure have undergone molecular rearrangement making them incapable of ionising.
The basis for these dyes colouring fats is that they dissolve into it. From another perspective, the fat is a solvent for the dye. The lyso part of lysochrome has the meaning of solution, and the chrome part refers to colour. The word therefore means dissolve colourer.
Lysochromes are mostly insoluble in strongly polar solvents, such as water, and somewhat more so in less polar solvents, such as ethanol. They are quite strongly soluble in non polar solvents, such as xylene. Triglycerides, being non polar compounds, dissolve them quite well. Other lipids, having fatty components, may also dissolve them. They may be used to colour compound lipids like lipofuscins and cerebrosides.
They are applied from solvents in which they are sparingly soluble, or from colloidal mixtures. As they come in contact with materials in which they are strongly soluble, they transfer to them significantly, often colouring them more strongly than the original solvent, a process called preferential solubility.
The common application media are:
- 70% ethanol
- Herxheimer’s fluid (equal parts 70% ethanol and acetone)
- Polyethylene glycol
- Aqueous gelatin solutions
The first three solvents are suitable for free floating sections. The last two are water miscible, and are valuable as they permit the use of cryostat sections which are already attached to slides or paraffin sections, without the likelihood of getting large amounts of dye precipitate all over the tissue.
Since lysochromes are soluble in non polar solvents, including xylene and toluene, and also in ethanol, these fluids must not be put on to sections after fats have been stained or the dye will be dissolved out. This means that resinous mounting media can’t be used. Instead, aqueous mounting media are required.
Although lysochromes stain lipids by dissolving in them, at least one lysochrome (sudan black B) can also stain materials ionically. In this case some background staining is to be expected, and interpretation of sections stained with it must take this into account.