Feulgen stain is astaining technique discovered byRobert Feulgen and used inhistology to identifychromosomal material orDNA in cell specimens. It is darkly stained. It depends on acidhydrolysis of DNA, therefore fixating agents usingstrong acids should be avoided.
The specimen is subjected to warm (60 °C)hydrochloric acid, then toSchiff reagent. In the past, asulfite rinse followed, but this is now considered unnecessary. Optionally, the sample can becounterstained withLight Green SF yellowish. Finally, it is dehydrated withethanol, cleared withxylene, and mounted in aresinous medium.
DNA should be stained red. The background, if counterstained, is green.
The Feulgen reaction is a semi-quantitative technique. If the only aldehydes remaining in the cell are those produced from thehydrolysis of DNA, then the technique is quantitative for DNA. It is possible to use an instrument known as amicrodensitometer or microspectrophotometer to actually measure the intensity of the pink Feulgen reaction for a givenorganelle. Using this procedure, it was early determined that interphase cells were composed of two populations, those withdiploid DNA and those withtetraploid DNA (two complete genomes). The nuclei looked identical, but one contained twice as much DNA. This gave rise to the division of the interphase period of thecell cycle to G1, S, and G2 phases based on the synthesis of that extra DNA.[1]