80 Microscopic Histochemistry 



The specificity of the Feulgen nucleal reaction has been 

 the subject of a hvely controversy until very recently. The 

 following arguments have been voiced against it: 



1. There need be no chemical interaction between nucleic 

 acid and the reagent. The active dye may be fuchsin itself, 

 adsorbed by the nuclei, just as, e.g., alumina can adsorb 

 fuchsin from Schiff's reagent. The effect of hydrolysis may 

 consist simply in a dissolution of the cytoplasm while the 

 nucleus is relatively resistant; in this way the contrast be- 

 tween the nucleus and the cytoplasm is enhanced. The fact 

 that an excess of bisulfite (which should block aldehyde 

 groups) does not prevent staining of the nuclei shows that 

 it is not due to an aldehyde reaction.^^ 



2. Recolorization of SchifF's reagent may be due to non- 

 aldehydic substances; pyridine and various purine bases will 

 give color reactions more or less readily.^^ 



3. It is admitted that the dye produced by the interaction 

 of Schiff's reagent and hydrolyzed DNA is a true aldehyde 

 addition product; however, it is soluble in water and a good 

 stain for chromosomes,^^ especially for chromosomin and his- 

 tone.^^ Therefore, DNA only contributes to the dye which 

 then stains something else.^^- ^^ 



The weak points of these arguments can be easily shown: 



1. The specificity of the nuclear staining is not a matter of 

 increase in contrast; without hydrolysis nothing whatsoever 

 will stain in a properly performed Feulgen test. As for the 

 blocking of aldehyde groups by an excess of SO2, it must 

 be remembered that aldehyde-bisulfite addition compounds 

 (especially those of aldehydes of higher molecular weight) 

 are very unstable and prone to break down.^^ 



2. Recolorization of fuchsin by nonaldehydic compounds 



93. Carr, J. G.: Nature, 156:143, 1945. 



94. Semmens, C. S.: Nature, 146:130, 1940. 



95. Choudhuri, H. C: Nature, 152:475, 1943. 



96. Sibatani, A.: Nature, 166:355, 1950. 



97. Stedman, E., and Stedman, E.: Symp. Soc. Exper. Biol., 1:232, 1947. 



98. Dodson, E. O.: Stain Technol., 21:103, 1946. 



