56 Microscopic Histochemistry 



Feulgen and Rossenbeck^^ in 1924 discovered that thymo- 

 nucleic acid yields on hydrolysis with dilute mineral acids 

 a substance which gives an intense aldehyde reaction, repro- 

 ducible both in the test tube and in microscopic sections 

 ("nucleal reaction"). The mechanism of this reaction has 

 been considered a mystery until very recently. According to 

 the latest reports of Overend and Stacey,^^ a large percentage 

 of desoxyribose and a few other carbohydrates exist naturally 

 in a noncyclic form which gives direct aldehyde tests (due 

 to a terminal -CH=0 group). This feature distinguishes 

 them sharply from the more common hexoses and pentoses 

 which have no overt aldehydic character. By controlled acid 

 hydrolysis it is possible to break just a sufficient number of 

 glycosidic linkages to expose enough aldehyde groups for 

 visualization of the reaction and at the same time retain 

 much of the polymerism and insolubility of the acid on which 

 good histochemical localization depends. Ribonucleic acid 

 gives no reaction under such conditions. 



The free aldehyde groups can be demonstrated by a num- 

 ber of specific aldehyde reagents. Theoretically, any alde- 

 hyde reaction used in organic analysis could be employed; 

 in practice, however, only a few have the desirable features 

 of a histochemical test. 



The most important reagent for aldehydes is Schiff's re- 

 agent, fuchsinsulfurous acid. 



The mechanism of the reaction has been clarified by Wie- 

 land and Scheuing.^* In an acid solution, fuchsin is trans- 

 formed by an excess of SO2 into a colorless N-sulfinic acid 

 which forms highly colored addition complexes with alde- 

 hydes. The addition complexes, of a purple shade different 

 from that of fuchsin itself, have an excellent staining power 

 and, being usually more insoluble than the corresponding 



12. Feulgen, R., and Rossenbeck, H.: Ztschr. f. physiol. Chem., 135:203, 

 1924. 



13. Overend, W. G., and Stacey, M.: Nature, 163:538, 1949. 



14. Wieland, H., and Scheuing, G.: Ber. deutsch. chem. Gesellsch., B, 

 54:2527, 1921. 



