15 



Organic Substances 57 



aldehyde, cling very tenaciously to the structures stained. 

 Schiff's reagent can be recolorized also by a different mecha- 

 nism and with different end-products. The reagent is stable 

 only in the presence of an excess of SO2 and at a high acid- 

 ity. Anything that removes or oxidizes SO2 or reduces acidity 

 will break the linkage between fuchsin and sulfurous acid 

 and restore the original dye. This type of recolorization, not 

 likely to take place under the conditions of a correctly per 

 formed histochemical test, is called a "pseudo-reaction. 

 It should not be confused with the genuine one, which is 

 practically specific for aldehydes. Lison lists a few nonalde- 

 hydic substances which may give colorations very similar 

 to those occuning with aldehydes; however, they are almost 

 all nonphysiological substances, not found in animal tissues. 

 The red shade of the pseudo-reaction is readily extracted by 

 acids and alcohol ( except in the case of acid-fast structures ) , 

 while the aldehyde-regenerated dye is extremely resistant 

 to both agents. 



It must be mentioned here that not only Schiff's reagent 

 but also undecolorized fuchsin is able to combine with alde- 

 hydes and yield purplish dyes which have staining properties 

 different from those of fuchsin itself. Such dyes are used in 

 Fite's modification^^ of the Ziehl-Neelsen stain, for the stain- 

 ing of nuclei (DeLamater)^^ and in the aldehyde-fuchsin 

 technique^^ for elastic fibers. In periodate-treated tissues a 

 fairly good localized staining of the aldehydes may be ob- 

 tained by fuchsin proper (Arzac);^^ however, the use of 

 Schiff's reagent is always preferable. 



Another good analytical reagent for aldehydes is an alka- 

 line solution of silver nitrate, first utilized for histochemical 



15. Lison, L.: Bull, d'histol. appliq. a la physiol., 9:177, 1932. 



16. Fite, G. L.: J. Lab. & Clin. Med., 25:743, 1940. 



17. DeLamater, E. D.: Stain Technol., 23:161, 1948. 



18. Gomori, G.: Am. J. Clin. Path., 20:665, 1950. 



19. Arzac, J. P.: Analecta Med., 9:15, 1948; Arzac, J. P.: Stain Technol., 

 25:187, 1950; Arzac, J. P.: J. Nat. Cancer Inst, 10:1341, 1950. 



