58 Microscopic Histochemistry 



purposes by BignardP^ and Clara^^ and later described inde- 

 pendently also by Gomori,^^ who was not then familiar with 

 Bignardi's work. Two more silver techniques for glycogen 

 and mucin were published by Mitchell and Wislocki^^ and 

 by Pritchard;^^ however, these latter are silver-impregnation 

 methods rather than histochemical tests. 



At this point the difference between the argentaffin reac- 

 tion and simple impregnation by silver ( argyrophilia ) must 

 be pointed out. Lison^^ has emphasized that some substances 

 ( ascorbic acid, polyphenols, aldehydes, uric acid, etc. ) have 

 the ability to reduce silver solutions under specific condi- 

 tions. This is the "argentaffin reaction" which may be used 

 for the histochemical characterization of reducing substances. 

 It should be distinguished sharply from impregnation by 

 silver such as is obtained in various techniques for nerve and 

 reticulum fibers, etc. In such techniques the silver is not re- 

 duced by substances contained in the tissue itself but by 

 extraneous reducers (formalin, hydroquinone, etc.). Some 

 of these methods are remarkably selective for various mor- 

 phological structures but cannot be called "specific" in the 

 chemical sense of the word. 



The silver technique for aldehydes can be used for glyco- 

 gen and mucin-like substances only, not for DNA. The reason 

 for this is not clear; it seems that acid hydrolysis makes DNA 

 somewhat alkali-soluble, and it will diffuse out of the section 

 before reduction of the silver can take place. 



PREPARA.TION OF THE ReAGENTS 



1. Periodic acid— A 0.25-0.5 per cent solution in distilled 

 water will serve all purposes. The use of alcoholic or buffered 



20. Bignardi, C: Atti Soc. nat. e mat. Modena, 70:97, 1939; Bignardi, 

 C: Boll. Soc. med.-chir. Pavia, 54:799, 1940; Bignardi, C: Atti Soc. ital. sc. 

 nat, 79:23, 1940. 



21. Clara, M.: Ztschr. f. mikr.-anat. Forsch., 47:183, 1940. 



22. Gomori, G.: Am. J. Clin. Path., 10:177, 1946. 



23. Mitchell, A. J., and Wislocki, G. B.: Anat. Rec, 90:261, 1944. 



24. Pritchard, J. J.: J. Anat, 83:30, 1949. 



25. Lison. L.: Histochimie animale (Paris: Gauthier-Villars, 1936). 



