78 HEWSON SWIFT 



IV. Other Cytochemical Reactions 



In addition to the Feulgen reaction, one other compound, which con- 

 denses with the sugar of nucleic acid, has been used as a cytochemical tech- 

 nique. This is 9-phenyl-2,3,7-trihydroxy-6-fluorone (or -xanthone), em- 

 ployed by Turchini and collaborators'^"-^- and also by Backler and 

 Alexander. '23 The compound when combined with PNA is yellow to pink 

 and with DNA, blue to purple. It also combines with other sugars to give 

 different shades. Turchini employed fixatives containing dichromate so 

 that PNA will remain in the section during the partial hydrolysis. Backler 

 and Alexander obtained good results with Zenker's, Bouin's, formalin, and 

 frozen-dried sections, that compared favorably with gallocyanine-chrome- 

 alum staining. 



A method for localizing nucleic acid purines and pyrimidines through azo 

 coupling has been proposed by MitchelP^^ and DanieUi.'"-i27 j^ this reaction 

 coupling with tyrosine, tryptophan, and histidine is blocked by benzoyla- 

 tion, leaving purines and pyrimidines to react with tetrazotized benzidine. 

 This compound, after coupling, can then be made an intense purple by 

 further coupling with naphthol. The specificity of the reaction apparently 

 has not been tested with nucleases. Gomori^" has strongly criticized this 

 method, stating that no coupling to bases can be expected to take place at 

 all under the conditions suggested, and that diazonium decomposition 

 products may be strongly and nonspecifically adsorbed to tissues. Clearly 

 more study of this method is needed before it can be used with any confi- 

 dence. 



V. Extraction Techniques 



In many cytochemical studies it is desirable to remove one or both 

 nucleic acids from tissue sections. Ultraviolet absorption does not distin- 

 guish between the types of nucleic acids. If it is to be made specific for DNA 

 or PNA in nuclei, these must be selectively removed. Also, evidence for the 

 specificity of cytochemical reactions is obviously gained if "blank" slides 

 from which the component studied has been extracted can be used for 

 comparison. 



20 J. Turchini, P. Castel, and K. Kien, Trav. soc. chim. biol. 35, 1329 (1943). 



^' J. Turchini, P. Castel, and K. Kien, Bull, histol. appl. 21, 124 (1944). 



22 J. Turchini, Expil. Cell Research Suppl. 1, 105 (1949). 



" B. S. Backler and W. F. Alexander, Stain Technol 27, 147 (1952). 



"^ J. S. Mitchell, Brit. J. Ezptl. Pathol. 23, 296 (1942). 



" J. F. Danielli, Symposia Soc. Exptl. Biol. 1, 101 (1947). 



" J. F. Danielli, Cold Spring Harbor Sijmposia Quant. Biol. 14, 32 (1951). 



" J. F. Danielli, "Cytochemistry, A Critical Approach." Wiley, New York, 1953. 



