PHENOLS, UREA, AND SULFONAMIDES 75 



PROCEDURE 



1. Fix tissue in one of the solns. indicated. The iodate gives a 

 less intense reaction but is less prone to pseudoreactions. 



2. Prepare sections and treat them with the 3% reagent for a 

 few hours. 



Result. A brownish color indicates a positive reaction. 



UREA 



Two methods have been proposed for the localization of urea in 

 tissue sections. The Leschke procedure is based on fixation of the 

 tissue in a half-saturated mercuric nitrate solution in 1% nitric 

 acid and subsequent treatment of the sections with a saturated 

 hydrogen sulfide solution. The mercury urea compound is converted 

 to black mercuric sulfide, which is easily visualized. The xanthydrol 

 method depends on fixation of the tissue in a solution of xanthydrol 

 in acetic acid in order to precipitate dixanthylurea which can be 

 recognized in sections by its double refraction when examined under 

 a polarizing microscope. Lison ( 1936, pages 165-170) critically dis- 

 cussed these methods. As he pointed out, the usefulness of the mer- 

 cury reaction is entirely vitiated by its extreme lack of specificity, 

 far too many tissue constituents being capable of precipitation by 

 mercury salts. The xanthydrol reaction is chemically specific, but 

 its serious fault lies in a combination of unfortunate factors including 

 the great diffusibility of urea, the poor penetrability of xanthydrol, 

 and the slowness of the reaction between the two. The result is that 

 the position of the crystals formed bears little or no relation to the 

 regions originally containing urea. A suitable method for the his- 

 tological localization of urea is not available at present. 



SULFONAMIDES 



^lacKee et al. ( 1943) described a test for sulfonamides in frozen 

 tissue sections depending on the formation of a yellow to orange 

 precipitate of the p-dimethylaminobenzylidene derivative when sul- 

 fonamides react with p-dimethylaminobenzaldehyde. It should be 

 borne in mind that procaine, phenacetin, acetanilid, and aromatic 

 amino compounds in general will also give the reaction. 



