STAINING, PRACTICAL AND THEORETICAL 



especially useful in determining the acidity or basicity of tissue 

 elements stained by it: the method we used is described on 

 page 422. 



DYES AS DIFFERENTIATORS AND DECOLORIZERS 



It has long been known to histologists that a few dyes when 

 applied to tissue-elements previously stained with some other 

 dye, will remove the first stain and leave the tissue-elements 

 colourless, if the process is stopped at this stage. If the process 

 is allowed to continue, however, the second dye will then stain 

 the tissues; that is, by replacement of the first dye. We can see 

 an example of this, in the standard Faviol method, which is 

 particularly vigorous when the protective acid bath is omitted. 

 In this case an aqueous solution of the acid dye, sun yellow G. 

 acts both as a differentiator and as a decolorizer. It acts on the 

 tissues by removing the dye which has already stained them. It 

 also acts on the displaced dye by decolorizing it. 



Lee (19 13) mentions the use of aurantia and of picric acid in 

 alcoholic solutions as differentiators of certain other dyes, but 

 this fact was well known before that time. Both aurantia and 

 picric acid are acid dyes, but unlike sun yellow G, which is a 

 sulphonated stilbene dye, these two are non-sulphonated and 

 belong to the nitro group. It appears also that they are used 

 in alcoholic solution and for the differentiation of basic dyes. 

 They decolorize the tissue-elements, if the differentiation is al- 

 lowed to proceed long enough, then stopped at that stage. If 

 their action is allowed to continue they stain the tissues after 

 displacing the basic dye. This applies to synthetic basic dyes 

 used without a mordant. It also applies to certain acid dyes 

 such as eosin and phloxine possessing carboxyl (acid) groups, but 

 devoid of sulphonic groups. Tartrazine in cellosolve has been 

 used (Lendrum, 1947) as a contrast stain to, and for differentiating 

 phloxine, as described on page 368. Tartrazine contains two 

 sulphonic groups attached directly to the aromatic nucleus and 

 one carboxyl group attached to a side chain; that is, not directly 

 attached to an aromatic nucleus. The structure of this dye is 

 given by Gurr (i960). In this case the dye removed (phloxine) is 

 not decolorized by the dye (tartrazine) used to displace it. The 

 reason for this is that neither dye possesses groups which would 



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