8Q CYTOCHEMISTRY OF ALDEHYDES 



TABLE VI 



Distinction between free aldehyde, acetal aldehyde, and aldehyde lib- 

 erated by oxidation. The numbers indicate the order in which the steps 

 should be carried out to obtain a reaction for one only of the three types 

 of aldehyde. 



Free Acetal Oxidation 



Aldehyde Aldehyde Aldehyde 



Cold 0.1 iVHCl .. 2 1 



Oximation . . 1 2 



Oxidation . . . . 3 



Reduced fuchsin 13 4 



To demonstrate free aldehyde only, the only procedure which 

 is necessary after cutting frozen sections is to expose the sections 

 to reduced fuchsin. This exposure should be of as short a dura- 

 tion as possible, and care must be taken that the fuchsin solution 

 is not too acid in reaction. It frequently happens that part of the 

 sulfur dioxide present in reduced fuchsin solution becomes oxi- 

 dised to sulfuric acid, thus making the solution much more acid 

 than it should be. It may be that, when free aldehyde alone is 

 to be demonstrated, it is best to use a buffered solution of re- 

 duced fuchsin. However, I have not investigated this point. 



To demonstrate acetal aldehyde only, the first step is to 

 block up the free aldehyde with either hydroxylamine or dime- 

 done. When this has been done, aldehyde is liberated from ace- 

 tal by treatment with cold 0.1 N hydrochloric acid, and the alde- 

 hyde so liberated is coloured by treatment with reduced fuchsin. 



To demonstrate aldehyde formed by oxidation procedures, it is 

 first desirable to eliminate both free aldehyde and acetal alde- 

 hyde. The procedure in such a case is first to expose to cold 

 0.1 N hydrochloric acid, to liberate aldehyde from acetal. Then 

 all the liberated and free aldehyde is blocked by treatment with 

 hydroxylamine or dimedone. After this, the oxidation procedure 

 may be carried through and then the aldehyde formed by oxida- 

 tion may be demonstrated by treatment with reduced fuchsin. 



Many of the papers on the cytochemical demonstration of alde- 

 hydes are extremely difficult to interpret, owing to the failure of 

 the authors to distinguish clearly between free aldehyde, acetal 

 aldehyde, and aldehyde formed by oxidation, both adventitious 

 and intentional. Part of the difficulty has been, as Feulgen and 



