82 CYTOCHEMISTRY OF ALDEHYDES 



chloracetic acid, either before or after preprecipitation of the 

 tissue with saturated calcium chloride, magnesium sulfate, or 

 ammonium sulfate, no variation in the distribution of aldehyde 

 was found except with those fixatives containing trichloracetic 

 acid. With the trichloracetic fixatives, after treatment with re- 

 duced fuchsin, some degree of nuclear staining was found which 

 was not encountered with the other fixatives. It seems likely that 

 this staining is caused by hydrolysis of the nuclear thymonucleic 

 acid, since trichloracetic acid is a fairly strong acid. Apart from 

 the nuclear staining, the aldehyde contained in the liver after 

 treatment with these different fixatives is always in the same 

 position, and it can always be removed by extraction with ace- 

 tone. It therefore seems justifiable to conclude that this alde- 

 hyde is not shifted from its physiological position by the action 

 of the fixative, with the reservation that strongly acid fixatives, 

 such as those containing trichloracetic acid, must be avoided. 



Specificity of the Detection of Aldehydes 



Of the naturally occurring compounds, there is no evidence 

 that any other than aldehydes will react with reduced fuchsin to 

 give a purple colour. Some unsaturated substances have been 

 alleged to react, but the reaction appears to be due to the pres- 

 ence of aldehydes or other products of oxidation, and not to the 

 original substance. This is the case with oleic acid. A few sub- 

 stances, such as those formed by atmospheric oxidation of sub- 

 stances containing double bonds, may perhaps be capable of 

 reacting with reduced fuchsin to give colours similar to those 

 given by aldehydes. But there is no evidence that such sub- 

 stances normally occur in tissues in an amount which would be 

 detected in a cytochemical study. However, there always exists 

 the possibility that some unusual substance may be encountered 

 or that a reaction with reduced fuchsin may occur as a result of 

 the presence of enzyme systems. Consequently, a number of sup- 

 plementary tests which can be used to check the nature of the 

 reacting substance are desirable. A number of reactions have 

 been tried for this purpose. They include the following pro- 

 cedures. 



1. A useful distinction can be made between substances which are sol- 

 uble in fat solvents and those which are not. For example, it is important 

 in the study of fat metabolism to be able to distinguish between the long- 



