CHAPTER ^ 



THE CRITICAL STUDY OF 

 THE CYTOCHEMISTRY OF ALDEHYDES 



Introduction 



Techniques will be considered which permit the study of the 

 following types of aldehydes and derivatives of aldehydes: 



1. Free aldehydes. Many tissues contain substances which have a very 

 reactive aldehyde group. This generally forms part of a fatty molecule. 



2. Another common component of tissues is aldehyde existing in the 

 form of acetal. 



3. Most sugars contain potential aldehyde groups of sugars which, how- 

 ever, are masked by ring formation. If at any time the ring opens, the 

 aldehyde group becomes reactive. 



4. In addition to the aldehyde groups just mentioned, which exist in- 

 trinsically in tissues, many recently developed cytochemical techniques 

 involve formation of aldehyde groups developed as a result of oxidation 

 procedures. 



The most successful reagent for studying these compounds is 

 reduced fuchsin, which was introduced by Feulgen, first for the 

 study of nucleic acid and later for the study of free fatty alde- 

 hydes. The chemical properties of the different groups of alde- 

 hydes mentioned above permit distinctions to be made between 

 them. Thus the reaction between reduced and free aldehydes 

 is rapid and may be complete in 15 minutes or less. The acetal 

 aldehydes can be split so as to release free aldehydes by a brief 

 treatment with 0.1 N hydrochloric acid. There is a great deal 

 of evidence that some of the tissue acetals can be split by the 

 action of mercury salts. But, as mercury may have other actions 

 besides that of splitting acetals, it is better to avoid mercury, and 

 to use hydrolysis with dilute hydrochloric acid as the diagnostic 

 procedure for acetals. Some glycosides may also be split by an 

 appropriate hydrolysis. In the case of the deoxy sugars the alde- 

 hyde group of a sugar liberated from the glycoside will react in 

 the cold with reduced fuchsin: This latter reaction is, of course, 



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