CHAPTER I 



THE NATURE OF THE PROCESSES 



OF IDENTIFICATION IN 



HISTOCHEMISTRY 



Most of the methods used in histochemistry have been bor- 

 rowed from other fields of chemistry and partly modified for 

 the purpose of apphcation to histological preparations. The 

 methods can be divided into several classes, according to 

 the nature of the procedures and phenomena utilized. 



1. Chemical— Some of the reactions of this class are the 

 same as those used in analytical chemistry or biochemistry 

 (for Fe+ ++, Cl~, enzymes, etc.). However, modifications of 

 the original technique are often necessary; e.g., determina- 

 tion of melting point cannot be used in histochemistry; color- 

 less or only slightly colored precipitates are transformed, 

 whenever possible, into intensely colored ones for better 

 visibility. 



2. Semichemical—Re2iCtions in this class are more or less 

 specific for certain chemically definable substances, but the 

 nature of the reaction is poorly understood (Best's carmine 

 for glycogen; mucicarmine for mucin ) . 



Mere staining with dyes of acid or basic character, al- 

 though it does reveal something about the acid- or base-com- 

 bining properties of the substance stained (basic dyes for 

 nucleic acids; acid dyes for globins and histones), cannot 

 be called a histochemical method in any true sense of the 

 word. 



3. Physical— This class can be subdivided into several sub- 

 classes. 



a) Staining of fat —This is a purely physical phenomenon 

 of solubility in oil, without any chemical reactions taking 

 place. 



