Theory of Staining 51 



When the first edition of this book appeared, in 1925, frequent 

 papers on the theory of staining were being written by biologists. 

 Today, although much valuable work on the mechanism of stain- 

 ing is being done, it is ordinarily recognized that the findings 

 relate only to specific cases, and arguments about the general 

 theory of staining, as then understood, have almost disappeared 

 from the literature. This is partly perhaps because as we know 

 more we theorize less ; but other factors seem to be more important 

 in contributing to the lack of interest. Chief among them is the 

 realization of the great complexity of staining processes, and of the 

 necessity of both chemical and physical theories to explain them — 

 a complexity so great that, until we possess many more facts than 

 at present, our theories are little better than guesses. 



A good illustration of the modern way of treating the subject is 

 the recent review paper by Singer (1952). In this paper the author 

 mentions the various theories without showing special favor to any 

 one of them, but gives his attention to the various controllable 

 factors which influence staining of tissues. He points out that the 

 extensive literature of the early part of this century on the theory 

 of staining has brought out many facts of value relating to the 

 interaction of dyes and tissue proteins, and that this interaction 

 deserves further study, without theorizing as to whether it is 

 chemical or physical. His paper is an excellent review of the in- 

 formation on this subject now available. 



Histochemistry. It is interesting that now, when a purely chem- 

 ical theory of staining is not generally accepted, specific instances 

 of what seem to be definite chemical reactions between dyes (or 

 related compounds) and tissue elements are becoming more numer- 

 ous. The new science of histochemistry depends on the fact that 

 some dyes, or uncolored chemicals which become colored under 

 certain definite conditions, can be used as reagents for giving in- 

 formation as to the chemistry of cell constituents. Although the 

 chemical nature of the observed reactions seems quite evident, 

 such findings have little bearing on the general theory of staining. 

 Ordinary staining follows a comparatively leisurely process of 

 fixation, during which degenerative changes of the tissue can take 

 place; while the histochemical procedure generally involves quick 

 freezing or instantaneous introduction into fixing reagents, or 

 both, to avoid any appreciable change in the nature of the constit- 

 uents of the tissue. It is clear that the histochemical reagents, 

 therefore, are given quite a different substance to act on from that 

 furnished the ordinary histological stains. Furthermore, the 

 histochemical procedures all employ specific reactions which have 

 little connection with the usual staining methods. An authorita- 

 tive text on the subject is the book by Gomori (1952). 



It is true that, as long ago as the mid-twenties, the chemical 

 theory of staining was leading specialists in the field to speculate 



