Uses and Standardization of Stains 13 



A third important group of methods in animal microtechnic in- 

 cludes those for neurological staining. These methods are very 

 numerous and all highly specialized. Many of them (as is true of 

 some methods for demonstrating connective tissue) are not actually 

 dyeing procedures, but depend on the use of silver salts with which 

 tissue elements are impregnated so that the desired structures are 

 made visible when the salts are converted into metallic silver. As 

 these silver methods do not involve dyes, no attention is given to 

 them in the following pages. (They are, however, described in 

 Staining Procedures,* leaflets I-B and I-C.) This is not to say, 

 however, that dyes are not used by neurologists in microtechnic; 

 many such methods, as a matter of fact, do call for dyes, either with 

 or without impregnation of the tissue with metallic salts. 



In addition to these three groups of procedures in animal his- 

 tology, there are stains for other types of tissue and body fluids. 

 Fats, for example, are stained by special oil soluble dyes, such as 

 Sudan IV. Blood has its own special methods of staining, most of 

 which depend on the use of compound stains, prepared by allowing 

 eosin or a related dye to react upon a mixture of dyes of the meth- 

 ylene blue group; such compound dyes are also useful in staining 

 bone marrow. Bone has its own special methods, many of which 

 depend on the use of alizarin red S, which is particularly valuable 

 for demonstrating bone in small specimens that have been cleared 

 by treatment in klkali. 



Histochemistry. Many of the above-mentioned specialized 

 staining methods come close to yielding actual chemical informa- 

 tion as to the nature of cell constituents. Some theories of stain- 

 ing do postulate chemical affinities to explain differential staining; 

 but they do not go very far in the matter, chiefly because stains 

 are ordinarily applied to fixed tissue, whose chemical nature is 

 realized to be very different from that which occurs in nature. 

 The introduction, however, of rapid freezing methods is now en- 

 abling histochemists to get away from chemical fixation and is 

 giving them material for study which is more nearly representative 

 of natural conditions. Also their study of enzymes on the one 

 hand, and thejir identification of the individual nucleic acids on the 

 other are gradually giving them criteria to show which stains or 

 other reagents are actually specific for definite cytochemical in- 

 gredients. Especially important has become the use of the Schiff 

 reagent (fuchsin-sulphurous acid), after some oxidizing agent, in 

 the identification of mucopolysaccharides. Developments in histo- 

 chemistry are now becoming so rapid that predictions are hard to 

 make. Whether the histochemical reagents of the future will be 

 primarily dyes or other chemicals, it is impossible to say at present. 



Plant Histology. A rough grouping of the most common plant 

 histological methods can be made by recognizing general tissue 



*Conn and Darrow (1943-52). 



