214 Microscopic Histochemistry 



of enzymes with somewhat divergent, but overlapping, sub- 

 strate specificities. 



2. The biochemical difference between "true" and "pseu- 

 do"-cholinesterase does not hold histochemically. Some of 

 the "true" cholinesterases (brain and muscle spindles of the 

 mouse, brain of the dog, etc. ) do split the long-chained fatty 

 acid esters, others (brain of man and the rat and of all species 

 phylogenetically below birds ) do not. The same sort of situ- 

 ation applies to the "pseudo"-cholinesterases. The abihty or 

 inability to hydrolyze these esters is a feature which bisects 

 the class of cholinesterases in a plane different from that 

 which divides them into the "true" and the "pseudo" types. 

 The two classifications have no common basis for compari- 

 son. 



The thiocholine method appears to give results in better 

 harmony with biochemical findings. At the present time, 

 however, the material studied and reported is too scanty to 

 permit any meaningful comparison of the activity patterns 

 with those of the other methods. It is for this reason that 

 the results of the thiocholine method will not be included 

 in the following attempt at the histochemical classification 

 of esterases. 



After a careful analysis of hundreds of slides containing 

 many tissues of a number of species, stained for the various 

 esterases, the writer came to the conclusion that animal tis- 

 sues contain three cardinal types of esterases, with narrowly 

 defined specificities, and, in addition, a number of interme- 

 diate types which hydrolyze the substrates of two, or even of 

 all three, of the main types. Whether the intermediate types 

 actually represent individual enzymes with transitional prop- 

 erties or only the topographic compresence of several of the 

 main types cannot be decided on the basis of data available. 

 Table 6 gives a tentative classification of esterases demon- 

 strable in paraffin sections by the Tween, azo dye, and the 

 long-chained fatty acid choline ester techniques. 



