Special Features of Histochemical Methods 13 



3. The end result produced must be reasonably stable. 

 Soluble or fleeting colors, such as that of the rhodanate re- 

 action^ for Fe+ + + or the change in the shade of indicators 

 caused by acid liberated enzymatically from esters, do not 

 localize with sufiicient accuracy. A reaction to be used in 

 histochemistry must produce highly insoluble and, prefer- 

 ably, intensely colored precipitates. 



4. When a reaction produces a crystalline precipitate, the 

 size of the crystals must be small enough to permit cytolog- 

 ical localization. This condition is not fulfilled in the gypsum 

 reaction for Ca^ or in the digitonine reaction for cholesterol.^ 



5. In the case of soluble substances, the reaction must be 

 prompt, of a speed not much inferior to that of ionic re- 

 actions. Slow reactions are unusable. To give an extreme 

 example, some otherwise excellent reactions for glucose 

 (osazone formation; the reduction of Benedict's solution) 

 are entirely unsuitable for histochemical purposes. Glucose 

 would diffuse far from its original site before either of these 

 reactions was completed. 



Failure to recognize these simple principles has resulted 

 in a number of publications reporting the histochemical 

 localization of substances for which suitable reactions are not 

 known.^ 



6. Schmelzer, W.: Ztschr. f. wissensch. Mikr., 50:99, 1933. 



7. SchujeninofiF, S.: Ztschr. f. Heilk., 18:79, 1897. 



8. LeuHer, A., and Noel, R.: Bull, d'histol. appliq. a la physiol., 3:316, 

 1926. 



9. Seeger, P. G.: Arch. f. exper. Zellforsch., 21:308, 1938; Ztschr. f. mikr.- 

 anat. Forsch., 48:181, 639, 1940, and 53:65, 1943. 



