112 Microscopic Histochemistry 



though they have been precipitated previously by histologi- 

 cal fixatives. 

 a) The ferrocyanide reaction of Hartig-Zacharias} 



Method 



Treat the section for 10 minutes with an acidified solution 

 of potassium ferrocyanide (1-5 per cent in dilute hydro- 

 chloric acid; concentrations not important ) ; wash thoroughly 

 and flood with a dilute solution of ferric chloride. Proteins 

 stained blue. Ferric compounds (hemosiderin) will be blue 

 before the application of ferric chloride. 



h) The tannin-ferric method of Salazar,^— 



Method \ 



Treat the section for 15-20 minutes with an acidified solu- 

 tion of tannin ( 5-10 per cent tannin in 5-10 per cent acetic 

 acid; concentrations not important); wash thoroughly and 

 flood with a dilute solution of ferric chloride. Some protein 

 structures, secretion granules, etc., stain gray-black. 



2. The digestion tests, once widely used, yield very little 

 information of a chemical nature, although they do differen- 

 tiate between various protein substances. Pepsin digests col- 

 lagen and reticulum; trypsin, elastic fibers. For details the 

 interested reader is referred to the review of the topic by 

 Spalteholz and others.^ This topic should be reinvestigated 

 with the aid of purified proteolytic enzymes which have 

 become available recently. 



Amino Acm Components of Proteins 



Of the reactions described for the histochemical identifica- 

 tion of amino acids, only two are reagents for amino acids 

 proper (ninhydrin and alloxan). The others are specific 



1. Zacharias, E.: Bot. Ztschr., 41:208, 1883. 



2. Salazar, A. L.: Compt. rend. Soc. de biol., 83:1655, 1920. 



3. Verdauung, kiinstliche. In Enzyld. d. mikr. Technik (3d ed.; Berlin 

 and Vienna: Urban & Schwarzenberg, 1927), 3:2220. 



