40 Microscopic Histochemistry 



The 8-hydroxyquinoline^^ and the dinitroresorcinoP^ tech- 

 niques ojBFer no advantages. 



Masked or occult iron.— A number of well-defined iron- 

 containing compounds, such as hemoglobin, malaria pig- 

 ment, formalin pigment, and possibly some other less well- 

 known ones, do not show any reaction with the Prussian blue 

 method; however, they can be made positive by destroying 

 the organic part of the molecule ("unmasking" the iron). 

 Ammonium sulfide and acids are sometimes referred to as 

 unmasking agents, but wrongly so. They do not liberate 

 demonstrable iron from any of the substances mentioned. 



A good demasking agent must spare tissue architecture 

 and leave the iron at its original site. No strongly acidic sub- 

 stance can satisfy the latter condition. The only reagents 

 which have been used successfully are free chlorine, bro- 

 mine,^^ and hydrogen peroxide.^^ Chlorine is applied either 

 in the form of a gas ( Okamoto ) ^^ or in a nonaqueous solvent 

 (Kockel).^^ These methods are cumbersome and not very 

 reliable. Even if they do work, they will transform iron into 

 highly hygroscopic and diffusible ferric chloride, which can- 

 not be localized with any accuracy. On the other hand, hy- 

 drogen peroxide produces insoluble ferric oxide. Whether all 

 or only some of the iron-containing biological substances are 

 unmasked by it and to what extent remains to be deter- 

 mined; hemoglobin gives a fairly intense Prussian blue re- 

 action after treatment with hydrogen peroxide. 



Method 



Apply a few drops of a 30 per cent solution of hydrogen 

 peroxide (e.g., Superoxol), alkalized with some dilute am- 

 monia or sodium carbonate, to the section and leave it on 



32. Thomas, J. A., and LavoUey, J.: Bull, d'histol. appliq. a la physiol., 

 12:400, 1935. 



33. Humphrey, A. A.: Arch. Path., 20:256, 1935. 



34. Klein, G.: Praktikum der Histochemie (Berlin: J. Springer, 1929). 



35. Brown, W. H.: J. Exper. Med., 13:477, 1911. 



36. Okamoto, K.: Acta Scholae Med. Univ. Kioto, 20:413, 1937. 



