166 Microscopic Histochemistry 



potassium permanganate (brown coloration) or by ferriferricyanide 

 (precipitation of Prussian blue), the latter by the recolorization of 

 leu CO methylene blue. 



The application of the modern redox-indicator dyes to the tissues*^"^ 

 should give results far more delicately graded than the methods of 

 Unna. However, it must be admitted that the oxidizability of organic 

 compounds by permanganate or by ferricyanide is theoretically mean- 

 ingful: it indicates the presence, in the structures stained, of com- 

 pounds with oxidation-reduction potentials lower than those of the 

 oxidants mentioned. Owing to its very powerful oxidizing properties, 

 the specificity of permanganate is practically nil, since it is reduced by 

 the majority of tissue components. Ferricyanide, being a much milder 

 oxidant, does give a fairly selective staining of the more strongly re- 

 ducing groups (-SH, polyphenols). On the other hand, the results of 

 staining with leuco methylene blue cannot by any means be construed 

 to reveal anything about the distribution of "free" oxygen (whatever 

 this term may mean) or of oxidizing systems in the tissues. Unna 

 stained sections in a solution of leuco methylene blue containing a 

 large excess of the reducing agent (formaldehyde sulf oxylate ) . While 

 the tissue is in this solution, it shows no indication of staining. On 

 rinsing the sections in water, the color of methylene blue becomes 

 apparent, and the picture is very similar to, although not absolutely 

 identical with, that obtained with a regular solution of methylene 

 blue. Obviously, the affinity of leuco-dye to basophilic structures is 

 practically the same as that of the parent-dye, allowing for minor 

 differences caused by structural changes in the dye molecule. When 

 the excess of the reducer is removed by rinsing and the tissue is ex- 

 posed to atmospheric oxygen, the latter will restore the bound leuco- 

 dye to its original color. There is absolutely no need to assume any 

 oxidizing activity residing in the tissue proper. Unna's work has been 

 received wtih much criticism"^^ but has been defended stubbornly by 

 the author, who ultimately resorted to such fanciful hypotheses as 

 the "oxypolar affinity,'"^^ meaning some sort of chemical attraction 

 between compounds rich in oxygen and poor in oxygen. 



A slight modification of Unna's leuco methylene blue technique was 

 revived by Roskin.^^ as a diagnostic aid for the recognition of malig- 



77. De Robertis, E., and Moura Gongalves, J.: Endocrinology, 36:245, 

 1945. 



78. Oelze, F. W.: Arch. f. mikr. Anat., 84:91, 1914; Schneider, H.: 

 Ztschr. f. wissensch. Mikr., 31:51, 478, 1914; Rothman, S.: Jadassohn's 

 Handb. d. Haut- u. Geschlechtskrankh., I, 2:330, 1929. 



79. Unna, P. G.: Arch. f. mikr. Anat., 87:96, 1915. 



80. Roskin, G.: Ztschr. f. Krebsforsch., 35:140, 1931; Roskin, G.: Ztschr. 

 f. Zellforsch. u. mikr. Anat., 14:781, 1931-32; Roskin, G., and Semenoff, W.: 



