84 Microscopic Histochemistry 



ylene blue, thionin, toluidin blue, pyronin, and safranin. 

 Methyl green stands in a class by itself on account of its 

 unique property of staining some, but not all, of the baso- 

 philic substances. It will stain high-polymer DNA as it occurs 

 in the nuclei or as obtained by gentle extraction procedures, 

 and also sulfate-polysaccharides, but not depolymerized 

 DNA or RNA in any form. The reasons for this curious be- 

 havior are not well understood. ^^^' ^^^ In practice, however, 

 a combination of methyl green and pyronin (Pappenheim 

 and Unna)^^^ is a most useful dye mixture, giving an excel- 

 lent contrast between green-staining DNA of nuclei and red- 

 staining RNA. All other dyes mentioned will stain both the 

 nucleotides and also acid polysaccharides; the latter may be 

 stained in a metachromatic shade. It is important to use the 

 dyes at a neutral or slightly acid ( pH 5-6 ) reaction, because 

 otherwise they may be taken up even by nonbasophilic struc- 

 tures. 



The nature of the binding of basic dyes by basophilic sub- 

 stances is not completely understood. There are good indi- 

 cations that in a solution there is a strict stoichiometric re- 

 lationship between the acidic groups and the amount of dye 

 bound.^^^ However, it is very questionable whether this re- 

 lationship holds for the insoluble nucleoproteins of the tissues 

 where an undetermined proportion of the acidic groups may 

 be tied down to strongly basic proteins in a stable, nonre- 

 active form. It is quite likely that in sections ionic forces, im- 



113. Pollister, A. W., and Leuchtenberger, C: Proc. Nat. Acad. Sc, 

 35:111, 1949. 



114. Vendrely, C, and Vendrely, R.: Compt. rend. Soc. de biol., 

 143:1388, 1949; Kumick, N. B.: J. Gen. Physiol., 33:243, 1950; Kumick, 

 N. B., and Mirsky, A. E.: J. Gen. Physiol., 33:265, 1950; Kumick, N. B.: 

 J. Nat. Cancer Inst., 10:1345, 1950; Taft, E. B.: Exper. Cell Research, 

 2:312, 1951. 



115. Unna, P.: Plasmazellen. In Enzykl. mikr. Technik (2d ed.; 1910), 

 p. 744. 



116. Chapman, L. M., Greenberg, D. M., and Schmidt, C. L. A.: J. Biol. 

 Chem., 72:707, 1927. 



