CYTOCHEMICAL OBSERVATIONS 3 



a Specific manner; but even in sea urchin eggs or pancreas, only the 

 nuclei — because of their high DNA content — took the stain. When, 

 as recommended long ago by Unna, a second basic dye (pyronine) 

 is added to methyl green, bright red staining of the cytoplasm and 

 nucleoli occurs. However, it took several years before the real meaning 

 of this staining with pyronine became clear. Purified ribonuclease, 

 i.e. the enzyme which breaks down RNA, became available around 

 1938 and it thus became possible to treat sections with solutions of 

 the crystalline enzyme. As can be seen from a comparison between 

 Figs. 1, 2 (pp. 9, 10), digestion of RNA with ribonuclease leads to the 

 complete disappearance of cytoplasmic and nucleolar basophilia, 

 without at all affecting the staining of chromatin with methyl green. 

 Development of a refined method of UV microspectrophotometry 

 by Caspersson (1936, 1940) led, at the same time, to similar develop- 

 ments. Nucleic acids, because of the presence in their molecule of 

 purine and pyrimidine bases, have a strong absorption in the UV. 

 It was soon found by Caspersson that the cytoplasm and the nucleoli 

 of the cells which have a high RNA content display strong UV 

 absorption, despite the fact that they are Feulgen-negative : RNA 

 must thus be localized primarily in the cytoplasm and the nucleolus. 

 These early cytochemical studies have led, as we shall see, to the 

 unexpected conclusion that RNA must play a role in protein syn- 

 thesis. After a brief survey of the results obtained by cytochemical 

 and biochemical methods, more direct evidence, derived from stud- 

 ies on plant viruses, on the action of ribonuclease on living cells 

 and on purified enzyme systems will be presented. These studies 

 lead to the same conclusion, i.e. that RNA plays a major and direct 

 role in the synthesis of specific proteins. 



2. CYTOCHEMICAL OBSERVATIONS 



As we have just pointed out, the conclusion that RNA is somehow 

 concerned with protein synthesis comes from the cytochemical ob- 

 servations of Caspersson (1941) and Brachet (1942). Using entirely 

 different methods for RNA detection, they reached the same con- 

 clusion independently and simultaneously. 



References p. 50/54 



