BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 487 



methyl green - pyronine), combined with digestion with ribonuclease. The 

 latter method does not lend itself so well to precise quantitative measure- 

 ments, but it has the advantage of simplicity and low cost. Both techniques 

 have been discussed in Chapter 17. 



It would be a long and almost impossible task to review here all the 

 cytochemical papers in which the intracellular localization of PNA in all 

 possible types of cell has been studied. All we can attempt to do is to give 

 a brief summary of the earlier results which led to the conclusion that PNA 

 plays an essential part in protein synthesis, and to mention a few of the 

 more recent papers supporting the same conclusion. 



As was shown first by Caspersson and Schultz,^° PNA, which had already 

 been demonstrated a few years earlier in the cytoplasm of the sea urchin 

 egg (Brachet"), is abundant in rapidly growing cells (onion root-tips, 

 imaginal disks of Drosophila larvae). Proliferating tissues, however, are by 

 no means the only ones to contain large amounts of PNA in their cytoplasm 

 and nucleoli: the same holds true for the exocrine part of the pancreais, the 

 cells producing pepsin in the gastric mucosa, liver cells, nerve cells, young 

 oocytes, and embryos undergoing differentiation, all of which are the site 

 of marked protein synthesis. On the other hand, many tissues which have 

 a very high physiological activity, but which do not synthesize large 

 amounts of protein, contain only small amounts of PNA: such is the case 

 for heart, muscle, or kidney (Caspersson, ^^ Caspersson, Landstrom-Hyd^n 

 and Aquilonius,^- Caspersson and Thorell,*'' Hyden,^'*'^^ Brachet-^). Micro- 

 organisms, which multiply very rapidly and thus synthesize very quickly 

 their own proteins (e.g., yeasts or bacteria), are also very rich in PNA 

 (Caspersson and Brandt,^* Malmgren and Heden^^). 



It is already obvious from this very brief description that all the organs 

 which synthesize large amounts of proteins, whether for growth or multi- 

 plication, are always rich in PNA, which is localized in the nucleolus and 

 the cytoplasm; all other cells and tissues have a much lower content in 

 PNA and much less conspicuous nucleoli. 



Further confirmatory evidence may also be cited: one of the organs 

 which has the largest PNA content is the silk gland in silkworms (Brachet,'^ 

 Denuc^*^), the only known function of which is the production of the pro- 



«« T. Caspersson and J. Schultz, Nature 142, 294 (1938). 



" J. Brachet, Arch. biol. (Liege) 44, 519 (1933). 



" T. Caspersson, H. Landstrom-Hyd^n, and L. Aquilonius, Chromosoma 2, 111 (1941). 



*' T. Caspersson and B. Thorell, Chromosoma 2, 132 (1941). 



" H. Hyd^n, Z. mikroskop. anat. Forsch. 54, 96 (1943). 



" H. Hyd^n, Acta Physiol. Scand. 6, Suppl. 17 (1943). 



^* T. Caspersson and K. Brandt, Protoplasma 35, 507 (1941). 



6' B. Malmgren and C. G. Hed^n, Acta Pathol. Microbiol. Scand. 24, 437, 448, 472, 



496 (1948). 

 «8 J. M. Denuce, Biochim. et Biophys. Acta 8, 111 (1952). 



