4 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



As was shown first by Caspersson and Schultz (1938), RNA is 

 abundant in rapidly growing cells (onion root-tips, imaginal discs 

 of Drosophila larvae). Proliferating tissues, however, are by no 

 means the only ones to contain large amounts of RNA in their 

 cytoplasm and nucleoli. The same holds true for the exocrine parts 

 of the pancreas (Fig. 3, p. 11), the cells producing pepsin in the 

 gastric mucosa (Fig. 4, p. 1 2), liver cells, nerve cells, young oocytes and 

 embryos undergoing differentiation, all of which are sites 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 proteins, contain only small amounts of RNA: such is 

 the case for heart, muscle, or kidney (Caspersson, 1941). Micro- 

 organisms, which multiply very rapidly, and thus very quickly 

 synthesize their own proteins {e.g., yeasts or bacteria) are very rich 

 in RNA (Caspersson and Brandt, 1941). In conclusion, all the 

 organs which synthesize large amounts of proteins, whether for 

 growth or multiplication, are always rich in RNA, which is localized 

 in the nucleolus and the cytoplasm. All other cells and tissues have 

 a much lower content in RNA and much less conspicuous nucleoli. 



Further confirmatory evidence may be cited. One of the organs 

 which has the largest RNA content is the silk gland in silk worms 

 (Brachet, 1941 ; Denuce, 1952), the only known function of which 

 is the production of the protein silk. While endocrine glands are 

 relatively poor in RNA, it is a striking fact that stimulation of 

 hormonal secretion in the pituitary is linked with a marked increase 

 in the RNA content (Desclin, 1940; Herlant, 1943; Abolins, 1952). 



There is thus no doubt that the RNA content of various cells 

 can be made to vary under different physiological conditions, but 

 always in relation to protein synthesis. 



There is, further, more recent evidence for this conclusion. The 

 effect of hypophysectomy and growth hormone on liver cells (Di 

 Stefano et al., 1952, 1955; Fiala et al., 1956), the normal growth 

 and compensatory hypertrophy of the kidney (Kurnick, 1955), the 

 action of cold on neurons (Gordon and Nurnberger, 1955), the 

 growth of feathers (Koning and Hamilton, 1954), and the secretory 

 cycle in pancreas (Oram, 1955), etc., have been studied and a close 



