40. RADIATION' AND NUCLEIC ACID METABOLISM 535 



(crypt cells) are in a very small minority compared with mucosal and sub- 

 mucosal cells. If cell death is occurring among the crypt cells — as indeed it 

 does — all the findings could be explained on selective death of cells capable 

 of DXA synthesis, without direct interference with the process of DXA 

 synthesis itself. 



b. Bone Marrow 



A significant depression of glycine-C 14 incorporation into rabbit bone 

 marrow following 800 r. TBR was found at 5 hours after irradiation, 33 and 

 of P 32 incorporation 2 hours after 1000 r. TBR 34 ; similarly in mice incorpora- 

 tion of both P 32 and adenine-C 14 was depressed after 300 r. TBR. 18 - 26 



However, the youngest bone marrow cells — the population with the 

 highest proportion of DXA (and RX'A) synthesizing cells — are the most 

 radiosensitive in respect of cell destruction, and within 3 hours their num- 

 ber drops to about 50% of normal. 35 ' 36 This is in agreement with earlier 

 findings of a 40% drop in DXA content of rabbit bone marrow 3 hours 

 after 1500 r. 37 Consequently, the same criticism as applied to intestinal 

 tissue is valid here: cell population changes alone could explain the experi- 

 mental findings. 



c. Spleen and Thymus 



More detailed information is available on the effect of radiation on DXA 

 synthesis in lymphatic tissue, spleen and thymus in particular. Both in 

 rabbit appendix 31 and rat thymus and spleen 38 the incorporation of P 32 was 

 found to be depressed by 50-70% within 2-3 hours after irradiation (1000 r. 

 abdominal to rabbits, 100 r. TBR to rats). However, Thomson et a/. 39 have 

 shown in rat thymus, that although 3 hours following 800 r. TBR the DXA 

 P 32 uptake is depressed by about 50%, by that time also the total DXA 

 content has dropped 15 %. As this 15 % loss of DXA may have meant selec- 

 tive loss of the most actively growing cells, the situation is rather similar 

 to that with the intestine and bone marrow. 



In the spleens of mice receiving 300 r. TBR, P 32 incorporation into DXA 

 was depressed by about 50 %. 18 Kelly, however, points out that changes 

 even within the first 2 hours after irradiation may reflect popula- 



33 R. Abrams, Arch. Biochem. Biophys. 30, 90 (1951). 



34 R. M. S. Smellie, G. F. Humphrey, E. R. M. Kay, and J. N. Davidson, Biochem. J. 

 60, 177 (1955). 



35 E. V. Hulse, Brit. J. Haematol. 3, 348 (1957). 



36 E. Harris, Strahlentherapie 38, Suppl. Ill, p. 6 (1957). 



37 C. Lutwak-Mann, Biochem. J. 52, 356 (1952). 



38 L. S. Kelly and E. L. Bennett, Radiation Research 5, 485 (1956). 



39 J. F. Thomson, W. W. Tourtelotte, and M. S. Carttar, Proc. Soc. Exptl. Biol. 

 Med. 80, 268 (1952). 



