256 H. J. CURTIS AND CATHRYN CROWLEY 



produced in each case, and also from the fact that the radiomimetic compound 

 nitrogen mustard produces very few aberrations and no shortenmg of the life- 

 span. 



These facts also supj)ort the concept that mutations are an important part 

 of the normal ageing process, but there must be other factors also involved in 

 ageing. This follows from the fact that young irradiated animals may have an 

 aberration frequency of nearly 100%, but quite a long life-exj)ectancy; whUe 

 old normal mice may have 30% aberrations and a short life-exj)ectancy. These 

 experiments give no information as to the other ageing factors. 



In these experiments liver cells are taken as typical somatic cells, so there 

 is every reason to believe that, following a large dose of radiation, almost all 

 somatic cells in the body contain disrupted chromosomes. Yet under these 

 conditions animals are functionally almost normal, and remain so for many 

 months. • 



EEFERENCES 



CaIiDECOTt, R. S. (1961). In "Effect of Ionizing Radiation on Seeds". International 



Atomic Energy Agency, Vienna. 

 Curtis, H. J. (1961). In "Radiobiology", p. 114. Third Australasian Conference. 



Butterworths, London. 

 Curtis, H. J., Person, S. R., Oleson, F. B., Henkel, J. F., and Delihas, N. (1956). 



Nucleonics 14, 26. 

 Russell, W. L., and Russell, L. B. (1959). Proc. 2nd International Conference on the 



Peaceful Uses of Atomic Energy 22, 360, 1959. 

 Steffenson, D., and Arnason, T. J. (1954). Genetics 39, 220. 

 Stevenson, K. G., and Curtis, H. J. (1961). Radiation Res. 15, 774. 



DISCUSSION 



gray: This is an extremely mteresting correlation that you've sho\vn but I wonder 

 whether it is m fact quite fair to compare liver with bone-marrow, because may it not be 

 that the nitrogen mustard wliich undoubtedly produces the cliromosome damage does so 

 by interference with the actual turnover, the dupUcation, of the clu-omosomes. When it 

 acts on ceUs which are proliferating it produces more damage. In the case of the radiation, 

 of course it is obviously mtroducing a defect mto the genome which expresses itself as a 

 cliromosome abnormaUty when the ceU divides. The X-ray damage may only be repaired 

 to a small extent. But nitrogen mustard which acts only in producing clu:omosomal 

 instantaneous breaks at the time of turnover wouldn't have its effect on the liver but it 

 could on proliferatmg tissue, so that tliis perhaps wouldn't mean that you couldn't 

 produce with nitrogen mustard chromosome defects which would persist if they'd been 

 initiated in cells which were turning over at the time of treatment. 

 CURTIS: Yes, I quite agree. I feel this work shows clearly that nitrogen mustard does not 

 break chromosomes m non-dividmg cells whereas it has been shown many times that it 

 does so in dividing cells such as those in bone-marrow or testes. But rapidly dividing 



