388 RADIATION BIOLOGY 



frequent mitosis are the facts (1) that radiation also induces aneuploidy 

 by nondisjunction and by lagging of whole chromosomes when it is 

 applied within a limited period (some days) before mitosis (see Sect. 3-2) 

 and (2) that unless a cell undergoes at least one mitosis subsequent to its 

 irradiation, any structural changes that may have been induced in it 

 must fail to result in either chromosome bridges or aneuploidy of any 

 kind, and hence must remain comparatively innocuous. 



These radiogenetic considerations, then, furnish an explanation of the 

 high correlation between the amount of proliferative activity of an 

 organism, organ, tissue, or type of cell, and its susceptibility to being 

 damaged by radiation or other agents (such as mustards) that cause 

 structural chromosome changes. It is only to be expected, on this basis, 

 that the younger an individual is, all the way down to the stages of early 

 cleavage, the greater is the damage caused by a given dose of radiation; 

 that those parts are most affected, and most checked in growth, which 

 grow more actively; that regeneration and wound healing tend to be 

 inhibited; and that in the adult the tissues selectively affected are 

 germinal tissues, blood-forming tissues, the epidermis and its derivatives, 

 mesodermal parts that require cellular replacement, proliferative endo- 

 thelia and endodermal epitheUa, and mahgnant growths of all kinds. 



On the other hand, it must not be assumed that, in general, develop- 

 mental abnormalities that result from irradiation of the embryo are 

 always expressions of chromosomal damage. Processes of morphogenesis 

 can be affected during their sensitive stages by the influence of radiation 

 just as by other toxic influences, such as high temperature or certain 

 chemicals, so as to shunt them into some abnormal direction, without 

 any significant genetic change having been induced in the nuclei of the 

 cells concerned. That this is the case in irradiated Drosophila embryos 

 was found by Lamy and Muller (1939) by comparison of the effects on 

 diploids and polyploids : these failed to show the consistent differences to 

 have been expected between them if the effects had had a genetic basis. 

 That the radiation damage to developing individuals which causes their 

 death is, however, in some cases due to genetic changes was shown by the 

 contrary results of A. R. Whiting and Bostian (1931) and of Clark and 

 Kelly (1950), who used similar techniques with immature stages of the 

 wasp Habrohracon which differed from one another in the number of 

 their contained chromosome sets. In the latter material the damage was 

 inversely correlated with chromosome number in just the manner to be 

 expected of the effects of induced chromosomal changes. In cases in 

 which the question is not concerned primarily with the determination of 

 the type of morphogenetic processes, but rather with the capacity of 

 tissues for proliferation and survival, we are on firmer ground in invoking 

 chromosome change as the usual means by which ionizing radiation pro- 

 duces its long-term damage. 



