HISTOLOGICAL CHANGES AFTER IRRADIATION 1133 



the body. Many such attempts have been made but all of them are 

 deficient, owing mainly to extensive gaps in available data and to a 

 failure to compare sensitivities for the same end effect. In Table 17-1 

 are listed the reactions of the cells when compared only for cell death in 

 nearly grown rabbits exposed to 800 r of total-body 200-kv X rays. A 

 similar table for lower doses would show many fewer dead cells. Data are 

 not available for a complete list at double or treble this dose or for 

 localized irradiation of each organ. Such tables do not consider many 

 important questions, such as the time of onset and the rapidity of cell 

 death or the rate of regeneration. 



As early as 1906, Bergonie and Tribondeau felt that enough was known 

 to enable them to make some generalizations as to the types of cells 

 readily affected by irradiation: "The greater the reproductive activity 

 of the cells, the longer their mitotic course ('leur devenir karyokinetique'), 

 and the less definitively fixed their morphology and functions, the more 

 intense is the activity of X rays on these cells." This so-called "law" 

 obviously does not apply to the relatively great radioresistance of reticular 

 cells which are more primitive than the free blood-cell-forming cells to 

 which they give rise. Nor does it hold for the radioresistance of primitive 

 ovarian follicular cells as compared with their derivatives, the growing 

 follicular cells which are much more radiosensitive. There are numerous 

 other instances in which this generalization does not hold. In our 

 opinion it should be remembered only as an early attempt to focus atten- 

 tion on the greater susceptibility of growing organs and tissues containing 

 dividing cells as against static ones. 



Although it is true that many strains of cells which are actively in 

 division at the time of irradiation are readily and quickly destroyed, and 

 while it is also true that in some cell strains the destructive effects of 

 irradiation become apparent only when the cells later undergo division, 

 the destructive effects of ionizing radiations are not always connected 

 with cellular reproduction. Thus, the small lymphocytes, which prac- 

 tically never divide, are among the most sensitive cells in the body. If 

 those theories of hematopoiesis which hold that lymphocytes are undiffer- 

 entiated cells are found to be true, then the sensitivity of these cells might 

 be a reflection of their mitotic potential. On the other hand, plasma 

 cells, which are recognized by all to be slightly changed lymphocytes, are 

 exceedingly resistant. Again, spermatocytes are in continuous division 

 (meiosis) and yet are not visibly affected by doses of radiation which will 

 destroy all spermatogonia. It is thus clear that mitosis per se is not a 

 prime factor determining radiosensitivity. 



As is well known, small amounts of radiation may have no visible effect 

 on the mature sex cells, and yet striking developmental changes may 

 appear in the progeny. These effects are usually lethal mutations or 

 malformations. Perhaps similar results may occur in somatic cells in 



