1092 RADIATION BIOLOGY 



tion is given at one time or in divided doses. Finally, medical interest in 

 radiation has focused much of the investigation on the higher animal 

 forms and on a relatively limited dose range determined by therapy in 

 man and toxicity and survival studies in animals. 



Depending in part on the amount of radiation applied and in part on 

 the degree of susceptibility of the cells irradiated, the resulting cellular 

 changes may be classified roughly into two groups. The first comprises 

 subtle effects recognizable only by alterations in the actual and potential 

 functions of the cells (including those which become manifest only in 

 their progeny). The second group consists of more obvious changes, 

 detectable by the optical microscope: (1) degenerative phenomena which 

 may be reversible but which usually lead to cell death, (2) inhibition of 

 mitosis, and (3) abnormal mitosis. Other morphological criteria must be 

 sought especially through study of intracellular enzymes and of changes 

 in submicroscopic structure. 



Since all cells near an embedded radium needle will die if exposed long 

 enough, there are no completely resistant cells. But with amounts of 

 radiation which permit survival of laboratory animals for only a short 

 time, many cells appear to be unaffected. Among the more sensitive 

 cells, however, gradations in susceptibility are revealed. 



In a number of organs the visible effect of a given dose of radiation is 

 the death of a given type of cell, while other cell types may show no 

 obvious damage although some of them may undergo great changes of 

 another kind. It is thus impossible to compare their reaction to radia- 

 tion using cell death as the sole criterion. For instance, spermatogonia 

 in most mammals are destroyed by exposure to 800 r (total-body irradia- 

 tion with 200-kv X rays) while spermia may show no obvious change. 

 Yet the spermia may have been injured, although the effects will appear 

 later only if one of them fertilizes a normal egg. For the other cells of 

 the testis (spermatocytes, spermatids, Sertoli cells, and interstitial cells) 

 we can find no comparison of relative radiosensitivities, as revealed by 

 cellular degeneration, beyond the observation that they are much more 

 resistant than the spermatogonia. Similar statements can be made 

 about the other organs, both the sensitive and the more resistant ones. 

 We are thus in a position to evaluate only roughly the relative radio- 

 sensitivity of a few types of cells while the other cells of the body may be 

 grouped together as comparatively radioresistant. The data so far 

 determined do not permit an evaluation of all cell types of the body in 

 exact terms such as might be expressed by LD.,o or LDioo values for those 

 cells. This lack was stressed by Stafford L. Warren (Duggar, 1936, 

 p. 475). 



Although relative radiosensitivity is a convenient term for distinguishing 

 varying degrees of susceptibility shown by the different parts of an 

 organism to the same amount of radiation, it is meaningless unless the 



