HISTOLOGICAL CHANGES AFTER IRRADIATION 1093 



cells or tissues are being compared for the same end effect of radiation and 

 unless the physical conditions of irradiation are the same. The idea of 

 radiosensitivity of a given cell (or organism) must be further qualified to 

 include consideration of its functional state at the time of irradiation and, 

 in many cases, its present or future reproductive activity, and the species 

 and age of the animal to which it belongs. Other terms should be used 

 for those conditions which do not satisfy these criteria. 



In contrast to a few types of injuries which cause immediate damage to 

 cells (thermal burn, for example) the effects of irradiation, like those of 

 many toxic agents, become apparent only after some time has elapsed. 

 This interval may vary from a few minutes to months or years, depending 

 mainly on the cell type, and the criterion of the injury. For instance, 

 vast numbers of lymphocytes are destroyed in a few hours by 600 to 800 r 

 of 200-kv X rays, while cells of the epidermis may not die for several 

 days after three times this amount of radiation, or months or years may 

 elapse before cataracts develop in the lens. 



With the possible exception of certain effects due to specific ionization, 

 the results of the several kinds of ionizing radiations from external sources 

 given in equivalent amounts are identical (W. Bloom, 1948a). The same 

 biological changes as those produced by X rays and neutrons result when 

 the radiations are from internal sources, but these vary with the distribu- 

 tion of the isotope (the chemical nature of the element determining its 

 localization, being diffuse in the case of Na 24 or highly localized with Sr 89 ), 

 the type of "particle" given off, and the half-life of the isotope (and its 

 daughters). The continuous nature of internal irradiation is in contrast 

 to that from most external sources which only exceptionally have been 

 applied continuously. 



The cold-blooded vertebrates are more resistant than the warm-blooded 

 ones and, going down the scale of animal (and plant) complexity, the 

 forms become increasingly resistant. Nevertheless, the kinds of cellular 

 changes seen in vertebrate cells also occur in lower forms, although the 

 amount of radiation necessary to elicit these changes will usually be much 

 greater. The younger the animal the more susceptible it will be; this is 

 also true for its cells, at least for extremes in age. It is well known that 

 embryos are much more sensitive than postnatal animals and that resist- 

 ance increases progressively with age. For instance, the nerve sheath 

 cells in a three-week chick suffer great damage with 800 r total-body 

 irradiation, whereas these same cells are not damaged with this dose when 

 the animal is 11 weeks old (Snider, 1948b). Changes in temperature and 

 metabolic rate affect the sensitivity of a particular cell strain, as shown by 

 greater resistance at lower temperatures and when the circulation of 

 blood is interrupted during irradiation. 



It must be kept in mind that all the effects of ionizing radiations on 

 cells, ranging from nonvisible effects on chromosomes to rapid cell death, 



