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G. W. CASARETT 



Sublethal, life-shortening total-body irradiation and localized irradiation, 

 by means of external or internal sources of radiation, produce permanent 

 changes in the tissues of experimental mammals preceding the appearance of 

 age-dependent disease entities, which, according to the histopathological 

 criteria discussed, seem to this author to be essentially identical with the 

 histopathological manifestations of premature ageing (Casarett, 1952, 1956, 

 1957, 1958, 1960). 



This radiation fibro-atrophy comprises an increase in amount and density 

 of connective tissue, and increase in amounts of interstitial mesenchymal 

 elements, constituting an increase in histohaematic barrier, reduction of fine 

 vasculature in a process of arteriolocapillary fibrosis, and reduction in number 

 of parenchymal cells. 



Fig. 3. Histopathological hypothesis for the tissue fibro-atrophy of ageing and "radiologic 

 ageing". 



The following is a description of the author's histopathological hypothesis 

 of ageing and premature ageing caused by radiation (Fig. 3). At radiation 

 dosage levels causing temporal advancement of age-dependent diseases and 

 life-span shortening, the nonspecific damage to the endothelium of fine 

 vasculature, by direct or indirect mechanisms, seems to be the change of 

 greatest importance, among the early radiation changes, in the eventual 

 development of the tissue fibro-atrophy of "radiologic ageing". Although 

 some of the damage to the fine vasculature may be due to relatively direct 

 eifects of the radiation on endothelial cells, much of it is probably indirect. 

 Some of the indirect damage to fine vasculature in some tissues seems to be 

 due to the initial destructive effect of radiation on parenchymal cells, which 

 varies in degree according to their nature and radiosensitivity from one tissue 



