1122 RADIATION BIOLOGY 



and endothelial proliferation may be so marked as to occlude the vessels 

 (Wolbach, 1909). Some of the small vessels may be closed by thrombi. 

 Multinucleated endothelial cells in the derma one year after an erythema 

 dose and arteriolar change a year and a half after the same dose in 

 human skin have been described. Some of the dilated vessels may 

 persist for years. An important observation is that some vessels always 

 seem undamaged. After injection of radium, the endothelial cells may 

 calcify and there may be calcification of the walls of small vessels 

 (Rhoades, 1948b). Although vascular radiation lesions are well estab- 

 lished, some believe that their importance as the cause of many of the 

 changes observed in various irradiated organs has been exaggerated 

 (Lacassagne and Gricouroff, 1941). These critics believe that the part 

 played by obliterating arteritis in radionecrosis has not been proved; 

 they hold, rather, that transformations of extracellular substances of the 

 connective tissue and associated infection are much more important 

 factors in the pathogenesis of this process. 



Some of the changes in the derma, especially in laboratory animals, 

 may be secondary to infections resulting from denudation of the derma 

 with ulcer formation. 



The late effect in the derma is progressive sclerosis, resulting from the 

 inflammatory changes and characterized by hyalinization of the collagen 

 and loss of elastic fibers. These changes, coupled with, and perhaps in 

 part responsible for, the long-persisting, incomplete regeneration of the 

 epidermis, leave the skin highly susceptible to further injury. 



A quite different picture is called forth in the connective tissue by the 

 local injection of those radioactive isotopes which adhere to collagen, such 

 as plutonium and Y 91 . In such areas the inflammatory reaction persists 

 for weeks or months, but is atypical in that there is a continued emigra- 

 tion from the vessels of granular and nongranular leukocytes, which are 

 apparently killed shortly after leaving the vessels. Consequently, there 

 is no flooding of the field with small, medium-sized, and large inflamma- 

 tory macrophages, which in ordinary inflammations develop from the 

 nongranular leukocytes of the blood. The local macrophages (histio- 

 cytes), which are relatively few for an inflamed tissue, do not divide by 

 mitosis but become enormously swollen and very basophilic; they do not 

 degenerate. The local fibroblasts likewise become swollen but show no 

 evidence of death. It is undoubtedly the continued irradiation which is 

 responsible for the failure of a typical inflammatory reaction to develop 

 around radium needles. 



EPIDERMAL DERIVATIVES 



Hair. It has been known for a long time that irradiation of the skin 

 causes destruction of the hair follicles and falling out of the hair. In 



