54 ROBERT H. BROWNSON 



the granule cells than in Purkinje cells, appearing in both as nuclear pyknosis 

 and hyperchromatic staining. There appeared to be an appreciable decrease 

 in cellularity most severely affecting the 5,000 r level at 1 16 days. 



Wilson ( 1960) irradiated monkeys with whole-body exposure to cobalt-60 

 (gamma) from 400 to 40,000 r and stressed the changes that occurred in 

 animals dying during the first 54 hours post-irradiation. Cerebellar granule 

 cells underwent nuclear pyknosis especially within the innermost layers 

 with a decreased cellularity explained on the basis of decreased nuclear area. 

 Vogel ( 1959) administered massive doses of gamma radiation to the head of 

 rabbits and dogs sacrificed at intervals up to 10 days. He reported that the 

 altered granule cells of the cerebellum demonstrated pyknotic and hyper- 

 chromatic nuclei most notably within the first 24 hours after exposure. His 

 opinion was that the recovery phase was completed by 72 hours after 

 radiation. 



The tissues studied in this project are decidedly those of chronic classifi- 

 cation sacrificed up to 228 days after exposure. The results from oiu' study 

 indicate that x-irradiation of the rat head will elicit slight changes in cere- 

 bellar neurons after 228 days and that a dose as high as 5,000 r is capable of 

 causing severe cell change during the 1 16 days. In all instances the changes 

 described through and including 228 days after radiation were accompanied 

 by decreased cellularity. It would only be speculation to propose cell loss. 

 The only absolute measure of this would ha\e to incorporate a quantitative 

 analysis. 



Wilson (1960) and Vogel's (1959) findings that the granule cells in the 

 cerebellum undergo acute changes followed by reco\ery during a slightly 

 later period after radiation, would seem to indicate the existence of secondary 

 effects. This effect noted months after exposure may further demonstrate 

 its recovery phase at a still later date. 



Neurons in the cerebral cortex, hypothalamus, and brain stem seldom ap- 

 peared to be necrotic and were probably of reversible nature. Such changes 

 were described as shrinkage in both cytoplasm and karyoplasm or pyknosis. 

 It is suspected that the decrease in cell or nuclear volume or both is para- 

 mount to the hyperchromatic staining quality, as well as to the appearance 

 of decreased cellularity. Peculiar to these alterations was the obvious scat- 

 tering of cell changes within identical structiue, thus indicating certain 

 differences in radiosensitivity within the particular structure imder observa- 

 tion. In view of the perfusion-fixation methods utilized for this study, it is 

 felt that these observations are reasonably free from artifact. These cell 

 changes were present in varying amounts throughout all radiation levels, 

 which seemingly indicates little or no direct qualitati\e relationship between 

 dose and time within the parameters of this study. Only at the 5,000 r level 

 were there evidences of significant neuronal necrosis. The absence of major 



