66 YAMAZAKI, BENNETT AND CLEMENTE 



philic accumulation in the dura and around the \a.sculature of the pia 

 mater, tended to be more focal with the lower radiation dose and more 

 intense and generalized with the higher radiation dosages (1,000 r). 



In the long term series (survival periods of 20 days to 14 months), 

 vascular pathology consisted principally ol an increase in the thickness of 

 vessel walls with a consequent narrowing of the vascular lumen. This was 

 especially evident in vessels that were seen within the meningeal layers on 

 the surfaces of the brain, although at times deeper vessels also showed im- 

 mistakable signs of damage. The thickening in the blood vessel walls was 

 most pronounced in the tunica media and in the tunica intima. so that the 

 total vessel diameter did not increase, but the lumen decreased. Aroimd such 

 fibrosed vessels there often were signs of a minor chronic inflammatory 

 process. In these instances, the inflammatory reaction, characterized by the 

 presence of mononuclear cells, was possibly the residual of the more acute 

 inflammatory reactions observed in the short term series. 



Lesions observed over a longer time revealed hemorrhage and necrosis in 

 areas where capillary damage was most e\ ident in the brain of animals 48 

 to 72 hours after irradiation, suggesting that this initial lesion was an acute 

 phase of the lesions observed at later periods. The areas that were most 

 often involved included the cerebellum, basal ganglia (especially the globus 

 pallidus and caudate nucleus), diencephalon, and medulla. In many rats, 

 sections of the brain stem showed an excessive number of neuroglial cells, 

 even though no distinct localized lesions could be seen. This hypergliosis 

 unassociated with apparent neuronal damage was most intense in white 

 matter and most frequent in the midbrain, pons, and medulla, but was also 

 seen in the caudate nucleus and globus pallidus. 



Evidence of early inflammatory lesions in tlie choroid plexus was visible 

 as small contracted scars characterized by connective tissue proliferation. 

 Marked enlargement of the lateral ventricles developed in animals receiving 

 1,000 r during the immediate neonatal period (Fig. 4). With 500 r the dis- 

 tension of the ventricles was not as noticeable. The third and fourth ventri- 

 cles were never as severely involved. With lateral ventricular distension, 

 there occiared a concomitant decrease in thickness of the cortical layers. The 

 packing density of the cortical neinons looked like that of the 3-day-old rat, 

 although some of the animals had lived from 3 weeks to 3 months. The 

 cortical neurons appeared intact; however, the cortical mantle resembled 

 that of a much younger animal. 



The dura and pial coverings of the brain increased in thickness with time. 

 Proliferation of connective tissue and a mild secondary mononuclear infil- 

 tration occurred, especially in animals irradiated with 1,000 r between the 

 5th and 14th day and surviving for 9 months to one year. With meningeal 



