70 YAMAZAKI, BENNETT AND CLEMENTE 



ever, it remains to be demonstrated whether radiation actually affects ma- 

 turing cerebral vessels in a like manner. The capillaries have been demon- 

 strated to be the most radiosensiti\e of the blood vessels, and the degree of 

 vascularity would seem to have a bearing on the pathologic picture. The 

 degree of vascularity varies widely in the rat brain; for example, the globus 

 pallidus has been demonstrated to be strikingly low in vascularity (Craigie, 

 1945). The globus pallidus was one of the areas where necrosis was most 

 frequently involved. 



These physiologic and morphologic changes in the rat brain during this 

 initial postnatal 2 weeks when radioresistance is rapidly developing are also 

 associated with dynamic biochemical transformation. Thus, oxygen ixptake 

 increases rapidly as does the lactic acid production ( Greengard and Mc- 

 Ilwain, 1955; Tyler and Van Harreveld, 1942). Marked increase in enzyme 

 activities occur; there is a threefold increase in the respiratory enzymes suc- 

 cinic dehydrogenase and cytochrome oxidase during the initial 2 weeks and 

 a fivefold increase of adenosine triphosphase activity, believed to be involved 

 in making energy available to the cell to accomplish its differential growth 

 (Potter, ct al., 1945; Flexner, 1953). Increased cholinesterase and pseudo- 

 cholinesterase activity is similarly observed i Elkes and Todrick, 1955). The 

 water content decreases significantly and levels of proteins and phosphatides 

 are beginning to approach adult levels. Howe\er, the cerebrosides which are 

 importantly related to myelin formation accimiulate only 119^ of the adult 

 weight by the 19th day (Folch-Pi, 1955). Deoxyribonucleic acid content 

 which continues to accumulate during this period increases fourfold between 

 the 2nd and 16th postnatal days, after which further increase is hardly no- 

 ticeable (Mandel and Bieth, 1951 ). These examples amply demonstrate that 

 the x-irradiation may well alter these manifold biochemical changes occur- 

 ring in the rat biain shortly after birth. 



Summary 



Newborn rats ranging in age from 8 hours to 15 days received single doses 

 of x-radiation to the head only. Doses of 125 r, 300 r, 500 r, and 1.000 r 

 were administered. One group of animals was sacrificed at 48 to 72 hours 

 after irradiation, and another group was autopsied at ages ranging from 14 

 days to 15 months. 



The study demonstrated the development of a marked radioresistance in 

 the brain by the 3rd postnatal week, compared to the easily damaged brain 

 of the rat during the first postnatal week. The development of this relative 

 radioresistance following the first week of life was abrupt. The radiosensitiv- 

 ity was manifested by an increased mortality, retarded growth, retarded 

 brain size, production of cataracts, and abnormal neurologic signs in the 



