EFFECTS OF HEAD IRRADIATION IN NEWBORN RATS 69 



trauma, or oeneralized radiation illness, but who were assumed to have been 

 radiated to the head alone, revealed that the most extensive involvement of 

 the brain was in children ( Uchimura and Shiraki, 1952). 



The relatively abrupt change in the degree of radiosensitivity of the neo- 

 natal rat brain occurs at a time when unique morphologic, functional, and 

 biochemical changes are taking place, and this seems to present an interest- 

 ing temporal correlation with the findings reported in this study (Waelsch, 

 1955: Richter. 1957). 



The general growth rate of the neonatal rat brain is reflected by the five- 

 fold increase in weight by the end of the 2nd postnatal week and this repre- 

 sents almost 809r of the weight of the adult brain (Potter ct al., 1945; Folch- 

 Pi. 1955). The cerebral cortex is gaining weight proportional to the growth 

 of the brain as a whole, but the cerebellum is gaining nearly three times as 

 much weight, and the brain stem is accumulating only one-half its birth 

 weight during the same period ( Sugino, 1917). However, the vascularity of 

 the brain undergoes little change during the first 5 days, but between the 5th 

 and 10th day a definite increase in vascular richness occurs. After the 10th 

 day the density of the capillary bed increases rapidly, and a concomitant 

 richness of the capillary bed increases in oxidase content and mitochondria 

 simultaneously i Campbell, 1939). An example of the neuronal difTerentia- 

 tion taking place during the neonatal period is presented by the change in 

 the packing density of the neurons in the cerebral cortex. This density de- 

 creased rapidly between the 3rd and 4th day after birth and then more 

 slowly, "no change taking place after the 17th day" ( Haddara, 1956). This 

 would indicate an increase in cytoplasmic constituents along with the devel- 

 opment of a more elaborate cortical dendritic system. 



It is during the first 2 week period postnatally that the electroencephalo- 

 gram becomes more regular and assumes the characteristics seen in adult 

 rats 1 Grain, 1952). The ability of the rat to withstand anoxia is greatest in 

 the immediate postnatal period, and shortly after birth during the first 5 to 

 6 days there is a loss of tolerance to anoxia (Fazekas ct al., 1941 i. 



Immature budding vessels may well be more differentially sensitive to nox- 

 ious agents than fully developed ones. It has been shown that growing retinal 

 vessels during the first postnatal week, but not the choroidal vessels, constrict 

 when exposed to oxygen and may e\en be obliterated with prolonged expo- 

 sure ' Ashton and Cook. 1954). .\s the vessel reaches maturity, which takes 

 place at about the 8th postnatal day in the rat. it gradually loses its ability to 

 constrict when exposed to oxygen. In this regard, it is felt that the oxygen ef- 

 fect is an appropriate example since oxygen enhances ionizing reactions 

 (Dowdy et al., 1950) . Moreo\er, the best protectors against x-rays, i.e.. cystea- 

 mine and cysteine, sulfhydryl containing amino acids, are also the best protec- 

 tors of mammals against oxygen poisoning (Bacq and .Alexander. 1955) . How- 



