FETAL X-RAY EFFECTS: CEREBRAL CORTEX 39 



tcrs as a frame of reference foi properly exaluatinti the chemical properties 

 and metabolic acti\ity of nerxous tissues. We emphasize this point because 

 of the increasiny interest in chemical and metabolic effects of irradiation. It 

 is our belief that more significant results will be derixed from such in\e.stiga- 

 tions if they are based on the cellular composition of the tissue, rather than 

 on tlu' more commonly used biochemical referents. 



With reference to the beha\ioral studies, since the sur\i\in<; cells in our 

 irradiated preparations in area 2 appear cytologically normal, it appears that 

 the explanation of the observed deficits in learning ability must rest on the 

 assumption that the sur\i\ino cells possess physiologic deficits not amenable 

 to measurement with quantitati\e histologic or ordinary cytologic methods, 

 or that the deficit is entirely or in part the result of the numerical deficit in 

 cells, or both. It is of considerable interest to note. howe\ er. that the loco- 

 motor deficit obserxcd in the youngest animals tested is largely cleared up in 

 the oldest group. Lonu-tcrm studies are now in progress to determine if a 

 similar phenomenon may occur with relerence to learning ability in older 

 animals following fetal iiiadiation. 



Summary and Conclusions 



Exposuie to fiactionated doses of total-body x-irradiation at 100. 200. and 

 300 r (60 r per minute) o\er a peiiod of 5 to 7 days alter the 10th day of 

 gestation results in no significant difference in neuron packing density in 

 area 2 in the first 20 days of postnatal life. Nemoglial packing density and 

 the neuroglial neuron index are higher in the 300 r group in 1 -day-old 

 animals than in controls, but at later stages the differences are not signifi- 

 cant. Mean \alues for neuron soma \oknne in ]-. 5-. and 10-day-old irra- 

 diated animals are consistently lower than in nonirradiated series, but are 

 higher at the 20-day stage in irradiated than in the control animals. The 

 differences are considered to be within the range of error of the methods 

 employed. The nucleocytoplasmic ratio decreased steadily fiom the 1st to 

 the 20th days, and no significant differences occurred between irradiated 

 and control series. 



Brain weight \aried in\eisely with irradiation dose at the 10- and 20-day 

 stages and in 200 and 300 r groups at the 1- and 5-day stages. Clortical 

 thickness in the 100 r group was not significantly different from controls. In 

 the 200 r and 300 r groups, however, cortical thickness varied inversely with 

 dose le\el, and significant diflerences betvyeen the 200 and 300 r groups and 

 between the irradiated and control groups occurred at the 20-day stages in 

 all areas. It is concluded that the doses of fetal x-irradiation employed pro- 

 duced no significant dilTerences in volumetric or density relationships or 

 general cytoarchitecture between irradiated and control animals in area 2 

 but resulted in a significant decrease in total number of cells in this zone. 



