Early effects of x-irradiation of the cerebellum in infant rats: decimation and reconstitution of the external granular layer.

The cerebellum of rats was irradiated daily with 200 r x-ray from the day of birth, with the number of exposures ranging from one to ten. The animals were killed 2 and 24 hours, and 4 days after the last exposure, and at the constant ages of 8 and 10 days. The cell population of the external granular layer was drastically reduced 24 hours after irradiation with a single dose of 200 r, and it was subtotally and maximally destroyed 24 hours after the second exposure. In animals exposed up to 5 X 200 r, recovery of the external granular layer was evident 4 days after the last irradiation; the extent of recovery was inversely related to the number of doses received. The width of the external granular layer was normal or supernormal by 10 days of age in all these groups, though planimetric measurements showed that the total area occupied by this proliferative matrix was subnormal (due to the decreased surface area of the cerebellum) and it was inversely related to the number of doses received. Some evidence was obtained that after some delay the recovered cells differentiated, and led to the development of the molecular and internal granular layers. Several abnormalities were noted in the morphology of the developing cerebellar cortex after exposure to a higher number of x-ray doses, such as abnormal folding of the cerebellar cortex and the massing and disoriented growth of Purkinje cells, and explanations were offered for these effects.