Irradiation of the cerebellum in infant rats with low-level x-ray: histological and cytological effects during infancy and adulthood.

We examined the morphological consequences of focal irradiation of the cerebellum with successive daily doses of low-level X-ray in lo-, 30-, and 90-dayold rats. The subtotally eliminated external granular layer was reconstituted by 10 days in animals exposed to l-2 X 200 r and there was little or no reduction in the total area of the cerebellar cortex and its various layers at 30 and 90 days. With higher number of successive daily doses there was (in the affected lobes) decreasingly less regeneration of the external granular layer at 10 days, and there was a proportional reduction in the areas occupied by the molecular and internal granular layers at 30 and 90 days. The reduction in the population of granule cells in adult rats was also proportional to the number of daily exposures during infancy. These lasting effects were attributed not to greater damage done to the germinal layer by the higher number of daily exposures but to a postponement of recovery and hence reduction in the time left for regeneration when cerebellar neurogenesis comes to an end (about 21 days). Even with the highest number of exposures, the number of Purkinje cells was not affected. In the animals exposed to more than two doses of ZOO r, lasting morphological and cytological changes were produced. The abnormalities could be traced to events associated with the time course of cell death and regeneration in the external granular layer and to the autonomous but disoriented growth of the Purkinje cells.