Abstract
Inbred F344/DuCrj rats aged 0, 3, 6, 10, 13 and 17 days were exposed to a single wholebody X-irradiation at several dose levels ranging from 3 to 100 rad. They were sacrificed 6 hours after exposure, and their cerebella were removed and processed for microscopic observation. In the mid-sagittal sections through the vermis, the incidence of pyknotic cells among matrix cells in the external granular layer along the primary fissure was estimated. The peak incidences of pyknotic cells were noted following exposure at 10 days of age to any dose levels of irradiation or sham-radiation. For every age-group of treatment the following results were obtained. (1) The incidence of pyknotic cells following exposure to the low dose of 3 rad was significantly higher than for sham-radiation. (2) The incidence was raised with the increased dose (rad); the diagram closely resembles two straight lines with an inflection at 25 rad. (3) The estimated regression coefficient for 3 through 25 rad was smaller than for 25 through 100 rad.
