Kinetics of radiation-induced apoptosis in the cerebellum of 14-day-old rats after acute or during continuous exposure.

We have studied, by histological methods, cytological progression, frequency and distribution of apoptosis in the external granular layer of the cerebellum after whole-body irradiation of 14-day-old rats by gamma-rays from 60 Co. After acute exposure to 0.25, 0.5, 1.5 and 3 Gy (18 cGy/min), the duration of the apoptotic process gradually increased with dose from 6-9 h after 0.25 Gy, to > 24 h after 3 Gy. Up to 1 Gy, maximal frequency was found 6 h after exposures, and at this postirradiation time a linear increase in apoptosis with dose was observed. No effect of dose-rate on apoptosis induction could be demonstrated 6 h after delivering 1 Gy at dose-rates from 2.2 to 18 cGy/min. Continuous irradiation at 1.8 cGy/h induced a gradual increase of apoptosis that remained at a plateau value of about 3% from 15 to 29 h (controls 0.12%, SD = 0.07) and then gradually decreased to 1% at 53 h. At this time the mitotic index was similar to that measured in controls. Apoptosis occurring 3 h after acute irradiation, confined to proliferative cells, was only observed for doses of 1.5 and 3 Gy.