CELL-KINETICS IN RAT SMALL-INTESTINE AFTER EXPOSURE TO 3 GY OF GAMMA-RAYS AT DIFFERENT TIMES OF THE DAY

Qualitative and quantitative morphological changes in rat jejunum were studied after a whole-body exposure to 3 Gy of gamma rays. Four group s of animals were irradiated at different times of the day, namely mid night, 06.00, 12.00 and 18.00 hours. The number of epithelial cells, l abelling and mitotic indices were evaluated in crypt sections and the spatial distribution of S-phase cells was determined. At 12 h after ir radiation a marked reduction was observed in all parameters, but the p roliferative activity was restored quickly and at 36 h after irradiati on the values were significantly higher than the controls. The frequen cy distribution of labelled cells at different positions in the crypt was reduced at 12h but a clear expansion of S phase cells to positions near to the crypt villus junction was observed during the recovery ph ase. The animals irradiated at different times of the day showed a sim ilar general postirradiation response in the number of cells along the side of the crypt, labelling and mitotic indices and in the distribut ion of S phase cells along the crypts. It is worth noting that the ani mals exposed at midnight had a distribution of S phase cells similar t o controls at 72 h post-irradiaiton, i.e. earlier than the other group s.