Cell Proliferation Studies in the Intestinal Epithelium of the Rat: Response to Continuous Irradiation

A proliferative steady state is established in the rat jejunum within 2 to 3 days of commencing whole-body irradiation at 350 rads/day with 137Cs 7-rays. The parameters measured in this study were the labeling and mitotic indices of cells in various parts of the crypt at frequent intervals after starting irradiation and during recovery; also the proliferative cell cycle and turnover time of mature cells after 5 days of irradiation. It was concluded that the cell cycle starts to lengthen within 6 hours of starting irradiation, reverses this trend after a day, and is finally shorter than normal. From the disparity between the labeling index and cell cycle data in the lower part of the crypt after 5 days, it was deduced that about 38 % of the cells there are sterilized by radiation but quite possibly able to differentiate. Within 6 hours of ceasing irradiation an increase in labeling index occurs which denotes rapid initiation of recovery by unknown mechanisms. It is proposed that intestinal cells have a greater ability than bone marrow cells to repair low-LET damage, which would account for much of the difference in radiosensitivity of these tissues under continuous irradiation.