Further Studies on the Response of the Small Intestine to Continuous Irradiation

Rats were irradiated continuously at 350 rads/day by a ¹³⁷ Cs source . The length of (G2 + 1/2M), the part of the cell cycle between the end of DNA synthesis and division, was measured in the epithelial cells of the jejunum. It increased on the first day of irradiation and then pratially recovered. The rate of cell production in the crypt showed a simultaneous dip and recovery. In another experiment mice were irradiated continously at 325 rads/day for 20 hours, 2 days, or 3 days. They then received one of a graded series of doses given at 30 rads/min to establish median doses lethal in 5 days ( LD₅₀₍₅₎ ). Only the 2-day group had a significantly lower LD₅₀₍₅₎ than the control group (P < 0.05). These results support the concepts that (1) there is a partial recovery in proliferative activity in the continuously irradiated intestine after the first day and (2) there is little damage accumulated in the intest...