Intestinal Proliferation after Multiple Fractions of Gamma Irradiation

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Abstract
The histological appearance of intestinal mucosa in BALB/c+ mice was studied after fractionated irradiation. Daily doses of 200 and 350 rad were given to the abdomen to investigate recovery mechanisms. After 200 rad/day (1000 rad/wk), the average number of nuclei per tangential section of crypts increased in 2 steps and reached 117, 112, 151, 140 and 145% of the control value after 2, 3, 4, 5 and 6 wk. The average numbers of mitoses and of 3H-thymidine-labeled nuclei per crypt also increased to 155, 160, 207 and 221% and 190, 140, 131, 192 and 230% of the controls, respectively. There was an increase in the stem cell compartment, which was estimated at 1.4 times its normal value after 4-6 wk. The length of the mitotic cycle (Tc) was measured using the usual 3H-thymidine labeling technique after 5000 rad/5 wk, given in daily fractions of 200 rad. Tc was reduced from 14 to 9 h, this reduction mainly concerning the G1 phase (from 5 to 1 h) and to a smaller extent the S phase (from 7 to 6 h). Intestinal mucosa tended to compensate for cell destruction by 2 mechanisms: a reduction in the duration of the mitotic cycle and an increase in the size of the stem cell compartment. At 200 rad/day, an important compensation was achieved and the animals were able to tolerate the irradiation for about 6 wk. At 350 rad/day increased cell production can no longer compensate for cell destruction and the animals died after the 3rd wk.