Late radiation effects were investigated in the mouse small intestine after a daily fractionated
radiation treatment. Mice were given 14 X 3 Gy in 2 weeks over a partial abdominal irradiation field. There was evidence for late injury in the intestinal epithelium, the submucosa, and the subserosa. Late damage in the epithelium was shown histologically by a reduced crypt number and villus
atrophy at 3 and 6 months but not at 24 h after the end of treatment. The reduction in crypt number was significant in the ileum at 3 and 6 months after irradiation: 100 +/- 4 and 98 +/- 5 (SEM) per circumference, respectively, versus 132 +/- 3 and 146 +/- 6 in age-matched controls (P less than 0.01, t test). The mitotic activity in the crypts of the irradiated animals was significantly increased at all investigated times, suggesting a prolonged but insufficient compensatory response to maintain the mucosal integrity. The repercussion on intestinal epithelial function was, at least in part, reflected by a progressively reduced
body weight gain up to 5 g at 3 months
after treatment. The ability of the surviving crypt stem cells to form microcolonies after irradiation, however, was not impaired. Evidence for injury in the submucosa was provided from macroscopic and histological examination. Macroscopically, at 6 months
after treatment, narrowed and rigid bowel segments surrounded by fibrotic adhesions were observed, causing partial
intestinal obstruction. In addition, sometimes focal areas of
hemorrhage and
infarction in small bowel segments were present. Histologically, diffuse and pronounced submucosal
edema without increased
fibrosis was seen, together with markedly dilated small blood vessels in focal areas of macroscopic intestinal
infarction. The intestinal perfusion, as assessed by 86Rb extraction, was significantly but transiently reduced at 3 months after irradiation. These data suggest mainly late effects in the small intestine after this daily fractionated irradiation treatment. The reduced number of epithelial cells and the submucosal
edema are possibly mediated by
radiation injury in the intestinal microvasculature.