Intestinal cell proliferation during fractionated abdominal irradiation

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Abstract
In the mouse, at normal steady state of cell proliferation, the compensatory proliferative response to intestinal irradiation is such that when radiation exposures totalling 1,000 R are concentrated over the first few days of the week, summated proliferative activity for the entire week is near control levels. Symmetrically distributed exposures over a five-day treatment week (200 R daily, and especially 333 R on Monday, Wednesday and Friday) result in depressed levels of overall weekly proliferation. In these instances, the weekend break is particularly crucial. Similar results were obtained when the one-week measurement period was inserted between the third and fifth week of abdominal therapy, except in this instance, 200 R per day did not result in sub-control levels of proliferation, whereas 333 R on M, W and F, continued to do so. The intestine seems able to maintain its barrier epithelium for extended periods of diminished cell input, provided such is not too severe and that it seems from decreased cell production rate per crypt rather than from crypt attrition. A partial explanation for this relative tolerance is given by the finding that the vast majority of proliferative cells, even those irradiated and rendered permanently incapable of further division, succeed in migrating up the villus and hence help to maintain a barrier epithelium. In that sense, nearly all cell divisions become useful, even in the face of repeated exposures.