The effect of proliferative status and clonogen content on the response of mouse jejunal crypts to split-dose irradiation

The rate of cell production in hierarchical tissues is affected by the differentiation probability after each clonogen division and the frequency with which divisions take place. We have modified the latter by a high-fiber diet, which caused an increase in the BrdUrd labeling index (LI) in jejunal crypts of mice, and have tested for a change in tolerance using the in vivo colony assay. Clonogens were counted using the in vivo colony assay of crypt regeneration with Poisson correction. The LI was estimated by counting BrdUrd-labeled cells in longitudinal sections of complete crypts. Arrest in mitosis induced by injection of paclitaxel was used to test for a difference in the rate of crypt cell production in mice fed low- and high-fiber diets. Split-dose studies were used to test whether the change in proliferative status of the crypts was accompanied by changes in either the number of clonogens per crypt or their radiosensitivity, or an increased proliferative response to radiation-induced cell killing. We found an increase in the crypt LI induced by the high-fiber diet was 15-25% and was dependent on the time of day. The data on arrest in mitosis did not demonstrate a difference in cell production rates based on diet, possibly because of insufficient resolution of the assay. We conclude that the high-fiber diet had no effect on radiosensitivity, the number of clonogens per crypt (again, within the resolution of the assay) or the 'repopulated dose,' the dose represented by the shift in the close-response curve for 2.5 days relative to that for 6 h. When the number of clonogens at the start of rapid proliferation was different (on account of different first doses), the repopulated dose was the same when 5 Gy X rays was given first but was higher for the animals on the high-fiber diet when 12 Gy was given first. The high-fiber diet caused an increase in the LI in the crypts that was not accompanied by any change in radiosensitivity or, within the resolution of the assay, numbers of clonogens per crypt. The increased LI also did not result in an increase in clonogen repopulation between split (and equal) doses. However, in split-dose experiments where the first dose was higher and as a consequence the number of clonogens at the start of the proliferative response was lower, there was evidence of a higher rate of clonogen production with the high-fiber diet than with the low-fiber diet.