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Renewal of the ileal epithelium has been studied in germ-free (GF) and conventional (CV) rats fed with a semi-synthetic diet in the following forms: a) pellets sterilized by autoclaving (Pel. Au. diet); b) powder sterilized by gamma irradiation; when given to the rats, its weight in water was added to make a paste (Pa. I. diet); c) powder treated as (b) but with 10 p. 100 lactose added (L. Pa. I. diet). Ileal epithelium renewal time was similar in the GF rats fed with the Pel. Au. and Pa. I. diets. It was shorter in the CV rats fed with these two diets than in the GF rats and also in the CV rats given the Pa. I. diet than in those given the Pel. Au. diet. In GF and CV rats fed with the lactose diet, L. Pa. I., ileal epithelium renewal time was similar. These different effects were mainly due to variations in the number of cells produced in Lieberkühn's crypts. They were not related to changes in the intestinal bile salts pool caused by the different diet treatments.

The length and diameter of enterocyte microvilli at mid-villus position were measured on electron-micrographs. The duodenum, jejunum and ileum of axenic (germfree) and holoxenic (conventional) inbred rats fed the same diet have been studied. The microvilli were significantly shorter in all these intestinal regions when the microflora was present. The decrease in microvillus length (due to the presence of microflora), expressed as a percentage of the length in axenic rat, was 5% in the duodenum, 9% in the jejunum and 18% in the ileum. This was not true of microvillus diameter where only the values found for the ileum of axenic rat were significantly lower, the others not differing among themselves. In conclusion, the presence of a total microflora led to a decrease in the length of enterocyte microvilli. These results have been discussed in relation with studies on renewal and enzyme activity in the intestinal epithelium.