Within 10 minutes after intravenous injection, horseradish peroxidase has been found in the lamina propria and in the intestinal epithelium, both in intercellular spaces and within the cytoplasm of epithelial cells. Examination of intestines of mice subjected to 3 krads of X-irradiation and sacrificed at 24-hour intervals up to 4 days revealed that most intact capillaries remained patent and were permeable to peroxidase. It was not possible to characterize damage to endothelial cells on a morphological basis alone, but definite changes in permeability to peroxidase were demonstrated. Neither capillary nor epithelial basement membranes seemed to constitute a significant barrier to the extravascular diffusion of peroxidase, but at the time of villus denudation, on day 4, epithelial basement membranes were disrupted in the denuded areas. Diffuse staining of epithelial cell cytoplasm by peroxidase reaction product and less accumulation of it in intercellular spaces within the epithelium was observed on days 3 and 4 postirradiation. Neither evidence of leakage through, nor secretion by, epithelial cells of peroxidase into the gut lumen nor localization within lacteals was detected. Some of the enzyme probably was secreted or diffused into the lumen before denudation occurred but escaped detection by being washed away by the tissue fixative.

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