Pathologic characteristics of gut-associated lymphoid tissues and lymphocyte apoptosis in mouse intestine after neutron-and γ-irradiation

Objective: To compare the pathologic characteristics of gut-associated lymphoid tissues and lymphocyte apoptosis in neutron-irradiated mouse small intestines with those in γ-irradiated ones. Methods: Altogether 350 BALB/c mice were irradiated with different doses of neutrons or γ-rays, and were sacrificed on 6 h,12 h,125 d, 7 d, 14 d, 21 d and 28 d after irradiation and their total intestines were removed. Then the pathologic changes and death mode of lymphocytes in gut-associated lymphoid tissues were studied comparatively with light microscopy, electron microscopy and in situ terminal labeling method. Results: The basic pathologic changes of gut-associated lymphoid tissues after neutron irradiation included degeneration, apoptosis and necrosis of lymphocytes. The number of lymphocytes also decreased. There was no obvious regeneration after 4.0 and 5.5 Gy neutron irradiation, while after 2.5 Gy regeneration and recovery appeared, which were, there fore, dose-dependent. In the 2.5 Gy neutron group, the numbers of lymphocytes of intramucosal and submucous lymphoid tissues decreased, and karyopyknosis and a great quantity of nuclear fragments could also be observed at 6 h-3 d after irradiation. However, on the 3rd day regeneration of crypt epithelial cells appeared. On the 5th day hyperplasia of submucous lymphocytic tissues appeared, but recovery to normal level was not achieved till 14 d after irradiation. The basic pathologic changes after γ-irradiation were similar to that of neutron irradiation. Regeneration and recovery appeared in the 5.5 Gy group while no obvious regeneration in the 12.0 Gy group. The results of in situ terminal labeling indicated that at 6 h after irradiation the number of apoptotic cells in gut-associated lymphoid tissues of each group increased obviously, while in 4.0 Gy neutron group and 12.0 Gy γ-ray group it was more abundant. Conclusion: Both 2.5-5.5 Gy neutron and 5.5-12.0 Gy γ-ray irradiation can induce obvious injuries in gut-associated lymphoid tissues, and the effect of neutron is more severe man that of γ-rays under the same irradiation doses. Injuries of immune tissues are more severe and recovery of them are more slowly in comparison with those of intestinal epithelial tissue. More lymphocytic necrosis can be observed after neutron irradiation above 4.0 Gy, while after 5.5-12.0 Gy γ-irradiation apoptosis appears to be most serious. (authors)