Seasonal involution of gut-associated lymphoid tissue of the European ground squirrel.

Small intestine-associated lymphoid tissue of immature, male ground squirrels of the genus Citellus citellus L. was studied histologically throughout the annual cycle. Circannual morphological changes of the lymphoid cells in the lamina propria of the mucosa as well as of the Peyer's patches in the submucosa, were established. The changes consisted mainly in a gradual decrease in the proliferative activity of the cells in the autumn and in arrest of this activity in hibernating animals during the first phase of hibernation. An involution of the follicles and their germinative centres, as well as lymphocyte depletion in the Peyer's patche parenchyma, and a marked increase in the lymphocyte number of the lamina propria in hibernating ground squirrels were found. It was shown that the activation of the lymphoid cell proliferation in both compartments of the small intestine begins in hibernating animals in January.