Ultrastructural changes in the thymus of the turtle Mauremys caspica in relation to the seasonal cycle.

Changes in the ultrastructure of the thymus of the turtle Mauremys caspica, with special reference to its non-lymphoid components, were studied in relation to the seasonal cycle. The thymic cortex contains framework-forming epithelial-reticular cells and free macrophages, while the medulla includes, in addition, mature and presumptive pro-interdigitating cells. The ultrastructural features of these cells are generally similar to those described for non-lymphoid components of the mammalian thymus. The turtle thymus undergoes cortical involution in spring, with recovery periods in May-June and during autumn. A moderate involution occurs in winter. At the beginning of spring, cortical (but not medullary) epithelial-reticular cells show degenerative changes, probably related to high levels of circulating testosterone. In spring and autumn, mature interdigitating cells are absent, but macrophages, monocytes, and pro-interdigitating cells are found. During May-June, the cortical epithelial-reticular population recovers and macrophages, monocytes, and interdigitating cells are actively phagocytic. In summer, the epithelial-reticular cells in both cortex and medulla display normal ultrastructural features; mature and immature interdigitating cells are absent and some macrophages are detected occasionally. The results suggest that non-lymphoid components of the reptilian thymus can play a role in governing T-lymphocyte differentiation, and that the thymic cortex and medulla exhibit different cycles of seasonal activity.