title: Ultrastructural changes in the thymus of the turtle Mauremys caspica in relation to the seasonal cycle.
abstract: 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.
authors.affiliation: Departamento de Biologia Celular, Facultad de Ciencias Biologicas, Universidad Complutense, Madrid, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Leceta
authors.forename: E
authors.initials: E
authors.lastname: Garrido
authors.forename: M
authors.initials: M
authors.lastname: Torroba
authors.forename: A G
authors.initials: AG
authors.lastname: Zapata
mesh_terms: D000818:Animals; D008854:Microscopy, Electron; D012621:Seasons; D013950:Thymus Gland / Q000502:physiology* / Q000648:ultrastructure; D014426:Turtles / Q000033:anatomy & histology / Q000502:physiology*