SEASONAL-CHANGES IN THE LYMPHOID ORGANS OF WILD BROWN TROUT, SALMO-TRUTTA L - A MORPHOMETRICAL STUDY

A morphometrical evaluation was made of the seasonal changes affecting the numbers of lymphocytes in the thymus, spleen and pronephros of wi ld brown trout, Salmo trutta, while the size of the thymus and the thr ee thymic zones were also determined. Results reveal statistically sig nificant changes throughout the year in the number of lymphocytes in t he lymphoid organs studied. The spleen and pronephros have similar ann ual patterns of lymphocyte distribution with high numbers in two seaso ns, spring and autumn, and two periods of lymphoid involution in winte r and summer. The highest numbers of thymocytes occur in trout caught in May and August, and the lowest in winter. In addition to normal lym phocytes, degenerated lymphoid cells that show pale cytoplasm devoid o f cell organelles, also occurred in all the lymphoid organs. A negativ e correlation exists between the numbers of normal lymphocytes and tha t of degenerated lymphoid cells. The thymic size, as well as that of t he subcapsular, inner and outer thymic zones, undergo very significant changes over the year. We discuss the relevance of cell proliferation , cell migration and in situ cell death for the circannual variations observed in the cell content of trout lymphoid organs, together with t he possible causes. (C) 1998 Elsevier Science B.V. All rights reserved .