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 wild brown trout, Salmo trutta, while the size of the thymus and the three thymic zones were also determined. Results reveal statistically significant changes throughout the year in the number of lymphocytes in the lymphoid organs studied. The spleen and pronephros have similar annual patterns of lymphocyte distribution with high numbers in two seasons, spring and autumn, and two periods of lymphoid involution in winter and summer. The highest numbers of thymocytes occur in trout caught in May and August, and the lowest in winter. In addition to normal lymphocytes, degenerated lymphoid cells that show pale cytoplasm devoid of cell organelles, also occurred in all the lymphoid organs. A negative correlation exists between the numbers of normal lymphocytes and that of degenerated lymphoid cells. The thymic size, as well as that of the 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 the possible causes.