ONTOGENY OF THE LYMPHOID ORGANS IN THE TURBOT SCOPHTHALMUS-MAXIMUS - A LIGHT AND ELECTRON-MICROSCOPE STUDY

A histological and ultrastructural study was made of the development o f the kidney, thymus and spleen in the turbot Scophthalmus maximus, fr om hatching until the end of metamorphosis. Primordial haemopoietic st em cells are first observed in the pronephric kidney very early after hatch and rapidly differentiate into different cellular types. The spl een develops later, soon becoming rich in blood capillaries, red blood cells and thrombocytes. The thymus is the last lymphoid organ to appe ar but shows a quick development. This organ seems to originate from h aemopoietic stem cells migrating from the head region of the kidney. L ymphoid organs become lymphoid in the sequence thymus, kidney and sple en, Although a small number of lymphocytes appear only in the later st ags, cellular types involved in non-specific defense mechanisms, such as macrophagic and reticular cells, originate in early stages. These o bservations suggest that non-specific systems may play an important ro le in the immunocompetence mechanisms of the turbot during early larva l development.