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 of the kidney, thymus and spleen in the turbot Scophthalmus maximus, from hatching until the end of metamorphosis. Primordial haemopoietic stem cells are first observed in the pronephric kidney very early after hatch and rapidly differentiate into different cellular types. The spleen develops later, soon becoming rich in blood capillaries, red blood cells and thrombocytes. The thymus is the last lymphoid organ to appear but shows a quick development. This organ seems to originate from haemopoietic stem cells migrating from the head region of the kidney. Lymphoid organs become lymphoid in the sequence thymus, kidney and spleen, Although a small number of lymphocytes appear only in the later stags, cellular types involved in non-specific defense mechanisms, such as macrophagic and reticular cells, originate in early stages. These observations suggest that non-specific systems may play an important role in the immunocompetence mechanisms of the turbot during early larval development.