MELANOMACROPHAGES OF THE SEAWATER TELEOSTS, SEA BASS (DICENTRARCHUS-LABRAX) AND GILTHEAD SEABREAM (SPARUS-AURATA) - MORPHOLOGY, FORMATION AND POSSIBLE FUNCTION

Melano-macrophages in the head-kidney, spleen and liver of sea bass an d gilthead seabream have been investigated by means of light and elect ron microscopy, histochemistry and phagocytic assays. The results demo nstrate the presence of both free and clustered melano-macrophages (me lano-macrophage centres), with similar ultrastructural features. These large cells are PAS-, hemosiderin- and melanin-positive, and contain large, alkaline- and acid phosphatase-positive lysosomes, whose reacti on intensity depends on the amount of accumulated pigment. The relatio nship between the cytochemical features of these lysosomes and the cap acity of the melano-macrophages to phagocytose bacteria and latex bead s, has been studied. The large melanomacrophage centres have a capsule of flattened cells, whose ultrastructural and cytochemical features a re similar to fibroblast-like reticular cells. Melanin is the main acc umulated pigment. A subpopulation of head-kidney mononuclear phagocyte s engulfs melanin associated with cell debris. The relationship betwee n the origin of the melano-macrophage pigment and the ability of monoc ytes/macrophages to phagocytise the melanin from melanocytes, is consi dered. The origin and possible function of melano-macrophage centres a re discussed.