Cytochemical characterization of leucocytes from the seawater teleost, gilthead seabream (Sparus aurata L.).

The cytochemical characterization of head-kidney and peripheral blood leucocytes of gilthead seabream (Sparus aurata L.) was studied by light and electron microscopy. Neutrophilic granulocytes show some cytoplasmic granules, which are positive for alkaline phosphatase and peroxidase but acid phosphatase negative. The scarce granules found in the cytoplasm of the circulating neutrophils and their cytochemical features seem to be indicative of an immature stage. Acidophils are also alkaline phosphatase and peroxidase positive at pH 11.0. They are strongly positive for acid phosphatase and acid phosphatase activity may thus be considered a cytochemical marker to characterize and differentiate neutrophilic from acidophilic granulocytes in this fish species. Three granule populations are characterized in the cytoplasm of the gilthead seabream acidophils: the first is positive only for peroxidase and the second contains a dense core with acid and alkaline phosphatase activities, surrounded by a thin peroxidase positive electron-dense halo. The third granule type contains an eccentric core, which is strongly positive for acid and alkaline phosphatase and peroxidase. As regards their cytochemical features, the first and second granule types seem to correspond respectively to the azurophilic and specific granules found in acidophils of mammals and could be involved in phagocytic processes, thus playing an important microbicidal role in this species. The monocytes, monocyte-macrophages and macrophages show different cytochemical features. The first have scarce acid phosphatase-positive lysosomes, while blood monocyte-macrophages and macrophages are positive for acid and alkaline phosphatases and for peroxidase; the monocyte-macrophages show scarce lysosomes.