Resident peritoneal cells of red sea bream Pagrus major

We found abundant peritoneal cells in normal red sea bream Pagrus major. The cells consisted of three cell types: 1) large round cells with a peripherally located small nucleus and pale cytoplasm (large cells), 2) medium-sized cells with a round or oval nucleus and eosinophilic granules in cytoplasm (eosinophilic cells), 3) medium-sized cells with a large round or oval nucleus and basophilic cytoplasm (basophilic cells). Electron microscopically, large cells had many large and slightly electron-dense granules, some of which were degranulated. Eosinophilic cells had abundant granules with a definite electron-dense core in the center or periphery of granules. Nuclei of the cells were polymorphic. Only eosinophilic cells had peroxidase activity and were stained strongly with Sudan black B, these characters are principal properties of vertebrate neutrophils. As there were neutrophilic or heterophilic granulocytes with similar morphology and staining properties to the peritoneal eosinophilic cells in blood smears and head kidney imprint preparations, we classified these eosinophilic cells as neutrophils. Basophilic cells had slender microvilli, micropinocytotic vesicle-like vacuoles, and bundles of microfibrils. Mitosis was often observed, especially in wild fish. These cells tended to adhere to slide glasses and showed phagocytotic activity to Latex beads. These data indicated the cells were resident peritoneal macrophages.