Response of eosinophilic granule cells of gilthead seabream (Sparus aurata, Teleostei) to bacteria and bacterial products.

Eosinophilic granule cells in the gills and peritoneal exudate of gilthead seabream (Sparus aurata L.) are characterized by the presence of prominent eosinophilic granules in their cytoplasm and are here described for the first time. The oval granules of these cells contain an electron-dense inclusion surrounded by a less dense filamentous matrix and are peroxidase- and acid phosphatase-negative. Unlike other granulocytes of gilthead seabream, eosinophilic granule cells do not ingest bacteria in vivo. The intraperitoneal injection of extracellular products of Pasteurella piscicida induces mobilization of eosinophilic granule cells to the blood and other tissues and causes changes in their structure. Shortly after injection, the granules of eosinophilic granule cells become swollen and some fuse with the cell membrane. From 7 h post-injection, many eosinophilic granule cells in the gills degenerate and are then phagocytosed by macrophages, which are especially abundant after 24 h. From 24 h to 72 h, eosinophilic granule cells from the gills contain abundant autolysosomes together with granules of a normal morphology.