A Light and Electron Microscope Study on the Phagocytosis of Leucocytes in Rockfish and Rainbow Trout

Abstract

Identification and characterization of leucocytes in two teleosts, rockfish Sebastes schlegeli and rainbow trout Salmo gairdneri were performed based on their morphology and phagocytic ability with bacteria, Escherichia coli and Staphylococcus aureus and zymosan particles. Cytochemical test for peroxidase was also applied to the leucocytes.
Light and electron microscopy revealed that both neutrophils and monocytes in the two teleosts actively phagocytized bacteria. Thrombocytes ingested bacteria, but their phagocytic activity was very low.
The process of phagocytosis was observed with neutrophils and monocytes. Fusion of cyto-plasmic granules into phagosomes was observed. Large phagosomes were present in neutrophils but entirely (rockfish) or almost absent (rainbow trout) in monocytes. Cytoplasmic granules in neutrophils of both fishes were positive for peroxidase. The peroxidase reaction product was also observed in phagosomes. Therefore, the process of phagocytosis was considered to be similar to that known with mammalian leucocytes.
The fate of bacteria phagocytized by fish leucocytes differed to that in mammals. At even 12 h (rockfish) or 6 h (rainbow trout) after incubation, bacteria in phagosomes were almost intact and they seemed to be digested slowly compared with those ingested in mammalian leucocytes.