Phagocytic defence mechanism in sea bass (Dicentrarchus labrax L.): An ultrastructural study

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Abstract
Background: The ultrastructure of the phagocytic process in fish has not been established in spite of the significant morphofunctional differences detected in the fish immune system with respect to the basic immunological pattern in vertebrates. We report the ultrastructure of the bacterial phagocytic defence mechanism in sea bass (Dicentrarchus labrax L.).Methods: Head‐kidney, blood, and peritoneal exudate leukocytes were challenged with Aeromonas salmonicida and Escherichia coli and processed for transmission electron microscopic study.Results: Macrophages challenged with bacteria showed changes in the cell outline, in the chromatin pattern, and in the ultrastructural features of the cytoplasm as a consequence of an activation process. The phagocytic process consists of the following: (1) Bacteria‐macrophage contact. One or more spot contacts between the bacterial wall and the phagocyte membrane are observed. (2) Bacteria engulfment. Slight depressions, membrane invaginations, or cytoplasmic processes are formed at the phagocyte surface. Macrophage processes occasionally surround the bacteria, overlaping and roaming parallel, or a single, long pseudopod encircles a bacterium several times. (3) Endocytic vesicle formation. Macrophages show one or more bacteria inside membrane‐bound cytoplasmic vesicles. (4) Phagolysosome formation. Some dense granules (lysosomes) fuse with the endocytic vesicle. (5) Intracelular killing/digestion. Bacteria inside the endocytic vesicles are observed both virtually intact or damaged at different digestion stages.Conclusions: Sea bass macrophages possess the mechanisms necessary to both engulf and kill bacteria. Cellular and subcellular events in the morphology of phagocytosis and lysosomal dissolution of bacteria fit the general pattern described for mammals.