PHAGOCYTIC DEFENSE-MECHANISM IN SEA BASS (DICENTRARCHUS-LABRAX L) - AN ULTRASTRUCTURAL-STUDY

Background: The ultrastructure of the phagocytic process in fish has n ot been established in spite of the significant morphofunctional diffe rences detected in the fish immune system with respect to the basic im munological pattern in vertebrates. We report the ultrastructure of th e bacterial phagocytic defence mechanism in sea bass (Dicentrarchus la brax L.). Methods: Head-kidney, blood, and peritoneal exudate leukocyt es were challenged with Aeromonas salmonicida and Escherichia coli and processed for transmission electron microscopic study. Results: Macro phages challenged with bacteria showed changes in the cell outline, in the chromatin pattern, and in the ultrastructural features of the cyt oplasm as a consequence of an activation process. The phagocytic proce ss consists of the following: 1) Bacteria-macrophage contact. One or m ore spot contacts between the bacterial wall and the phagocyte membran e are observed. 2) Bacteria engulfment. Slight depressions, membrane i nvaginations, or cytoplasmic processes are formed at the phagocyte sur face. Macrophage processes occasionality surround the bacteria, overla pping and roaming parallel, or a single, long pseudopod encircles a ba cterium several times. 3) Endocytic vesicle formation. Macrophages sho w 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 posse ss the mechanisms necessary to both engulf and kill bacteria. Cellular and subcellular events in the morphology of phagocytosis and lysosoma l dissolution of bacteria fit the general pattern described for mammal s. (C) 1994 Wiley-Liss, Inc.