A SEQUENTIAL LIGHT-MICROSCOPIC AND ULTRASTRUCTURAL-STUDY ON THE UPTAKE AND HANDLING OF VIBRIO-SALMONICIDA IN PHAGOCYTES OF THE HEAD KIDNEY IN EXPERIMENTALLY INFECTED ATLANTIC SALMON (SALMO-SALAR L)

The uptake and handling of Vibrio salmonicida in phagocytes of the hea d kidney of Atlantic salmon (Salmo salar L.) were evaluated by light a nd electron microscopy, including in situ identification of the bacter ium by immunolabeling at the light microscopical and the ultrastructur al level. Fish were injected with live bacteria, and 4, 24, 48, and 72 hours after inoculation, samples were collected after perfusion fixat ion. Morphologically, the most prominent change in the course of the e xperiment was an increasing number of intrasinusoidal, endothelial cel l-adherent phagocytes and the elevated number of interstitial melanoma crophages. Immunohistochemically, bacterial antigens were initially id entified in intrasinusoidal phagocytes, and at 24 hours post-infection in endothelium-adherent phagocytes and intrasinusoidal melanomacropha ges. Later, (48 and 72 hours postinfection), the interstitial melanoma crophages were also found to harbor bacterial antigen. Ultrastructural ly, bacteria were identified in phagosomes in intrasinusoidal phagocyt es, and morphological findings also indicated an increased cellular de gradation, including autophagocytosis. Immunoelectron microscopy indic ated that bacterial antigens were associated with melanomacrophages, s pecifically in their electron-dense cytoplasmic granules. These findin gs indicate that intrasinusoidal phagocytes and melanomacrophages part icipate in the rapid and active clearance of particulate material from the circulation, i.e., pathogenic microorganisms, and in the scavengi ng of cellular degradation products. The process of formation of melan omacrophages and their possible function is discussed.