In vitrodifferentiation of eosinophilic granular cells inRenibacterium salmoninarum-infected gill cultures from rainbow trout

Gill explants were maintained for one week in culture, and infected withR. salmoninarum. The explants were collected at two weeks post-infection and processed for ultrastructural examination. A marked tissue reaction occurred in the infected explants in contrast with the control (non-infected) cultures, partially resembling an inflammatory response, and involving a marked increase in the number of eosinophilic granular cells. Several stages were identified from non-granulated precursors to mature eosinophilic granular cells, the earlier ones being closely apposed to fibroblasts. The ultrastructural images suggest that mature eosinophilic granular cells showed a chronic, non-anaphylactic, degranulation. Although the major extracellular products ofR. salmoninarummay elicit the differentiation and degranulation of the EGCs, the possibility of these phenomena being cytokine-driven is discussed. These results demonstrate that eosinophilic granular cells are able to differentiatein situ, thus reinforcing the analogies between this cell type and mast cells.