| Summary: | The peripheral blood cells, kidney hematopoietic cells, and eosinophilic granule cells (EGC) of the different tissue sites, as well as related tissues of Atlantic salmon, rainbow trout and goldfish were studied in an attempt to clarify the existence, identity, and function of the cell types by the routine hematological and histological methods, certain special histochemical techniques, enzyme cytochemistry and lectin histochemistry. The blood and hematopoietic cell lineages of the two salmonids were similar and no coarse granulocytes were found. One type of coarse granulocyte was present in goldfish which was different from the mammalian eosinophils and basophils with respect to the enzyme activity and cellular components. Eosinophilic granule cells of different tissue sites of a particular species were classified in different lineages according to their lectin binding patterns, histochemical properties, and origins. The gastrointestinal EGC of the three species studied were very similar to one another in the carbohydrate residues of the glycoconjugates of the cytoplasmic granules. The pathogenesis and microscopic lesions of infectious hematopoietic necrosis (IHN) by artificial infection in Atlantic salmon yearling and rainbow trout fingerlings were described. Similar microscopic changes were present in most of the affected organs (kidney, spleen, liver, and pancreas) of both species except for the gastrointestinal tract and gills. The sinusoidal lining cells of kidney hematopoietic cords were found to be the initial targets of IHNV. The pathological changes of these lining cells corresponded well with the major IHN clinical pathological and histopathological changes in these two salmonids. No degeneration or necrosis of gastrointestinal EGC occurred in the Atlantic salmon IHNV challenge group. EGC in necrotic gill tissues of the rainbow trout appeared after IHNV challenge and were a cell type other than the gastrointestinal EGC. Lesions in the gills and gastrointestinal tract indicated that both organs were entry sites for IHNV. The severity of renal lesions was highly correlated with that of hematopoietic tissues. The lesions of renal corpuscles lend support to the hypothesis that fish death is related to the imbalance of electrolytes and body fluid.
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