Fish granulocytes: Morphology, distribution, and function

Abstract

In the last 20 years interest in fish granulocyte identification, distribution, and function has burgeoned, as evidenced by the increasing number of reports in scientific literature. Neutrophils, eosinophils, and basophils are present in peripheral blood and certain organs of fish. Some species of fish have all three cell types, while other species may only possess one cell type. Granulopoiesis in fish occurs in the spleen, kidneys, epigonal organ, organ of Leydig, and other specialized tissue, with the specific locations depending on the species. In some fish, neutrophils are actively phagocytic. In vitro assays indicate phagocytosis, mobilization in response to chemotaxins, detectable chemiluminescence, and an active respiratory burst when appropriately stimulated. In fish species without neutrophils, the eosinophil may be responsible for phagocytosis. Eosinophils also function in antiparasite immunity in certain species. Basophil function has not been investigated. Responses of fish neutrophils and eosinophils can be altered by season of the year, environmental pollutants, disease, and others stressors. Differences among fish species in type, cellular distribution, and function of granulocytes are the focus of this review.