Melanin and inflammation in fish 3519

Abstract Description In ectothermic vertebrates, inflammatory reactions may occur with a dark colorization due to its content of melanin. Melanised cells, commonly referred to as melano-macrophages, populate the immune organs of ectothermic vertebrates, where they may form melano-macrophage centres, speculated to be the analogues of germinal centers of homeothermic vertebrates. The presence of such cells accounts for the discoloration observed in certain inflammatory reactions, however, it has been un-known if melanin is produced on site. Muscle inflammatory lesions with melanization is a common finding in farmed salmon, and the pathogenesis is related to adipocyte necrosis in certain locations of the fillet. Here we investigate key genes for melanin production by in situ hybridization and follow the development of emerging inflammation in the muscle of Atlantic salmon. First, cells positive for the expression of melanin-synthesizing genes appear in an a-melanoctic phenotype in early inflammatory changes. In more advanced changes, positive cells are faintly black, and in granuloma, being the end stadium of these changes, positive cells are heavily melanized. The results show that melanin-producing cells compose an important component in the inflammatory reaction, making it significantly different from granuloma formation of mammals. The rationale for pigment production in fish inflammatory responses remains unclear. Funding Sources Fiskeri- og Havbruksnæringens Forskningsfond Topic Categories Veterinary and Comparative Immunology (VET)