Lymphoreticular system in fish: erythrocyte-mediated immunomodulation of macrophages contributes to the formation of melanomacrophage centers

In fish, melanomacrophage centers (MMC) represent the equivalent of germinal centers of birds and mammals. They consist of aggregates of macrophages filled with fragments of cells, mainly erythrocytes, and various pigments such as haemosiderin, lipofuscin and melanin. The kidney, the spleen and the liver contain many MMC either moving freely or encapsulated in the context of the lymphoreticular tissue. Here, we have demonstrated for the first time that MMC exist in the kidney, spleen and liver of the rainbow trout Salmo gairdneri Richardson by using traditional stainings and cytoenzymatic methods: Periodic Acid-Shiff, α-Naphthyl Acetate Esterase and Peroxidase. Finally, in the view of the already described peripheral interaction between fish erythrocytes (FE) and macrophages (MØ), it will be postulated that these cellular aggregates (rosettes) travel to distant organs, thus contributing to the formation of MMC in the kidney, in the spleen and in the liver.