The term melanosis is used to describe the presence of unusual amounts of melanin in abnormal areas. Melanin is considered to plays a defensive role, since it is capable of absorbing and neutralizing free radicals, cations and potentially toxic compounds derived from the breakdown and phagocytosis of cellular material.

Inflammatory reactions and tissue degeneration in bony fish frequently involve melanomacrophages. These macrophages contain fragments derived from phagocytized debris and cells, often erythrocytes, and pigments such as melanin, lipofuscin and haemosiderin.

In salmonids these cytoplasmic granules are dark brown/black.

In most higher teleost fish (not salmonids), melanomacrophages are found within membrane-limited structures called melanomacrophage centers (MMC), most commonly found in kidney and spleen, but also other tissues in which there is high cell turnover e.g. ovary. The pigment in melanomacrophages of higher teleosts is usually golden brown, but can be pink or dark brown.

REFERENCES

• Agius, C., & Roberts, R. J. (2003). Melano‐macrophage centres and their role in fish pathology. Journal of fish diseases, 26(9), 499-509.
• Roberts, R. J. (1975). Melanin-containing cells of teleost fish and their relation to disease. The pathology of fishes, 399, 399-428.
• Passantino, L., Cianciotta, A., Jirillo, F., Carrassi, M., Jirillo, E., & Passantino, G. F. (2005). Lymphoreticular system in fish: erythrocyte-mediated immunomodulation of macrophages contributes to the formation of melanomacrophage centers. Immunopharmacology and immunotoxicology, 27(1), 147-161.