Muscular melanosis.

Muscular Melanosis – Gross Pathology

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.

Atlantic salmon, with muscular melanosis.
Atlantic salmon, with muscular melanosis.

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.

Atlantic salmon, with muscular melanosis due to fracture.
Atlantic salmon, with muscular melanosis due to fracture.

Atlantic salmon, with peduncular melanosis.
Atlantic salmon, with peduncular melanosis.

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.

causes of muscular melanosis


  • Agius, C., & Roberts, R. J. (2003). Melano‐macrophage centres and their role in fish pathology. Journal of fish diseases26(9), 499-509.
  • Roberts, R. J. (1975). Melanin-containing cells of teleost fish and their relation to disease. The pathology of fishes399, 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 immunotoxicology27(1), 147-161.

By: Carlos Sandoval

Carlos Sandoval Hurtado is a Veterinary doctor at Universidad Católica de Temuco, Masters in Science, specializing in Animal Health at Universidad Austral de Chile. He has over 15 years experience in the area of pathology and histopathology of fish and other species, becoming one of the most renowned histopathologists in Chile. He is currently the Director of VeHiCe (Veterinary Histopathology Center) a leading laboratory in histopathology whose clients include laboratories, universities, aquaculture industry and avian industry mainly.