Piscine macrophage aggregates: A review

Abstract

Piscine macrophage aggregates (MAs), alternatively known as melano-macrophage centers (MMC), are focal accumulations of macrophages usually containing the pigments hemosiderin, lipofuscin and ceroid, and melanin. The structures are not confined to fish and have been observed in other poikilothermic vertebrates. The aggregations are most commonly present in the spleen, pronephros, mesonephros and liver but may be found in other organs, especially in relation to inflammation. In the lower fishes (Agnatha, Chondrichthyes) the pigmented cells tend to be solitary or in small (<30 cells/aggregation), irregularly shaped aggregations primarily in hepatic tissue. In the Osteichthyes, greater numbers of cells and aggregates are present than in the lower fishes. These aggregates are more nodular, and they occur more commonly in the spleen and kidney rather than the liver.

An exception to these observations if found in the Clupeiformes and the Salmoniformes, considered lower teleosts, who have poorly organized and irregularly shaped aggregations that are smaller than the higher teleosts.

Macrophage aggregates function in normal physiological processes and in the body's defense against injurious agents. Evidence indicates that these functions are multiple, complex, and not well understood. They may be classified as, (a) immune, including humoral and inflammatory responses; (b) storage, destruction, or detoxification of exogenous and endogenous substances; and (c) iron recycling. There is a growing body of information that elucidates these functions.

Macrophage aggregates qualify as anatomical and cytological biomarkers since they are known to change in number, size, and pigment content in relation to fish health and environmental degradation. Their value lies in their ubiquity, availability, and ease of measurement. There have been few extensive, controlled attempts to produce MAs or to study their kinetics by chronic exposure to contaminants known to exist in polluted environments. Clearly, without such investigations the value of MAs as monitors (biomarkers) of fish health and environmental degradation remains questionable.