Microscopic examination of the pigment cell aggregations in the liver and spleen of mud turtles (Kinosternon flavescens) suggests that the morphology and behavior of these cells is consistent with the melanomacrophages described in teleost fishes and the pigmented "Kupffer cells" described in frogs and reptiles. These cells contain massive amounts of melanin, substantial lipofuscin, and some hemosiderin consistent with their phagocytic function. Similar-appearing isolated pigmented macrophages are solitary in the liver, spleen, lung, and kidney. Number and size of the largest hepatic aggregations increase almost linearly with turtle age so that in old turtles they may constitute up to 20% of the liver volume. This increase may result from hepatic recruitment of macrophages throughout the life of the turtle and suggests that size and number of melanomacrophage aggregations may serve as a marker for senescence in otherwise healthy turtles of this species.

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