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Melanin pigments in lower vertebrates are often found in locations other than the skin, thus forming an extracutaneous pigmentary system of unknown function. The cellular and biochemical structure of this system is still poorly characterized. This paper deals with the ultrastructural and biochemical features of the melanogenic system of Xenopus laevis. Melanin containing cells were identified in the dorsal and ventral skin, and in the lung, spleen, liver and connective tissue surrounding blood vessels. The pigment cells in the skin and the lungs appeared to be typical melanocytes. The spleen contained isolated melanocyte-like cells, but most of the pigment cells present in this organ were associated with melanomacrophage centers. Conversely, the liver appeared devoid of melanocytes and only displayed melanomacrophage centers. Tyrosinase activity was found in all pigment-containing organs except the liver. All organs containing tyrosinase activity also displayed melanin formation potential from L-tyrosine. Therefore, tyrosine hydroxylase and melanin formation activities could be detected only in those organs containing typical melanocytes but not in locations such as the liver, where only melanomacrophages centers were found.

The Melanogenic System of Xenopus laevis.