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Anatomical localisation of neuromelanin in the brains of the frog and tadpole. Ultrastructural comparison of neuromelanin with other melanins.

Neuromelanin is a dark pigment which occurs in some groups of neurons of vertebrate nervous systems. This neural pigment owes its name to its similarity in colour and histochemical reactions to cutaneous and other melanins. However, neuromelanin differs from melanin pigment occurring elsewhere in its mode of formation (Foley & Baxter, 1958), its chemical composition (Lillie & Yamada, 1960) and its ultrastructure (Moses, Ganote, Beaver & Schuffman, 1966). Mammals have neuromelanin occurring in a few areas of the nervous system, mainly in the 'substantia nigra' and in the 'locus coeruleus' which are located in the brain stem. Although these nuclei have been widely studied with respect to their functions and pathological changes, the reasons for their cells being pigmented are obscure. Amphibians have a large quantity of neuromelanin distributed throughout their central nervous system (Adler, 1939). Such neuromelanin granules are similar in many respects to horseradish peroxidase granules and may represent a source of possible errors in the interpretation of neuronal connections (Mensah & Finger, 1975) when this enzyme is used as an intra-axonal transport tracer. In the present paper, the localisation of neuromelanin has been studied in the central nervous system of the frog Rana esculenta and its distribution in tadpoles at various stages of development. The aim of this work is to define the exact sites where neuromelanin granules occur in the brain of a low vertebrate and to try to give a phylogenetic and ontogenetic meaning to their distribution. In addition, the ultrastructural differences between neuromelanin and melanin occurring elsewhere in the frog have been defined.