ABSTRACT
The organisation and development of the dopaminergic (DA) system in the retina of the adult brown trout were studied with tyrosine hydroxylase immunocytochemical techniques. Adult DA cells are rather homogeneous in appearance and possess thick dendritic processes running to the ganglion cell layer and thinner axonal processes which run to the horizontal cell layer, where they form a rich plexus of varicose fibres closely associated with the surface of these cells. Contact of DA fibres with photoreceptor processes was not observed. We therefore consider this DA population to consist mainly of interplexiform cells. These cells appear late in development, being first observed in prehatching (16 mm) embryos (after photoreceptors have begun to differentiate). DA cells increased in number throughout the fry and juvenile stages, but even in the largest juveniles studied (30-35 mm) the size of the DA cell population was only about 20% of that in adults. DA cells appear to arise in the marginal retina. In developing stages (embryos and fry) only inner nuclear layer processes were observed, the horizontal cell layer DA plexus appearing late in development (28 mm juveniles).
