Photoreceptor proliferation in the mature retina and pineal organ of the teleost fish.

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1)In parallel with histological observation on the increase in number of the rod cells, the distribution of proliferating cell nuclear antigen (PCNA)-immunoreactive cells was investigated in the mature retina of the teleost fish. A great number of PCNA-immunoreactive cells were demonstrated in both the outer and inner nuclear layers in the adult retina of the river eel Anguilla japonica migrating thousands of kilometers to breed in the ocean, and throughout the outer nuclear layer in the juvenile retina of the lefteye flounder Paralichthys olivaceus adapted to the benthic habitat and nocturnal feeding. These results suggest that the retinal rod cells proliferate throughout their life, following migration of the rod precursors from the progenitor clusters in the inner nuclear layer in the eel and from the basal to the apical region in the outer nuclear layer in the flounder.2)Together with opsin-immunoreactive photoreceptor outer segments, a tremendous number of PCNA-immunoreactive cells were demonstrated throughout the end-vesicle and stalk in the pineal organ of the 2 years-old rainbow trout Oncorhynchus mykiss lacking a circadian clock of melatonin synthesis. No or little coexistence of PCNA and opsin was demonstrated, although the close approaches of opsin-positive photoreceptor outer segments and PCNA-immunoreactive cells were found sometimes. These findings suggested that the pineal photoreceptor cells also proliferate in the adult rainbow trout, thus keeping highly sensitive signal-transduction mechanisms for active melatonin synthesis.