The distribution of proliferating cell nuclear antigen-immunoreactive cells in the pineal organ of the rainbow trout Oncorhynchus mykiss.

Cell proliferation in the pineal organ of the immature rainbow trout Oncorhynchus mykiss was investigated by immunocytochemical demonstration of the proliferating cell nuclear antigen (PCNA) together with photoreceptor-specific opsin. Numerous PCNA-immunoreactive cells were found throughout the pineal end-vesicle and stalk. Two types of PCNA-immuno-reactive cells were distinguished: intensely stained, large ovoid and round cells, and mildly stained, slender fusiform cells. The ovoid type of the former cell was found often in the apical region and the round type in the basal region of the epithelium, while the latter fusiform cells were scattered through the apical and middle regions. Occasionally, close approaches were found between the opsin-immunoreactive photoreceptor outer segments and the PCNA-immunoreactive cells, which expressed mildly stained, nuclear and cytoplasmic signals. In addition, overlaps of the opsin-immunoreactive outer segments with the BrdU-labelled cells were occasionally found within the pineal epithelium. These findings suggest that the proliferation and neurogenesis of the pineal photoreceptor cells might persist also in the adult rainbow trout, thus maintaining highly sensitive, photo-signal transduction mechanisms for melatonin synthesis.