Three-dimensional reconstruction of serotonin-immunoreactive photoreceptors in the pineal organ of the river lamprey, Lampetra japonica.

Serotonin-immunoreactive (5-HT IR) photoreceptors are present in the pineal complex (pineal and parapineal organ) of the river lamprey, Lampetra japonica. They are so-called modified pineal photoreceptors and have been regarded as photoneuroendocrine cells which secrete melatonin. We reconstructed 5-HT IR cells with a computer to demonstrate their three-dimensional structures from optical sections taken by a confocal laser scanning microscope. The 5-HT IR cell possesses a basal process, and it appears that the process does not branch out. These processes contact each other at the basal region of the end vesicle, and a process extends to the soma of the neighboring 5-HT IR cell. These findings were obtained by three-dimensional analysis with a computer, which is a useful technique to demonstrate the interaction between cells. We suggest that the 5-HT IR photoreceptors interact with one another.