Neural projections of the pineal organ in the larval sea lamprey (Petromyzon marinus L.) revealed by indocarbocyanine dye tracing.

The distribution of the central neural connections of the pineal organ of the larval sea lamprey was investigated by means of anterograde and retrograde tracing with the fluorescent lipophilic dye, DiI (1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate). Pinealofugal projections are well developed in larvae, extending from the posterior commissure into the diencephalon and mesencephalon. Small numbers of neurons were retrogradely labelled in the transition zone between the diencephalon and the mesencephalic tegmentum. These cells may constitute the first pinealopetal system described in anamniotes.