Neural connections of the pineal organ in the primitive bony fish Acipenser baeri: a carbocyanine dye tract-tracing study.

The pineal organ of the sturgeon (Acipenser baeri; Chondrostei) consists of an elongated terminal vesicle and a long stalk that runs to the region of the posterior commissure. In order to investigate the neural connections of the pineal organ of the surgeon, a fluorescent indocarbocyanine (DiI) was applied to the pineal stalk of paraformaldehyde-fixed brains. This application strongly labeled the pineal tracts (right and left), which form an extensive bilateral projection to the brain. The fibers of these tracts could be followed to their different targets in the brain, namely the preoptic region, the rostral habenulae, the medial subhabenular and retrohabenular (post- and subcommissural) region, the medial thalamus, the dorsal hypothalamus, the pretectal area, and medial and dorsolateral tegmental mesencephalic regions (interstitial nucleus of Cajal, oculomotor nucleus, mesencephalic reticular area, nucleus profundus mesencephali, and central gray). A few fibers reach the interpeduncular nucleus and the rostral optic tectum. Pinealofugal fibers branched profusely, and some of them crossed the midline ventrally in the optic chiasma, the postoptic commissure, the conspicuous preinfundibular commissure or, occasionally, the mesencephalic tegmentum. In addition to pinealofugal fibers, this type of DiI application labeled a few cells, here interpreted as pinealopetal neurons, in the posthabenular-dorsomedial thalamic region and the central gray of the mesencephalon. The wide distribution of pinealofugal projections in sturgeon is reminiscent of that in the lamprey. Comparison of the present findings with those of other authors in teleosts, amphibians, and reptiles also suggests that there has been a reduction in the number of pineal projections in these lines.