PINEAL ORGAN OF THE PIKE (ESOX-LUCIUS, L) .3. INTRAPINEAL PATHWAYS FOR CONDUCTION OF PHOTOSENSORY MESSAGES

In order to elucidate the sensory function of the pineal organ of the pike, Esox lucius, the acetylcholinesterase (AChE) reaction of Karnovsky and Roots (1964) was used to demonstrate the pineal neurons. Thirty to 40 sensory neurons 66 to 20 μm in diameter), more numerous in the ventral than in the dorsal wall (ratio: 3/1), were evidenced in the rostral (distal) part of the pineal vesicle (distal pineal). None or very few AChE-positive neurons were observed in the middle pineal. Two hundred to 744 neurons (4 to 11 μm in diameter) were identified in the proximal pineal (pineal stalk). Multipolar elements were generally observed in the distal pineal, while those in the proximal region were mainly pseudounipolar; bipolar neurons were seen in the distal and proximal pineal. At the level of the subcommissura organ, the pineal tract running towards the posterior commissure was accompanied by some AChE-positive neurons whose axons also ran towards the posterior commissure or the pineal organ. The ultrastructural features of sensory neurons were similar to those already described in other vertebrates. Typical photoreceptor cells, mainly in the proximal region, showed ribbon synaptic connections with dendritic processes of sensory neurons. The pineal tract was composed of two types of axons differing in structure and diameter. Some of the fibers of one type were myelinated at the middle of the pineal stalk, but most were myelinated at the caudal part. This study completed that of photoreceptor cells and sensory channels; it was concluded that the photosensory function of the pike pineal organ was more developed in the proximal then the distal regions, and regressed in the middle pineal.