The neural organization of the pineal complex in the frog: stratification and regional differences

Plexiform areas are functionally important units of the pineal complex. The silver impregnation technique by Matsuyama (1972) reveals in the frog that nerve fibres branch and terminate in these areas and that they are presynaptic to nerve cells which are part of the plexiform areas. Histochemically, the nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-diaphorase) method according to Scherer-Singler et al. (1983) indicates that pinealocytes and nerve cells are both stained, a strong stratification of different cell types can be visualized. Plexiform areas of different size exist at the dorsal and ventral surface of the organ; the biggest are located rostrodorsally. Approximately twenty large multipolar nerve cells are localized at the ventral surface of the pineal organ mainly at the rostral and lateral borders. They send long processes mainly in caudal direction. These processes are post-synaptic to the boutons of the endfeet belonging to photosensitive pinealocytes. Processes of several multipolar nerve cells converge to one plexiform area, branch and obviously terminate there. As in horizontal cells the processes of the multipolar cells seem to be capable of both receiving and transmitting signals. On the postsynaptic side of the processes of the multipolar cells are smaller nerve cells juxtaposed to the plexiform areas. They are known as pseudounipolar, unipolar or bipolar cells (Wake et al., 1974); the latter are free of boutons of the pinealocytes, their axons forming the pineal tract. Three different types of photosensitive pinealocytes can be distinguished: a slender, a spherical and a double-cone type. Also, the number of boutons varies in the endfeet: boutons occur alone, in groups of two or three, and in clusters.(ABSTRACT TRUNCATED AT 250 WORDS)