Abstract
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 postsynaptic 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. The number of pinealocytes per area is bigger in the ventral than in the dorsal wall, the different types of pinealocytes show a characteristic distribution pattern: circularly arranged, a bouquet of slender-type pinealocytes is localized in the centre, the spherical type more to the periphery of the circles. The number of double-cone type pinealocytes is low. Modified pinealocytes are observed next to the plexiform areas and without contact to the pineal lumen. Sometimes they contact the process of a photosensitive pinealocyte or send a process into the plexiform area. If the modified pinealocyte can transfer a message from a photosensitive pinealocyte to a nerve cell, then it functions like an interneuron; it suggests that a paraneuron transforms into a neuron in such an instance.
