Histochemical and ultrastructural study of the nervous elements in the pineal organ of the eel, Anuilla anguilla.

The pineal organ of the eel, Anguilla anguilla was studied light-and electron-microscopically. The tissue consists predominantly of glia-like supporting cells and photoreceptor-like sensory cells, which establish synaptic contacts with pineal nerve cells. Histochemically, a positive acetylcholinesterase (AchE) reaction can be observed in the perinuclear cytoplasm of the nerve cells and in the inner segments of the photoreceptor cells. Based on the intensity of the AchE reaction and the dimensions of the perikarya, at least two different types of nerve cells can be distinguished: type I with a large, intensely staining perikaryon, located mainly in the pineal end-vesicle, and type II with a smaller, moderately staining perikaryon, found predominantly in the pineal stalk. The total number of AchE-positive neurons in the pineal organ is approximately 370. About 140 neurons are found in the end-vesicle and 230 in the pineal stalk. Electron microscopical observations of transverse sections through the proximal-most part of the pineal stalk near the subcommissural organ in the diencephalic roof reveal the presence of approximately 530 nerve fibers was found distally from the site of transection. Consequently, it is assumed that the majority of the pineal-tract fibers is afferent (pinealofugal) and that the difference between the number of AchE-positive perikarya and the number of pineal-tract fibers in mainly caused by the presence of AchE-negative perikarya. It is possible, however, that in the proximal-most part of the stalk, inside the diencephalic roof, nerve fibers from other parts of the brain establish synaptic contacts with pineal elements.