Both intracerebroventricular injection of 5,7-dihydroxytryptamine and electrolytical midbrain-raphe lesions in rats induce degeneration of supraependymal axons (SEAs) normally occurring in large numbers upon the ependyma of the medial habenular nucleus and habenular commissure. It is concluded that the intraventricular axon plexus in the epithalamic region is comprised of serotonergic (5-HT) fibers originating in the dorsal and/or median raphe nuclei. Besides the elimination of SEAs, conspicious features were a marked reduction in the number of cilia, degenerative signs in the habenular ependyma, and the emergence of large numbers of supraependymal macrophages, being most probably involved in phagocytosis of the axonal debris. It is suggested that the nucleus habenulae medialis is influenced serotonergically by the midbrain raphe via (1) a direct projection upon its neurons and (2) an indirect projection by way of the intraventricular axon plexus. The origin of intraventricular macrophages is discussed in relation to recent data in the literature.

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Tanycytes with foot processes contacting capillary basal membranes were identified in the rat medial habenular nucleus. They constitute a relatively small but constant population of cells among the conventional ependymal cells. In contrast to tanycytes in most circumventricular organs, habenular tanycytes possess cilia. Superimposed upon the cells are nerve fibers belonging to the serotonergic supraependymal axon plexus. Their ultrastructure differs in many respects from that of hypothalamic tanycytes.

Cerebellar collaterals of the mesencephalic trigeminal root have been studied by the use of various histologic techniques in adult and fetal brains of rat and mouse. In addition, brains of rats with lesions in the mesencephalic trigeminal system were studied.Cells of the mesencephalic trigeminal system can he found in the brachium conjunctivum near the cerebellar hilus in the adult and fetal rat and in the fetal and full-term mouse. Such cells located in the anterior medullary velum were found in fetal rat material and in the human brain stem. In the rat collaterals of the mesencephalic trigeminal root course through the brachium conjunctivum and anterior medullary velum, terminating in the nodulus cerebelli and possibly also in the neighborhood of the primary fissure. These fibers coursing through the anterior medullary velum decussate and intermingle with fibers of the vcntral spino-cerebellar tract and fibers of the trochlcar nerve. They course ventrally from the tectocerebellar tract, which consists of smaller myelinized fibers. Termination in the cerebellum is by nieans of mossy fibers. The findings are discussed as they relate to previous studies.It is probable, that the cerebellum utilizes proprioceptive impulses conducted by mesencephalic fifth fibers in its coordinating influence o n the activity of the muscles of mastication.The mesencephalic root of the trigeminal nerve has been described in many We noted, incidentally, the presence of cells of the mesencephalic trigeminal nucleus located in the anterior medullary velum and brachium conjunctivum of the rat. Therefore, we have paid special attention to the midbrain-cerebellar fiber connections in a new series of brains of adult rats with lesions, which had partly destroyed the mesencephalic trigeminal root and nucleus.Normal mature and immature mouse and rat brains were also investigated. MATERIAL AND METHODSA total of 102 rat brains were used in this study. Unilateral lesions were established in the midbrain by means of electrocoagulation with a stereotaxic instrument. In order to obtain a complete fiber fragmentation of the mesencephalic trigeminal root a six-day postoperative survival period appeared suitable. At autopsy the animals were perrused with isotonic saline, followed by a 10% formalin or Bouin-solution, depending on the programmed stain-

In the medial habenular nucleus of the rat, ependymal and endothelial membrane specializations were studied with TEM and freeze-fracturing. They comprise ependymal adherent junctions - not manifest in freeze-fracture replicas-, gap junctions, and membrane-associated orthogonal particle complexes ("assemblies") - not identifiable in thin-sectioned material. Ependymal tight junctions being absent, no brain-liquor barrier exists. The capillary endothelium is provided with tight junctions only. Intraventricularly injected HRP was transported in large amounts through the ependyma, mainly through the intercellular spaces and additionally by way of massive pinocytosis through the cytoplasm of particular ependymal cells only, and finally through the parenchymal intercellular compartments towards habenular capillaries. Following intravenous injection of HRP, considerable transport of the enzyme took place by means of transendothelial pinocytosis, followed by some pinocytotic transport through diverse parenchymal elements and markedly profuse incorporation and lysis within pericytes. The habenular blood-brain barrier appeared to be considerably leaky with respect to HRP.