The fiber architecture of the rat median eminence with some accidental observations on the significance of tanycyte processes.

Abstract

The anterograde transport of horseradish peroxidase was used to stain nerve fibers in the hypothalamic median eminence of the rat. Bundles of commissural fibers were found coursing in the subependymal layer. They approach the median eminence along the basal surface of the diencephalon, and seem to terminate contralaterally in the medio-basal hypothalamus. The relatively thick fibers of the magnocellular fiber system were seen in the internal zone. Along their rostro-caudal trajectory the fibers join into bundles which surround elongated fields. The varicose fibers of the parvicellular system run rostro-caudally in the dorsal part of the external (palisade) zone, while they become randomly oriented more ventrally, i.e. adjacent to the portal capillaries. It is tentatively suggested that tanycyte processes around the median eminence form a barrier system localizing the substances secreted by the nerve terminals of the parvicellular fiber system. Tanycyte processes within the median eminence seem to divide the palisade zone into rostro-caudally elongated compartments allowing only transverse diffusion in the extracellular space.