Fig. 19.(A) Schematic diagram of the median eminence showing the organization of its three major zones: ependymal zone (E), internal zone (ZI), and external zone (ZE). ZE is invigilated by portal capillaries which are contacted by axon terminals of the tuberoinfundibular system and by processes of specialized ependymal cells, the tanycytes. (B) Fibers coursing through the ZI are seen immunocytochemically in the rat using antiserum to vasporessin. (C) Fibers terminating in the ZE in close association to portal capillaries (PC) are seen immunocytochemically in the rat using a proTRH-directed antiserum. III = third ventricle. (From Lechan RM, Functional Microanatomy of the Hypophysial-Pituitary Axis, in Melmed, S (Ed), Oncogenesis and Molecular Biology of Pituitary Tumors, Frontiers of Hormone Research, 20: 2-40, 1996.)