A Morphological Survey of the Median Eminence: Fluorescence Histochemistry, Electron Microscopy and Immunohistochemistry

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This review is a morphological survey of the median eminence of the rat as revealed by fluorescence histochemistry, electron microscopy and immunohistochemistry. Strong catecholamine fluorescence was diffusely distributed in the external layer of the median eminence especially around the primary capillaries of the portal vessels. In the internal layer, there was a scattering of varicosities or strands with catecholamine fluorescence. The origin of these catecholamine nerve fibers and the function of dopamine (DA) nerve fibers in the external layer were discussed. In the external layer of the median eminence, a large number of nerve endings and processes of various sizes ranging from 0.5 to 2.0 micrometer in diameter were found around pericapillary spaces of the hypophyseal portal system. These nerve endings have been classified into three types according to the morphological features of the vesicles contained in them. Small and large dense cored vesicles (40-60 nm, 70-100 nm in diameter) with very high electron density appeared in the first type nerve endings after 5-OHDA treatment; these endings were regarded as monoaminergic, and more specifically, catecholaminergic. Topographical distribution of luteinizing hormone-releasing hormone (LH-RH) nerve fibers in the median eminence was revealed by immunohistochemistry using the peroxidase antiperoxidase (PAP) method. LH-RH nerve processes and terminals appeared as brown beaded strands and dots in the PAP method. LH-RH terminals in the median eminence were seen to be encircling the infundibular radix. However, they were not evenly distributed; the most notable accumulation of LH-RH reactive dots were located in the region extending from the dorsal part of the tuberoinfundibular sulcus to the lateral part of external layer of the superior infundibular labium at the infundibular radix. Considering the distribution of LH-RH, the growth hormone-release inhibiting hormone (GIH) and the monoamine nerve endings in the median eminence, the function of the anterior pituitary appeared to be regulated by a delicate interrelationship of nerve endings.