Fine structure and vascular supply of the median eminence (ME) in Acipenser ruthenus (Chondrostei).

The fine structure and vascular supply of the median eminence (ME) was studied in 57 specimens of Acipenser ruthenus (Chondrostei) by means of light microscopy (normal histology, horseradish-peroxidase (HRP), formaldehyde induced fluorescence (FIF), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as well as by SEM of vascular corrosion casts. Light microscopy revealed that the ME increases in thickness from caudal to rostral with capillaries invading the subependymal layers only at the rostral median eminence (rME). At the middle (mME) and caudal median eminence (cME) capillaries are limited to the ME outer surface. Short term HRP-application (systemically as well as intraventricularly) resulted in reaction product in the intercellular space throughout the ME after a survival time of 2 hours. The ME tanycytes were distinctly marked after 7 days survival. FIF revealed aminergic cerebrospinal fluid (CSF-) contacting neurons; also the apical part of tanycytes showed specific fluorescence. SEM of vascular corrosion casts showed that the arterial supply of the whole ME results from branches of the hypothalamic artery and/or from those arteries supplying the saccus vasculosus. The whole 2-dimensional primary capillary plexus of the ME drains via portal vessels into the adenohypophysis. As demonstrated by SEM, the ME ependymal surface is made up by oligociliated tanycytes, abundant crown cells and intraventricular protrusions of CSF-contacting neurons. Supraependymal cells are lacking, supraependymal fibres are very sparse. TEM revealed the ME tanycytes with long ciliary rootlets. Their basal processes split into numerous delicate branches forming many end-feet, which end at the basal lamina opposite the fenestrated capillaries of the primary plexus. Like tanycytes, also the crown cells possess branching processes which may contribute to the outer glial membrane. CSF-contacting neurons of the type I (with dense core vesicles ranging in diameter from 100-120 nm and of presumed aminergic nature) and type II (with dense core vesicles ranging in diameter from 160-180 nm and of presumed peptidergic nature) were found. Both types are most abundant in the cME. Small astrocytes reveal delicate processes which cover parts of the CSF-contacting neurons and of tanycytes. Another glial cell type is situated near the basal lamina and forms processes parallel the latter. There are few axo-somal and axo-axonal synapses pointing rather to a humoral regulation of the ME with signals from the ME blood vascular bed or from the CSF acting on CSF-contacting neurons.