The ultrastructural characteristics and their functional significance of the arcuate nucleus and median eminence

Abstract

The ultrastructural characteristics of the arcuate nucleus and median eminence in rats have been studied by means of formaldehyde-osmium tetroxide fixation method. The observations showed that there are two kinds of neurons (dark and light) in the arcuate nucleus which might be responsible for producing both dopamine and releasing hormones. The tanycytes of the ependyma of the third cerebral ventricle pass longitudinally through the various zones of the median eminence and reach at pericapillary space of the portal vessels. The neurosecretory substance-containing nerve terminals may travel between ependymal cells and even enter the cavity of the third ventricle or end around the basal membrane of the capillaries of the median eminence. The axo-somatic and axodendritic synapses are formed at the perikaryon and dendrites of neurons in the arcuate nucleus. Both agranular type and granular type of axo-axonic synapses are encountered in the fibrous zone of the median eminence. There are also synaptic connections between the basic processes of tanycytes and the large granular vesicle-containing nerve terminals in the palisade zone of the median eminence. The ultrastructural characteristics mentioned above suggest that (1) the releasing (or inhibiting) hormones of the hypothalamus might be released through two routes: into the portal capillaries from nerve terminals directly or into the cerebrospinal fluid of the third ventricle first and uptaken by tanycytes, then transported to the portal capillaries by the basic processes of tanycyte; (2) each step of synthesis, storage, transport and release of the releasing (or inhibiting) hormones could be regulated by nervous mechanism.

Keywords

arcuate nucleus / median eminence / tanycyte / releasing hormones / axo-axonic synapse / neurosecretory

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