THE ULTRASTRUCTURAL CHARACTERISTICS AND THEIR FUNCTIONAL SIGNIFICANCES OF THE ARCUATE NUCLEUS AND MEDIAN EMINENCE / 解剖学报

ABSTRACT

The ultrastructural characteristics of the arcuate and median eminence in the rat have been studied by means of folmaldehyde-osmic acid 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 ventricle run longitudinally through various zones of the median eminence and reach the perivascular space of the portal capillaries. The neurosecretory substance-containing nerve terminals may travel between ependymal cells or end around the basal membrane of the capillaries of the median eminence, even keep close to the endothelial fenestrae of "open-type capillary". The axo-somatie and axo-dendritic synapses are formed at the soma and dendrites of neurons of the arcuate nucleus. Both agranular type and granular type axo-axonic synapses are encountered in the fibrous zone of the median eminence. There are also synaptic connections between the basic processes and the large granular vesicles 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 into the portal capillaries from nerve terminals directly or into the cerebro-spinal fluid of the third ventricle and then uptaken by the tanycytes, and transported to the portal capillaries by their processes. (2) each step of synthesis, storage, transport and release of the releasing (or inhibiting) hormones could be regulated by nervous mechanism