ELECTRON MICROSCOPIC OBSERVATION ON THE HYPOTHALAMO-HYPOPHYSEAL SYSTEM IN THE RAT I. THE ULTRAFINE STRUCTURE OF THE CONTACT REGION BETWEEN THE EXTERNAL LAYER OF THE INFUNDIBULUM AND PARS TUBERALIS OF THE ANTERIOR PITUITARY

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The ultrafine structure of the contact region between the infundibulum and Pars tuberalis of the anterior pituitary was investigated in rats, and following results were obtained.<BR>1) Pars tuberalis of the anterior pituitary consists of several layers of relatively small and variously shaped cells with large nuclei and conspicuous cell membrane. They contain neither secretory granules nor vacuoles except sparse mitochondria.<BR>2) Around both the capillary loops and the capillaries of the primary plexus of the portal vessels, a wide pericapillary space is always present. Its maximal width measures several micra. Collagen fibrills and numerous extremely fine granules are distributed in this space, and fibroblasts as well as the endfeets of glial processes are also occasionally observed.<BR>3) The external layer of the infundibular wall consists of numerous fibres with various calibre range. Most of these fibres are nothing but the processes of the astro lia which contain very few cytoplasmic element. The presence of nerve fibres in this layer was asccrtained. They are arranged verticaliy to the outer limiting surface of the external layer and thus parallel to each other. In addition to the sparse mitochondria, they contain a large number of small vesicles. The vesicles measure about 300-500Å in diameter and even the maximal diameter does not exceed 1000Å. They resemble "synaptic vesicles". The fibre which contains neurosecretory granules of high electron density could not be found in the external layer.<BR>These characteristic structures of the contact region strongly support the concept that some chemical substance (s) is released from the nerve endings in the external layer of the infundibulum into the capillaries in the median eminence. Although the small vesicles described above, which might be regarded as the morphological representation of this chemical substance, are similar in many respects to the so-called "synaptic vesicles", the final conclusion on the nature of the small vesicles cannot be derived at present. From the present observation, however, it is very likely that, even if the small vesicles are not identical with the synaptic vesicles, but are of neurosecretory nature, they are somewhat different in character from the ordinary neurosecretory granules contained in the nerve axons of the hypochalamo-neurohypophyseal tractus.
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ELECTRON MICROSCOPIC OBSERVATION ON THE HYPOTHALAMO-HYPOPHYSEAL SYSTEM IN THE RAT I. THE ULTRAFINE STRUCTURE OF THE CONTACT REGION BETWEEN THE EXTERNAL LAYER OF THE INFUNDIBULUM AND PARS TUBERALIS OF THE ANTERIOR PITUITARY

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