The tanycyte of the rat median eminence

Summary The present ultrastructural study proves the existence of nerve terminals closely apposed to the plasmalemmata of tanycytes in the rat median eminence. Several of these axo-tanycytic endings display remarkable accumulations of agranular endoplasmic reticulum in the form of pleomorphic vesicles which are closely apposed on either side of the plasma membrane of each cell compartment. Some of these vesicular profiles give the impression of structural continuity across both membrane systems. This phenomenon is discussed in the context of being a potential substratum for communication between both cell compartments.Supported by NIH Grant NS13717 and NSF Grant BNS 78-11820     

Ultrastructure of the median eminence of the rat

Summary The ependymal cells bordering the median eminence to the third ventricle are characterised by many microvillus-like projections and bulbous cell processes of the luminal plasma membrane. The latter contain many vesicles 500–1,000 Å in diameter. Cilia with 9+2 fibrillar pattern are seen occasionally. Adhesive devices in the from of zonula adhaerens and zonula occludens are found in the apical part of the intercellular junction. Unmyelinated nerve fibres with a mean diameter of 1 and containing many electron dense granules of 830–1,330 Å are often seen between the ependymal cells.Two types of glial cells are found in the median eminence. One is characterised by a nucleus with dense blods of chromatin and dense cytoplasm, and it is associated chiefly with the nerve fibres in the region of the hypothalamo-hypophysial tract. The other type of glial cell is characterised by fine, uniformly distributed chromatin in the nucleus and a relatively pale cytoplasm and branched processes which terminate perivascularly in the base of the median eminence.Myelinated nerve fibres are seen only in the region of the hypothalamo-hypophysial tract. Only a part of them contain electron dense granules 1,330–2,330 Å in diameter.Three types of unmyelinated nerve fibres can be distinguished in the median eminence according to the size of the electron dense granules they contain: 1. Nerve fibres containing granules 1,330–2,330 Å in diameter. They are seen primarily in the hypothalamo-hypophysial tract, but also in the zona externa; 2. those containing granules with a mean diameter of 1,330 Å; and 3. those containing granules with a mean diameter of 1,000 Å. The last two types are both encountered in the hypothalamo-hypophysial tract, the zona externa and the perivascular region of the base of the median eminence. Under high magnification, the membrane of the granules show evidence of a trilaminar structure and the content of the granules with a low electron density appeares to consist of small microvesicles or globular components. Besides granules, these nerve fibres contain vesicles mostly 420 Å in diameter whose relative number increases towards the perivascular nerve endings. 53 per cent of the inclusions in the hypothalamo-hypophysial tract are granules and 47 per cent vesicles, while the corresponding percentages for the zona externa are 40 and 60 and for the perivascular nerve endings 20 and 80.The mean width of the pericapillary space is 1 , but it varies greatly. It containes many collagen fibrils and fibroblasts. The capillary endothelium is frequently fenestrated and contains many vesicles of various sizes.Two types of granules-containing cells are found in the pars tuberalis depending on the size of the electron dense granules: 1. cells containing granules with a mean diameter of 1,330 Å: and 2. cells containing granules with a mean diameter of 2,000 Å. In addition, there are occasional follicular cavities filled with amorphous material, microvilli and cilia of 9+2 fibrillar pattern.Aided by a grant from the Sigrid Jusélius Stifteise.     

Ultrastructure of the rat median eminence after superfusion

Summary Isolated medio-basal hypothalami of adult rats were continuously superfused in a chamber with controllable inputs and outputs, for periods from 30 to 240 min. The median eminence was prepared for transmission electron microscopy under carefully controlled conditions by immersion fixation with osmium tetroxide. The ultrastructure of superfused median eminence was compared with that of directly fixed, non-superfused median eminence. Even after 4h of superfusion, the median eminence displays remarkably well preserved histological and cytological patterns; cytomembranes, cell organelles, intercellular relationships, and extracellular spaces were remarkably similar in superfused and non-superfused tissues. As a consequence of osmium tetroxide fixation, microtubules were not observable. The ultrastructural information obtained from unstimulated rat median eminence superfused in vitro provides a basis for future morphofunctional correlations in the study of neurosecretion.     

Further data on the origin of gonadotropin releasing hormone in the median eminence of the rat hypothalamus

Gonadotropin releasing hormone (GnRH) content of the two halves of the median eminence of the rat hypothalamus was determined by radioimmunoassay three weeks after three different unilateral knife cuts around the preoptic area. A unilateral cut in front or above the area caused a more than 25% decrease in the GnRH content of the two halves of the median eminence. A cut lateral to the preoptic region had only a slight effect similar to that observed after sham operations. The data suggest that probably more than 50% of the rat median eminence GnRH derives from outside the preoptic-suprachiasmatic region. The GnRH fibres projecting to the median eminence but arising from outside the preoptic region, probably mainly from GnRH perikarya in the limbs of the diagonal band of Broca and septum, enter this area partly from rostral and partly from above, but not from lateral direction. partly from rostral and partly from above, but not from lateral direction. Several of these fibres probably cross before terminating in the median eminence.     

The effects of orchidectomy and replacement therapy on the ultrastructure and gonadotropin-releasing hormone content of the median eminence of the rat

The effects of 2 weeks of orchidectomy and replacement therapy with testosterone upon the content and distribution of gonadotropin-releasing hormone (GnRH) in the median eminence were determined by means of radioimmunoassay and electron microscopy. Photographic montages were prepared from electron micrographs of the lateral median eminence at the point of deepest invagination of the tuberoinfundibular sulcus. Morphometric analysis of photographs of tissues immunohistochemically stained for GnRH was performed to determine changes in the volume density of GnRH-containing axon profiles following the experimental treatments. A decrease in GnRH content after orchidectomy was observed both by morphometric analysis of axon volume density and radioimmunoassay of total GnRH content. Testosterone treatment of orchidectomized animals prevented the postorchidectomy loss of GnRH. Morphometric analysis of conventional electron micrographs revealed an increase in the number of axons containing no dense-core vesicles following orchidectomy, but no decrease in volume density of the neuropil. The results indicate that the change in volume density of immunostained axons was related to the loss of immunostainable dense-core vesicles and not to a change in the size or number of axons. The area corresponding to the location of the highest concentration of GnRH-containing axons was observed to be largely avascular and separated from the vessels of the tuberoinfundibular sulcus by a "border zone" composed of glial foot processes. The unique morphology of the GnRH area has suggested the name "compact zone" to distinguish it from the palisade zone with which it is continuous medially. GnRH axons in this region are probably part of a tract extending farther caudally rather than a terminal field.     

Immunohistochemical localization of thyrotropin-releasing factor in the rat median eminence

Summary Antibodies raised in rabbits against a synthetic preparation of thyrotropin-releasing factor (TRF) were used in association with the immunoperoxidase histochemical technique to localize TRF in the rat median eminence. Using coronal sections cut through the mid-arcuate region, specific immunoreactive material was observed in the medial and lateral regions of the superficial layer of the external median eminence.This work was financed by a grant from the Medical Research Council of New Zealand     

Ultrastructural evidence for exocytosis in the median eminence of the rat

Summary Exocytosis has been demonstrated by electron microscopy in the external zone of the median eminence of the rat. Exocytotic profiles have been observed in nerve fibres characterized by the presence of granular vesicles with median diameters of 90–103 nm and agranular vesicles of about 50 nm. In addition to the small agranular vesicles, coated vesicles of the same size have been found in many nerve fibres, suggesting that at least part of the agranular vesicles in the median eminence originate by micro-pinocytosis. The nature of the fibres showing exocytosis is discussed. Attention is drawn to the possibility of identifying types of fibres in the median eminence by the occurrence of exocytosis.The technical assistance of Mrs. R. M. Y. Hartsteen is acknowledged with gratitude. We would like to thank Prof. J. Moll for his helpful criticism. We also thank Miss P. Delfos and Mr. W. van den Oudenalder for photographic assistance.     

Endothelin-1 and endothelin-3 stimulate prostaglandin E2 secretion from the rat median eminence.

The in vitro effects of endothelin-1 (ET-1) and endothelin-3 (ET-3) on the release of prostaglandin (PG)E2 from the rat median eminence were investigated. The addition of ET-1 from 10(-9) M to 10(-6) M stimulated PGE2 release in a dose-dependent manner (from 10.5 +/- 2.1 to 54.4 +/- 5.6 pg/ME fragment/30 min; mean +/- SEM, p less than 0.001). ET-3 also stimulated the release of PGE2 from 10(-7) M to 10(-5) M dose dependently (from 18.1 +/- 0.7 to 60.9 +/- 17.4 pg/ME fragment/30 min p less than 0.05). The time course effect of ET-3 (10(-6) M) showed that PGE2 release was stimulated within five minutes (control, 1.5 +/- 0.5; ET-3, 15.8 +/- 3.0 pg/ME fragment/5 min, p less than 0.01). These results suggest that ET-1 and ET-3 have some physiological effects on the rat median eminence.     

Localization of serotonin within tanycytes of the rat median eminence

Summary Formaldehyde and glyoxylic acid histochemical methods were employed to examine monoamine fluorescence of the rat median eminence. Tanycytes of the median eminence contained a yellow histofluorescence which was verified with microspectrofluorometry as due to the presence of serotonin. Catecholamine-containing varicosities, arranged in linear profiles throughout the depth of the median eminence, were observed. These linear profiles appeared to follow the contours of serotonin-containing tanycytes. Organculture experiments supported the hypothesis that the serotonin associated with tanycytes is localized within the tanycytes and does not arise from an extrahypothalamic source of nerve terminals. These data provide evidence that a tanycytic catecholamine-indoleamine morphological juxtaposition occurs in a manner reminiscent of that of another circumventricular organ, the pineal.Supported by USPHS Grants NS11642 and AM-19761USPHS Career Development Awardee NS-00259     

Cholinergic afferents to gonadotropin-releasing hormone neurons of the rat

Gonadotropin-releasing hormone-synthesizing neurons represent the final common pathway in the hypothalamic regulation of reproduction and their secretory activity is influenced by a variety of neurotransmitters and neuromodulators acting centrally in synaptic afferents to gonadotropin-releasing hormone neurons. The present study examined the anatomical relationship of cholinergic neuronal pathways and gonadotropin-releasing hormone neurons of the preoptic area. The immunocytochemical detection of choline acetyltransferase or vesicular acetylcholine transporter revealed a fine network of cholinergic fibers in this region. At the light microscopic level, the cholinergic axons formed appositions to the gonadotropin-releasing hormone immunoreactive cell bodies and dendrites. Results of electron microscopic studies confirmed the absence of glial interpositions in many of these neuronal contacts. Classical cholinergic synapses, which belonged to the asymmetric category, were only observed rarely on gonadotropin-releasing hormone neurons. The lack of synaptic density in most contacts corroborates previous observations on the cholinergic system elsewhere in the brain. Further, it suggests a dominantly non-synaptic route also in this cholinergic neuronal communication. This study provides direct neuromorphological evidence for the involvement of the cholinergic system in the afferent neuronal regulation of gonadotropin-releasing hormone neurons. The sources of cholinergic afferents and the receptorial mechanisms underlying this interaction will require further clarification.