The neurohypophyseal capillary bed. II. Specializations within median eminence

Vascular casts of the pituitary gland, median eminence and hypothalamus from several mammalian species were examined by scanning electron microscopy. These observations were correlated with light microscopic studies of injected, cleared median eminence-pituitary specimens and with light microscopic examination of serial sections of injected hypothalamic, median eminence, and pituitary specimens employing reflected lighting or epi-illumination. Transmission electron microscopy was employed to study long portal vessels on the ventral surface of the rat median eminence. In each of the species examined, the median eminence (infundibular) capillary bed is subdivided into an external and an internal plexus. The external plexus (the neurohemal contact zone) receives an arterial supply from the superior hypophyseal arteries and is continuous with the capillary bed of the infundibular stem and process. Egress from the external plexus is possible via three vascular routes: (a) by fenestrated portal vessels and capillaries to the adenohypophysis, (b) by capillary connections to the medial basilar hypothalamus and, (c) by internal plexus capillaries to the ependyma of the median eminence. Median eminence vasculature is structurally organized to deliver (1) hypothalamic and neurohypophyseal peptides to the glandular pituitary via portal vessels, (2) hypothalamic and pituitary secretions to the medial basilar hypothalamus via capillaries, and (3) hypothalamic and pituitary secretions to distant brain sites through cerebrospinal fluid via ventricular and subarachnoid routes.

Fine structure of the palisade zone of the rat median eminence as revealed by freeze-fracturing

Nerve terminals in the palisade zone of the rat median eminence were investigated with freeze-fracture electron microscopy. Fracture face P of the specific terminals showed two populations of intramembranous particles (IMP): a large and a small variety. The large IMP-s often formed small irregular clusters. In nerve terminals the total number of both populations of IMP-s was considerably less than that found on P membrane faces of ependymal feet. On fracture face E of the nerve terminals, the number of IMP-s was about a quarter of that seen on fracture face P. On both fracture faces of most terminals a few small round impressions (or elevations respectively) were found which may be interpreted as broken necks of either exo- or endocytotic vesicles. Neither gap nor tight junctions occurred at lateral membranes of the specific axon terminals. Similarly, no membrane specializations were observed with freeze-fracturing on membrane areas adjacent to the basement membrane. The findings are discussed in relation to a possible exocytosis mechanism of the hypothalamic releasing and inhibiting hormones.

[Immunohistochemical localization by electron microscopy of GnRH in the median eminence of Xenopus laevis daud]

Using antibodies against mammalian LHRH with the unlabeled antibody enzyme method immunoreactive nerve fibres were localized in the median eminence of Xenopus laevis at the electron microscopic level. These fibres contain irregularly shaped neurosecretory granules with an average diameter of 890 A.

Ipsilateral diminution of CRF-granules after unilateral hypothalamic lesions

Axons terminating in the outer layer of the median eminence of rats contain light microscopically visible granules. The granules are assumed to represent a corticotropin-releasing factor and, therefore, are called CRF-granules. To find out whether neurons containing CRF-granules originate and run together with the neurons of the hypothalamus-neural lobe system (HNS), the effect of unilateral lesions in the HNS on the amount and distribution of CRF-granules was studied in bilaterally adrenalectomized rats. HNS lesions prevented the adrenalectomy-induced increase in CRF-granules on the side of the lesion. Lesions outside the HNS or sham lesions did not influence the amount and distribution of the granules. The findings suggest that CRF-granules are located in terminals of neurons whose perikarya are situated in magnocellular hypothalamic nuclei. It can also be concluded that the axons of these neurons run within the HNS and do not decussate.

Direct projection from the medial preoptic area to the median eminence of the cat

The projection from the medial preoptic area to the median eminence of the cat was clarified by electron microscopy. After placing the electrolytic lesion in the preoptic area several kinds of degenerating neuronal processes and terminals were observed in the external layer of the median eminence. The one was dark shrunk terminals containing dense cored vesicles, the other was the dark ones containing myeline figure-like structure. The relationship between catecholamine-containing nerve endings and RH/IH-containing endings in the external layer of the median eminence was discussed.

Scanning electron microscopy of the third ventricular floor of the rat

By utilizing a horizontal dissection technique the entire floor of the third ventricle has been examined. When viewed in toto the ventricular floor was seen to have an hourglass shape with the supraoptic and infundibular recesses equalling the widened portions. Consistent regional differnces were also noted. The rostral half of the floor was densely ciliated while the caudal portion, containing ependymal elements of the underlying median emience, possessed few cilia. The ciliated cells ended in an abrupt transition zone located about halfway along the floor. The rostral portion of the infundibular recess had many more apical blebs and microvilli than did the caudal areas. Supraepen dymal cells of both the phagocyte-like and neuron-like variety were observed in all of the animals examined. In some animals, complex, branching, interconnecting networks of fine calibered fibers interconnected neuron-like cells that occurred singly and occasionally in clusters. Female rats examined at all phases of the estrus cycle demonstrated no cyclic alterations of the ependymal surface.

Fine structure of ependymal cells in the median eminence of the frog and mouse revealed by freeze-etching

Freeze-etched preparations of the ventricular surfaces of ependymal cells clearly reveal the presence of pinocytotic vesicles opening into the third ventricle and large vacuoles formed by broad cell projections. The density of the vesicular openings is approximately 20 per micron2. The ependymal cells in the median eminence of the frog are adjoined by tight junctions comprised of five to eight interconnected junctional strands, whereas near the median eminence in the mouse only one to two such strands form the tight junction of the ependymal cells. Gap junctions between the adjacent ependymal cells are detected near the median eminence in the mouse but not in the frog.

Morphological aspects of the hypothalamic-hypophyseal system. VII. The tanycytes: Their relation to the hypophyseal adrenocorticotrophic function. An ultrastructural study

The ultrastruct of the tanycyte ependyma in male 160-180 g Wistar albino rats was studied under normal conditions and in experiments involving long-term suppression of ACTH secretion and its long-term stimulation. The former was accomplished by daily (for 8 days) intraperitoneal administration of dexamethasone phosphate at low (5 microgram/100 g) and high (100 microgram/100 g) concentrations. The effectiveness of suppression of the hypothalamic-hypophyseal-adrenal system in the experimental animals was judged by their reaction to two-minute ether stress (determination of plasma corticosterone) and by the results of measurement of the adrenal weights. Stimulation of ACTH secretion was achieved by bilateral adrenalectomy; the animals were examined on days 8, 10, 14, and 22 following the operation. The results obtained were in agreement with the previously established fact that there is a negative correlation between tanycyte activity and hypophyseal adrenocorticotropic function (Akmayed and Fidelina, 1974). They also testified to the predominant involvement of the median eminence tanycyte ependyma (beta-tanycytes according to the author's nomenclature) in these relationships. It is supposed that correlations are regulated by a feedback mechanism and attest to the involvement of beta-tanycytes in the inhibiting control of hypophyseal adrenocorticotrophic function. The mechanism of this control may be explained alternatively: either the tanycytes transport ACTH-suppression substances (catecholamines, corticosteroids, ACTH) from the CSF to the hypophyseal portal system or they themselves secrete substances possessing ACTH-suppressive activity. The authors distinguish several types of vesicles in the beta-tanycytes, the number of which changed with experimentally induced shifts in hypophyseal adrenocorticotrophic function. These vesicles are discussed in connection with the transport and secretory activity of the tanycytes and are considered to be a possible substrate of the hypothalamic inhibiting effect on ACTH secretion.

Hormonal content and ultrastructure of the hypothalamo-neurohypophysial system of the rat after injection of vinblastine into the median eminence

The effects of an injection of vinblastine (Vbl) into the median eminence on the structure, fine structure and antidiuretic hormone (ADH) content of the hypothalamo-neurohypophysial system in the rat is reported. The animals were studied on days 3, 8 and 25 after the injection of 1 or 5 mM-Vbl (3micronl). Significant changes were observed only in the 5 mM-Vbl-injectecd animals. Their median eminence extracts showed a progressive accumulation of ADH whereas ADH depletion occurred in the neural lobe extracts. On day 8 after injection, the animals exhibited strong polidipsia although considerable amounts of ADH still remained within the neural lobe. The ADH content of the plasma samples was consistently below the sensitivity of the method (5 micronu). The light microscopic analysis showed accumulation of Gomori-stainable products in the median eminence and a striking depletion of this material from the neural lobe. Electron microscopy revealed accumulation of neurosecretory vesicles and other inclusions proximal to the site of injection in the median eminence together with some evidence of nerve fibre degeneration. Few neurosecretory terminals were found in the neural lobe of the 8-day experimental rats. They had been engulfed by pituicytes for digestion. Recuperation of the normal ADH content of both median eminence and neural lobe was found to occur on day 25 after the Vbl injection. Simultaneously, the neural lobe refilled with Gomori-positive materials and neurosecretory terminals reappeared. The results suggest (1) reversible blockade of axoplasmic transport at the site of the Vbl injection; (2) reversible degeneration of neurosecretory terminals and (3) reversible blockade of ADH release in the surviving terminals of the neural lobe.

Ultrastructure of homografts of the rat median eminence into the anterior chamber of the eye

Median eminences (MEs) were transplanted into the anterior chamber of the eye and collected at times varying between 10 and 72 days. At 10 days, all axons had degenerated leaving behind a well-vascularized epitheloid glial organ. With increasing age of the grafts, the characteristic lipid inclusions in the glial cells diminished and even disappeared, and the clear ultrastructural distinction between ependymal (tanycytes) and other glial cells became difficult or even impossible. In both the perikarya and perivascular terminals, an increased number of membrane-bounded, pleomorphic, electron-dense granulated vesicles occurred, the significance of which is unknown. Regrafting of these transplants into various hypothalamic and hypophysial sites is expected to yield information on the function of the glial cells of the ME.

A morphological survey of the median eminence: fluorescence histochemistry, electron microscopy and immunohistochemistry

This review is a morphological survey of the median eminence of the rat as revealed by fluorescence histochemistry, electron microscopy and immunohistochemistry. Strong catecholamine fluorescence was diffusely distributed in the external layer of the median eminence especially around the primary capillaries of the portal vessels. In the internal layer, there was a scattering of varicosities or strands with catecholamine fluorescence. The origin of these catecholamine nerve fibers and the function of dopamine (DA) nerve fibers in the external layer were discussed. In the external layer of the median eminence, a large number of nerve endings and processes of various sizes ranging from 0.5 to 2.0 micrometer in diameter were found around pericapillary spaces of the hypophyseal portal system. These nerve endings have been classified into three types according to the morphological features of the vesicles contained in them. Small and large dense cored vesicles (40-60 nm, 70-100 nm in diameter) with very high electron density appeared in the first type nerve endings after 5-OHDA treatment; these endings were regarded as monoaminergic, and more specifically, catecholaminergic. Topographical distribution of luteinizing hormone-releasing hormone (LH-RH) nerve fibers in the median eminence was revealed by immunohistochemistry using the peroxidase antiperoxidase (PAP) method. LH-RH nerve processes and terminals appeared as brown beaded strands and dots in the PAP method. LH-RH terminals in the median eminence were seen to be encircling the infundibular radix. However, they were not evenly distributed; the most notable accumulation of LH-RH reactive dots were located in the region extending from the dorsal part of the tuberoinfundibular sulcus to the lateral part of external layer of the superior infundibular labium at the infundibular radix. Considering the distribution of LH-RH, the growth hormone-release inhibiting hormone (GIH) and the monoamine nerve endings in the median eminence, the function of the anterior pituitary appeared to be regulated by a delicate interrelationship of nerve endings.

The primate median eminence. II. Correlative high-voltage transmission electron microscopy

The ultrastructure of the normal median eminence of the male rhesus monkey (Macaca mulatta) is described using high-voltage electron microscopy. Surface specializations of ependymal cells lining the infundibular recess included cilia, apical extrusions, and microvilli. Supraependymal cells were predominantly macrophage-like, but examples of lymphocytic types were also seen. Tanycytes had long, branching, basal processes filled with numerous microtubules, some lipid droplets, and granules. The zona interna was composed of large unmyelinated neurosecretory fibers. A few myelinated fibers were also seen, but their character as neurosecretory fibers could not be established. The zona externa was composed of densely-packed profiles of neuro secretory fibers of small diameter, was well-vascularized and contained the terminations of tanycytes. Perivascular glial cells, vesiculated elements, pituicytes, and cellular elements common to connective tissue were observed. The intricate relationships between both the cellular and fibrous elements of the median eminence can be appreciated with the capability of high-voltage electron microscopy to discern ultrastructure in sections 10 times thicker than those used for low-voltage electron microscopy. The median eminence of this primate species has an ultrastructural organization similar to that described for most other species.

The zinc iodide-osmium tetroxide (ZIO) reaction on nerve endings in the median eminence of the rat under normal and experimental conditions

The reaction of nerve endings in the median eminence of the rat to zinc iodide-osmium tetroxide (ZIO) staining was examined electron microscopically under normal and experimental conditions. The experimental condition of catecholamine exhaustion in the nerve endings was induced by the administration of H44/68 and reserpine. Vesicles in the terminals of catecholaminergic nerves reacted similarly to ZIO staining in both normal and experimental material. The majority of synaptic vesicles in various terminals gave a positive ZIO reaction. The neurosecretory elementary granules, however, failed to react with ZIO. On the other hand, some nerve terminals in the external layer of the median eminence showed a strong positive reaction in the cytoplasmic matrix, in mitochondria as well as in synaptic vesicles. These findings strongly suggest that the ZIO-positive substance in nerve terminals is not the transmitter itself, i.e. the monoamine, but rather represents a range of substances commonly found in various kinds of synaptic vesicles and is probably proteinaceous in nature. A brief discussion is also given on the difference in ZIO reactivity between neurosecretory elementary granules and small vesicles in the hypothalamo-hypophyseal tract.

Monosodium glutamate induced lesions of the arcuate nucleus. I. Endocrine deficiency and ultrastructure of the median eminence

Monosodium glutamate was administered daily on days 5 through 10 postnatally at a dose of 2.5 mg/gm body weight. Counts of remaining perikarya in the arcuate nucleus of adult mice indicated approximately an 80% decrease in the number of perikarya. The arcuate lesion resulted in endocrine deficits; reporductive capacity was reduced, animals were smaller in stature and obese, and the weights of the anterior pituitary, ovaries and testes were significantly decreased while adrenals were unaffected. Light microscopic studies revealed no significant changes in thickness or general histological appearance of the median eminence. At the electron microscope level, there were no alterations in the number of nerve terminals or dense core vesicles per unit area in the contact zone. These observations suggest that afferents to the median eminence from the arcuate nucleus may form a relatively small portion of its total nerve terminal population.

Quantitative ultrastructural observations on the inhibited recovery of the hypoglossal nucleus from the axotomy response when regeneration of the hypoglossal nerve is prevented

Electron micrographs of the left hypoglossal nucleus were quantitatively analyzed in adult rats 52 to 98 days after transection of the left hypoglossal nerve and implantation of the proximal stump into the already innervated ipsilateral sternomastoid muscle, a procedure which prevented the transected nerve from regenerating. Many presynaptic boutons with clear spherical synaptic vesicles and symmetrical synapses were lost from the injured perikarya and dendrites. Some perikarya and dendrites (and, rarely, boutons) became electron dense, and astrocyte or microglial sheaths partly surrounded them. Numbers of dendrite profiles in the neuropil decreased. These statistically significant effects persisted throughout the postoperative period, whereas after axotomy with unimpeded nerve regeneration, these features would have returned to normal by 84 days postoperatively. It was therefore suggested that their recovery depended upon successful regeneration and reconnection of the hypoglossal nerve with the tongue. Subsurface cisterns, and profiles containing unusual inclusions, were numerically normal 52 to 98 days postoperatively, so it was suggested that their early response and recovery after simple axotomy might be entirely dependent on nerve disconnection and not on nerve reconnection.

Scanning microscopy of pituitary vascular casts

Vascular casts of the pituitary-median eminence complex from seventeen adult female rabbits were examined with the scanning electron microscope. The results of this study confirm the presence of a single capillary bed common to the entire neurohypophysis. Arterial supply to the rabbit pituitary is only to the neurohypophysis. A direct supply to adenohypophysis was not found. Within the median eminence there are an external and internal capillary plexus. The internal capillary plexus is directed toward the infundibular recess of the third ventricle. It does not receive a direct arterial supply but derives its blood supply from the external plexus before draining to the adenohypophysis. Vessels of the posterior median eminence are confluent with vessels of the infundibular stem. On the basis of these studies, it is proposed that the entire neurohypophysis, not simply the median eminence, serves as the final common pathway to the glandular pituitary. It is also proposed that in the median eminence, vessels are organized to deliver blood containing hypothalamic releasing and inhibiting hormones as well as posterior lobe neural hormones (antidiuretic hormone and oxytocin) to the ventricular surface for subsequent transport to cerebrospinal fluid and distribution to the brain.

Organization of the hypothalamic-pituitary system: current concepts from immunohistochemical studies

Immunoperoxidase technique and light microscopy have been used to localize neurosecretory systems for vasopressin, oxytocin and related neurophysins (neurohypophysial peptides)) and gonadotropin-releasing hormone (Gn-RH) in the rhesus monkey brain. All the neurohypophysial peptides were found in the magnocellular nuclei (suproptic and paraventricular) of the hypothalamus and in their projections to the posterior pituitary gland, the zona externa of the median eminence and the organum vasculosum of the lamina terminalis (OVLT). Gn-RH was found in smaller cell bodies which were widely scattered in the hypothalamus. Some of these were found in the media-basal hypothalamus in the infundibular nucleus and lateral and dorsal to it, while others were found in dorsal hypothalamus. Numerous cells were also located in the preoptic area close to the OLVT. Gn-RH-containing fibers projected to the OVLT and the zona externa of the median eminence. The two neurosecretory systems studied have two common features: magnocellular perikarya containing the neurohypophysial peptides and smaller elements containing Gn-RH are found near and appear to terminate around the fine vessels of the OVLT. In addition, cells of both systems send fibers to the hypophysial portal capillary system in the zona externa of the median eminence. Many more vasopressin-than oxytocin-containing fibers end in the entire expanse of the zona externa, where they are mainly concentrated in the anterior and middle parts, while Gn-RH fibers project to all portions.

Immunohistochemical localization of hypothalamic hormones

Using an immunoperoxidase technique at the light and electron microscope levels, the localization of somatostatin and luteinizing hormone-releasing hormone (LH-RH) has been performed in the brain, as well as the pancreas and stomach. The following generalizations may be drawn on the localization of LH-RH and somatostatin. (a) LH-RH and somatostatin are contained in axons and nerve endings of the median eminence and organum vasculosum of the lamina terminalis. (b) LH-RH and somatostatin are present in different nerve endings. (c) In the subcommissural organ, subfornical organ and area postrema, the two neurohormones are present in the cytoplasm of ependyman and subependymal cells. (d) In the endocrine pancreas and gastric mucosa, somatostatin is localized within the secretory granules of specific endocrine cells.