Radioautographic studies on the neurohypophysial projections of the supraoptic and paraventricular nuclei in the rat

The distribution of labelled axonal pathways was studied after unilateral stereotaxic injection of 3H-leucine into either supraoptic (SON) or paraventricular nuclei (PVN). In addition to extrahypothalamic projections of both nuclei, the main efferents appeared to run towards the neurohypophysis, yet with a strikingly different pattern. At the neurohypophysial levels, the SO-neurohypophysial tract crossed the inner layers of the median eminence (ME) before scattering in the neural lobe. The PV-neurohypophysial pathway, by contrast, provided an exclusive innervation to the external layer of the whole neurohypophysial organ, including the median eminence, infundibular stalk and neural lobe. The functional correlates of the clear-cut anatomical distinctness between the two magnocellular neurosecretory systems are discussed.

Myelinated herring bodies in the posterior part of the median eminence of the mouse

The posterior part of the median eminence of the albino mouse (CF # 1-JCL) contains a cluster of myelinated axons beneath the tanycyte layer. Among them, small Herring bodies surrounded by myelin sheaths are revealed by electron microscopy. These structures contain electron-dense bodies, lamellar bodies, autophagic vacuoles, and neurofilaments. A few neurosecretory granules and mitochondria are also present. Some myelinated axons contain mostly accumulated neurosecretory granules.

The hypothalamo-hypophysial system of hypophysectomized rats. II. Structure and ultrastructure of the median eminence

The median eminence (ME) of hypophysectomized rats was studied by means of light and electron microscopy. Paraldehyde-fuchsin (PAF)-positive material is seen in the external zone (EZ) of the ME 2--5 days after the operation. Its amount gradually increases especially in the caudal part of the ME during the following few days. Some PAF-positive fibers make contact with the subependymally located blood capillaries. In the most caudal region of the recessus infundibuli they penetrate into the third ventricle. PAF-positive material decreases markedly from the ME of rats two months after hypophysectomy and exposure to a 1% salt load. Fibers of types A1, A2 and B containing granules of 120--220 nm, 100--150 nm and 80--100 nm in diameter, respectively, are seen in the EZ of the ME in hypophysectomized rats, although almost exclusively A2- and B-type structures make contact with the primary portal capillaries in intact animals. All types of neurosecretory fibers establish contact with the subependymal nonfenestrated blood capillaries and penetrate the recessus infundibuli. Some neurosecretory terminals of different types make direct contact with the glandular cells of the pars tuberalis or are separated from them by a thin basal lamina. It is assumed that mainly neurosecretory fibers of types A2 and B are permanently connected with the primary portal capillaries in the EZ of the ME in intact mammals, while the overwhelming majority of fibers of A1-type shows ingrowth during the course of postoperative reparation. The possible physiological significance of the described changes is discussed.

[Status of neurocytes of the arcuate and suprachiasmatic nuclei of the hypothalamus and median eminence in rats exposed to prolonged internal irradiation]

Ultrastructural changes caused by single administration of 75Se-selenomethionine (1.22 X 10(4) Bk/g bw) were studied in neurocytes of the arcuate and suprachiasmatic nuclei of the hypothalamus and medial eminence in random-bred male rats. Within the first three months after administration of the radioactive agent, there was an activation of the synthesis of neurocyte secretion granules and release of their content from nerve terminals into the channel of medial eminence portal capillaries. Afterwards (over 6-12 months) the synthesis of of neurocyte secretion material and the release into medial eminence portal capillaries were suppressed. By the 18th month after 75Se-selenomethionine administration there occurred a relative normalization of the structure and function of neurocytes, and activation of the release of the contents of secretion granules from nerve terminals into the channel of medial eminence portal capillaries.

Age related changes in the endocrine hypothalamus: I. Tanycytes and the blood-brain-cerebrospinal fluid barrier

The fine structural organization of the floor of the third cerebral ventricle (dorsum of the median eminence of the hypothalamus) of 2 normal adult mice Fisher 344 rats was compared and contrasted with that of 2 aged rats 30 months old. Closely juxtaposed tanycytes (specialized ependymal cells) of normal young adults in the lower walls and floor of the third ventricle. In contrast, tanycytes in aged rats demonstrated significant intracellular separations, with only fine cytoplasmic processes remaining to interlink them. The phenomenon of mechanical separation between tanycytes in aged animals is discussed with respect to a potential impairment in the integrity of the blood-brain-cerebrospinal fluid barrier.

The median eminence in normal, ovariectomized, and ovariectomized-estradiol-treated hamsters: an ultrastructural study

In an effort to define more clearly the effect various plasma concentrations of estrogen have on the morphology and function of tanycytes, the present investigation examined the median eminence (ME) of normally cycling, ovariectomized, and ovariectomized-estradiol-treated hamsters. In normally-cycling animals, when endogenous estrogen was at its highest level (day 4 or proestrus), numerous microappendages arose from the luminal surfaces of tanycytes located in the ventrolateral region of the ME. Large blebs (1.0-5.0-micrometers diameter), miniblebs (1.0-micrometer diameter), and microvilli dominated the surfaces of these cells. Large blebs appeared to have been formed by the coalescence of several miniblebs and were composed of cytoplasmic ground substance devoid of organelles. The peduncular shape of many of these blebs suggested their involvement in an apocrinelike secretion by the tanycyte. When endogenous estrogen levels were low (day 1 of the estrous cycle), the tanycytes of normally cycling hamsters possessed slightly fewer microappendages. Following ovariectomy, large blebs were nearly absent from the luminal surfaces of tanycytes, and the number of miniblebs and microvilli were also greatly reduced. Subcutaneous injections of 17-beta estradiol benzoate restored the large blebs to the tanycyte surface. The number and variety of tanycytic microappendages in these animals resembled those in normally cycling hamsters on day 4 of the estrous cycle. The present study demonstrates that tanycytes of the hamster ME are sensitive to estrogen and vary in their morphology in relation to the animal's reproductive status. These changes in tanycyte morphology can be correlated directly to functions of absorption (microvilli) and secretion (blebs). The sensitivity of tanycytes to estrogen suggests that these cells may also play a role in the hypophyseal-ovarian feedback mechanism.

Light and electron microscopic observation on the appearance of immunoreactive LHRH in perinatal rat hypothalamus

The appearance and localization of LHRH were studied in the developing hypothalamus of perinatal rats using the unlabelled antibody method. By light microscopy, immunoreactive LHRH was first detected as brown dots on day 18.5 of gestation in the OVLT and on day 19.5 in the median eminence, respectively. When the median eminence was examined by the preembedding immunohistochemistry technique for electron microscopy, the occurrence of immunoreactive LHRH fibers could be demonstrated on day 18.5. These fibers were thin and very occasionally encountered near the surface of the lateral regions of the median eminence. The axoplasm contained a few immunopositive secretory granules and also extragranular immunoreactive products. With development, a gradual increase was noted both in number and size of nerve fibers with a concomitant accumulation of secretory granules within the axoplasm. A possible physiological significance of LHRH is discussed in relation to the onset of hypothalamo-hypophysial system in fetal life.

Effects of p-chloroamphetamine, a serotonin-depleting drug, on the median eminence and pituitary pars intermedia

p-Chloroamphetamine (PCA), an agent known to cause depletion of levels of brain serotonin in rodents, was administered to rats in three sequential injections (10mg/kg) to study effects on the hypothalamic median eminence and pituitary gland. One week following the initial sequence of injections of PCA, light and electron micrographs revealed degenerate fibers in the outer zone of the median eminence. Lower drug doses or single 10-mg/kg doses did not lead to morphologic changes. Neuronal processes located in the pituitary intermediate lobe appeared normal although there was a significant increase in the numbers of secretory granules contained within intermediate lobe cells drug-treated rats, as compared to controls. Fluorometric analysis of levels of catecholamine and indoleamine showed a decrease in serotonin in median eminence and pons-medulla, but no change in that of the pituitary. Levels of dopamine and norepinephrine remained unchanged after PCA treatment. The data suggest that fibers affected in the median eminence contain serotonin. Processes in the intermediate lobe may be resistant to the serotonin-lowering effects of PCA observed in brain tissue. In addition, PCA may directly affect granule release from pituitary cells, or may alternatively act on hypothalamic regions which affect the release of intermediate lobe cell hormones.

Functional difference between "classical" neurosecretory material and vasopressin-like substances of the outer layer of the median eminence

In female Wistar rats the influence of adrenalectomy and NaCl administration on the amount of "classical" neurosecretory material (cNSM) in the supraoptico-hypophysial system and on vasopressin-like substance-containing granules (vlG) in the outer layer of the median eminence has been studied. In conjunction with appropriate sodium replacement, adrenalectomy induces an increase in the amount of vlG but does not alter the amount of cNSM. Administration of hypertonic saline diminishes cNSM but has not or only little influence on the amount of vlG. From the findings it is concluded that cNSM and vlG, in spite of their identical histochemical and immunohistochemical properties, have different functions. The functional significance of the vlG is discussed.

Prenatal and early postnatal development of the glial cells in the median eminence of the rat

The development of the glial cells of the rat median eminence (ME), including the supraependymal cells, was investigated from embryonic day (ED) 14 through postnatal day (PD) 7, and pituicyte development from ED 12 through ED 17. The anlage of the ME and neurohypophysis shows a neuroepithelial-like structure at ED 12. From ED 13 to 15, the cells of both regions start to differentiate. At the ultrastructural level, only one cell type appears. At the beginning of ED 16, glioblasts of the oligodendrocyte and astrocyte series migrate laterally (from the region of the arcuate nucleus) into the ME. Also at this time the first distinctive structural features appear in the neurohypophysial anlage, the cells of which later develop into pituicytes. Starting at ED 18, tanycytes and astrocytic tanycytes arise in the ME from local glial cells, and somewhat later oligodendroblasts and astroblasts are formed from immigrant glioblasts. Due to their common features, the pituicytes, tanycytes and astrocytic tanycytes apparently represent different forms of the same parent cell type. Microglial and supraependymal cells are first seen at ED 12. Initially, they resemble the prenatal phagocytic connective tissue cells and mature in fetus into typical electron-dense microglia and macrophage-like supraependymal cells. Both cell types are apparently of mesodermal origin. The microglial elements of the ME probably migrate from the mesenchyma through the basement into the nervous tissue. The intraventricular macrophages of the infundibular region may originate from microglia, epiplexal cells and subarachnoid macrophages.

Electron microscopic immunocytochemical investigation on the postnatal development of the vasopressin system in the rat

The present ultrastructural results indicate that, in the rat, the vasopressin-synthesizing perikarya of the supraoptic nucleus (NSO) attain a certain degree of maturity earlier than those of the paraventricular nucleus (NPV). In the neonate rat, the stainability of the nuclear areas is very weak; in the perikarya of the NSO a few labeled granules can be found, whereas the perikarya of the NPV often display only a labeled Golgi area, the cytoplasm being devoid of granules. At the end of the first (NSO) and the second (NPV) postnatal weeks, the filling of the neurosecretory granules with vasopressin is inhomogeneous with irregular spots of reaction product distributed on the granules. This feature is less obvious during the following week and has nearly disappeared after the third and fourth postnatal weeks. Already in the neonate two types of vasopressin-positive fibers are observed in the median eminence, characterized by the different diameters of their granules and by their typical location in the internal and the external pericapillary contact zone. Especially in one and two week-old animals, in the internal zone of the median eminence and, to a lesser degree in the neural lobe, the immuncytochemical reaction product is deposited on an axonal tubular network. Judging from the presence of very few vasopressin-negative fibers in the neural lobe of the neonate, the development of the oxytocin system appears to be delayed. A characteristic relationship between pituicytes and the neurosecretory fibers can be observed during the first two postnatal weeks. After the third postnatal week the immunocytochemical features of the vasopressin system correspond approximately to that in adult rats.

Hypothalamic lesions in rats with long-term streptozotocin-induced diabetes mellitus. A semiquantitative light- and electron-microscopic study

Sixteen male Wistar rats, 1 year after injection of streptozotocin or vehicle, were fixed by whole-body perfusion, the brains were removed and processed for light and electron microscopy. Study of semithin sections from the hypothalamic area revealed changes in the arcuate nucleus and median eminence. The lesions, in comparison with controls, were subjected to a blind semiquantitative evaluation. The following changes were observed by light microscopy in diabetic rats: accumulation of glycogen (P < 0.01), degeneration of neurons (P < 0.05), hypotrophy of tanycytes (P < 0.01), and axonal changes. Electron microscopy of diabetic rats revealed that glycogen was increased in neuronal bodies and processes (axons, synapses), also in tanycytes, and glia cells. In neurons were seen: dilated and fragmented endoplasmic reticulum, degranulated ergastoplasm, loss of organelles, increased number of microtubuli, myelin figures, irregularities in the form of nuclei, and appearance of chromatin. The tanycytes in diabetic animals were reduced in volume, had an increased nuclear cytoplasmic ratio, a reduced number of organelles, short basal processes, and almost complete loss of the apical processes. These changes demonstrate the existence, under experimental conditions, of an encephalopathy pathogenetically related to streptozotocin-induced diabetes.

Quantitative radioautographic light and electron microscopic analysis of the localization of monoamines in the median eminence of the rat. I. Catecholamines

By means of light and electron microscopic radioautography a quantitative study of the regional distribution of catecholamines in the median eminence of the rat was carried out. One hour after intraventricular injection of 3H-dopamine the highest radioautographic reaction was recorded in the external zone, especially in the lateral palisade zone where many neurosecretory terminals are separated from the basal lamina of the portal pericapillary space by a glial "cuff". This area showed the highest percentage (52%) of labelled catecholamine containing neurosecretory terminals as well as the maximal silver grain density per one terminal. In the medial palisade zone where direct neurovascular contacts with the capillary loops prevail, only 27 per cent of neurosecretory terminals were found to harbour tritiated dopamine. On the average 35 per cent of neurosecretory terminals in the median eminence of the rat contain catecholamines (both dopamine and noradrenaline). Pretreatment of animals with recretory terminals as well as grain density over terminals were decreased in both the medial and lateral palisade zones, although to a lesser degree in the latter. The site of origin of catecholamine fibers as well as the mode of catecholamine action at the level of the median eminence are discussed.

Quantative radioautographic light and electron microscopic analysis of the localization of monoamines in the median eminence of the rat

Serotonin containing structures in the median eminence of the rat have been studied by quantitative light and electron microscopic radioautography following intraventricular infusion of tritiated 5-hydroxytryptophan. One hour after injection of the tracer the highest density of silver grains was recorded in the ependymal and external zones, especially in the lateral palisade zone. The proportion of labelled neurosecretory terminals was also larger in the lateral palisade zone (29%) as compared with the medial palisade zone (13%), although the mean number of developed silver grains per one terminal was higher in the latter. On the average, 16% of neurosecretory terminals sequestered radiolabelled 5-hydroxytryptophan in the external zone of the rat median eminence. It is suggested that serotonin, like catecholamines, is discharged from neurosecretory terminals localized in the external zone and via the portal circulation affects the function of the anterior pituitary. The sites of origin of serotoninergic structures of the median eminence as well as the possible role of monoamine (catecholamine and indolamine) neurohormones in a dual peptidergic and monoaminergic control of anterior pituitary functions are discussed.

The hypothalamo-hypophysial system of the lamprey, Lampetra fluviatilis L. III. High-resolution radioautography of monoaminergic structures in neurohemal regions

The distribution of monoaminergic structures was studied in the proximal neurosecretory contact region and neurohypophysis of the lamprey by light and electron microscopic radioautography. Only weak radioautographic reactions were found in the proximal neurosecretory contact region 1 h after injection of 3H-dopamine. High-resolution radioautography revealed some labeled neurosecretory terminals mainly in contact with the basement membrane of the connective tissue layer separating the proximal neurosecretory contact region from the hypophysial pars distalis. The number of silver grains as well as the number of neurosecretory terminals marked by the presence of labeled dopamine was much higher in the neurohypophysis of the same species. In the latter, labeled neurosecretory terminals were found in contact with the connective tissue layer containing blood vessels of the general circulation. Some neurosecretory terminals make synaptoid contacts with tanycyte perikarya and their basal processes. According to their ultrastructure and the size of their granules, the labeled neurosecretory terminals are identical with the B type terminals described in both neurohemal regions (transmission electron microscopy). No labeled neurosecretory terminals were observed in the proximal neurosecretory contact region and the neurohypophysis of lampreys treated with the serotonin precursor, 3H-5-hydroxytryptophan.

The tanycyte of the rat median eminence. I. Synaptoid contacts

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.

Organization and development of the perivascular space system in the neurohypophysis of the laboratory mouse

The organization of the system of perivascular space around the capillaries in the neurohypophysis was studied in the adult and developing laboratory mouse by the use of histological silver impregnation and electron microscopical techniques. In the median eminence short and long extensions, arising mainly from the shallow space around capillary loops of the primary plexus of the portal system, formed radiations into the adjacent neural tissue of the external zone. The tissue of the neural lobe was separable into non-vascular regions dominated by undilated portions of neurosecretory nerve fibres and pituicytes, and neurovascular regions with perivascular space extensions forming an extensive system of connections between neighbouring capillaries. In the median eminence, the system of extensions of the perivascular space was estimated to increase the neurovascular contact surface area by at least 50%, implying an increased efficiency of the organ without a notable increase of its volume. The possibility that the ramifications of the perivascular space imply an enhanced uptake rate into the bloodstream and a subsequent increased concentration of the neurohormones in the portal blood, was discussed. During development of the median eminence, differentiation of perivascular space extensions of the adult type started in the juvenile of about three weeks of age, when shallow capillary loops had been formed. In the neural lobe, perivascular space ramifications were already present when the internal capillaries were formed and were fairly frequent in ten-day young. At the age of three to four weeks the organization of the system was similar to that of the adult animal.

A system of intraependymal cisternae along the margins of the median eminence in the rat: structure, three-dimensional arrangement and ontogeny

Structure, three-dimensional arrangement and ontogeny of large intracellular cisternae located in the median eminence region of the rat hypothalamus were studied using toluidin-blue stained semithin sections and electron microscopy. The cisternae occur along the projections of ependymal cells lining the ventral portion of the third ventricle (infundibular recess). Small cisternae can be seen close to the ventricle, whereas larger ones, divided into smaller compartments by thin septa, cluster near the surface of the hypothalamus. The cisternae are encompassed by a thin layer of cytoplasm to which axon terminals containing synaptic and dense core vesicles are closely related. Cisternae are arranged around the median eminence in a characteristic pattern. They occupy the midline in the retrochiasmatic area, flank both margins of the median eminence and extend caudally behind the origin of the pituitary stalk. The cisternae appear first between the 15th and 17th postnatal days. At about the 30th day their size and distribution resemble the situation observed in adult animals. The ependymal cisternae are suggested to be closely related to the luteinizing-hormone releasing-hormone (lH-RH)-containing fibers.

[Changes in the hypothalamo-hyophyseal neurosceretory system of mammals following pituitary stalk transection and hypophysectomy]

Hypophysectomy and pituitary stalk section result in dramatic morpho-functional changes in all parts of mammalian hypothalamo-hypophyseal neurosecretory system. Reorganization of the hypophyseal stalk consists of several interconnected but differing in time processes. Simultaneously with the developing traumatic changes (degeneration of the sectioned neurosecretory fibers, secretory disorders) proliferation of pituicytes with characteristic phagocytic activity is observed. A little bit later, intensive mitotic division of endothelial cells and capillary formation piercing the stalk periphery begins. At the same time, a new way for blood outflow from the capillaries of the primary portal plexus into the synuses of the brain pias is restored. Degenerated neurosecretory fibers are gradually substituted by regenerating fibers forming a dense network in heavily vascularizated stalk parts. As differentiation of endothelial cells and regeneration of neurosecretory fibers procede, axovasal contacts are gradually forming. At that time the hypophyseal stalk begins functioning as a neurohumoral organ but morpho-functionally less perfect than the posterior hypophyseal lobule. In the median eminence of the operated animals, unlike the intact ones, neurosecrete is accumulating around the capillaries of the portal plexus. Mechanical damage of neurosecretory fibers during the operation results in degeneration of a greater number of neurosecretory cells in the supraoptical and paraventricular nuclei. Preserved cells have an increased functional activity because of neurohormonal deficiency in the organism. As a result of the structural changes mentioned, diabetes mellitus develops, subsiding gradually with time course.

Serotonergic projections to the suprachiasmatic nucleus and the median eminence of the rat: identification by fluorescence and electron microscope

The identification of serotonergic input to the rat median eminence and nucleus suprachiasmaticus was carried out by fluorescence and electron microscopy. The clear differentiation of serotonin (5-HT) from catecholamine (CA) neurons was possible with an improved new filter system. 6-hydroxytryptamine (6-HT) was selectively taken up by 5-HT terminals and produced an intense yellow fluorescence. The yellow fluorescence of 5-HT disappeared 2 weeks after treatment with 5,6-dihydroxytryptamine (5,6-DHT), whereas the blue-green fluorescence of CA persisted. Combined treatment with either 6-HT and reserpine with addition to alpha-methyl-p-tyrosine, or with 6-HT and 6-hydroxydopamine, caused a marked reduction of CA and yellow fluorescence prominent only in 5-HT terminals. Under the electron microscope, 6-HT and 5,6-DHT were selectively taken up by the 5-HT terminals and produced electron-dense cores in the small vesicles with a diameter of 50 nm. The quantitative study revealed that the density of serotonin boutons marked with such dense-cored vesicles was 4.3 +/- 0.5% (mean +/- S.E.M.) in the suprachiasmatic nucleus and 4.8 +/- 0.8% in the perivascular region of the median eminence. Several studies have indicated an important functional role of 5-HT in the control of gonadotrophin secretion. The present study suggests that the serotonergic terminals exert their influence on the anterior pituitary function, not only at the level of the suprachiasmatic nucleus, but also at the level of the perivascular region of the median eminence, either through the axo-axonic synapses with the axons containing the releasing hormone or by the direct release of 5-HT into the portal vessels.

[Ultrastructural study of the median eminence of the cat]

The electron microscopic study of the median eminence in cats sacrified during the growth period (36 hours to 30 days after-birth) and in adult cats leads to the following conclusions: 1) The external infundibular zone of newborn kittens (36 hours postpartum) shows very few capillary loops and it lacks a palissade appearance. On the seventh day a palissade layer and penetrating capillary loops are observed. 2) There are no myelinated fibers in the fibrillar layer of the median eminence external infundibular zone during the growth-period. The number and size of Herring bodies increase after the seventh day. In adult cats, Herring bodies may be 50 microns long and some show myelinic sheaths. 3) In adult cats, the nerve endings which surround the capillary loops penetrating into the median eminence, are in direct contact with the lamina basalis, while the nerve endings of the palissade layer are always separated from the capillary space of the hypophyseal portal plexus by glial cell prolongations.

The hypothalamic neurosecretory systems of the Japanese quail as revealed by retrograde transport of horseradish peroxidase

Within 24-48 h after injection of horseradish peroxidase (HRP) into the neural lobe or into the median eminence of adult Japanese quail dense accumulations of its reaction product (HRP-RP) can be demonstrated in axons of the hypothalamo-hypophysial tract and in the magnocellular neurosecretory perikarya of the supraoptic and paraventricular nuclei as well as in scattered neurons of the accessory hypothalamic neurosecretory nuclei. The HRP-RP-containing nerve fibers, which are beaded in appearance, occur prominently in the internal zone of the median eminence. They turn dorsally at its anterior border to become widely distributed in the retrochiasmatic region and extended to the paraventricular, supraoptic areas. These observations confirm more directly conclusions drawn earlier from Gomori-type preparations and from immunologic demonstration of arginine vasotocin, mesotocin and neurophysin. HRP-RP was also found in perikarya of parvocellular secretory neurons in the infundibular nucleus 48 h after injection of HRP into the median eminence but not after injection into the pars nervosa. This provides direct evidence that a conspicuous component of the tubero-infundibular tract is formed by axons of tuberal neurons that originate from the infundibular nucleus and pass directly into the median eminence.