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The article presents a study which examines the cerebrospinal fluid (CSF) in patients with ependymal neoplasms and illustrates the ependymoma cytomorphologic features and its variants. The ependymal neoplasms are rare tumors, which arise from the cerebral ventricles' ependymal lining and remnants of spinal cord's central canal and mostly affects children and young adults. Moreover, the ependymomas exfoliated cells of patients with myxopapillary tumors are recognizable in CSF samples.

A wealth of information now exists showing that glial cells are actively involved in the cell–cell communication process generating and disseminating information within the central nervous system. In the hypothalamus, two types of glial cells, astrocytes and ependymal cells lining the latero-ventral portion of the third ventricle (known as tanycytes), regulate the secretory activity of neuroendocrine neurones. This function, initially described for astrocytes apposing magnocellular neurones, has been more recently characterised for neurones secreting gonadotrophin hormone-releasing hormone (GnRH). The available evidence suggests that glial cells of the median eminence regulate GnRH secretion via two related mechanisms. One involves the production of growth factors acting via receptors with tyrosine kinase activity. The other involves plastic rearrangements of glia–GnRH neurone adhesiveness. GnRH axons reach the median eminence, at least in part, directed by basic fibroblast growth factor. Their secretory activity is facilitated by insulin-like growth factor 1 and members of the epidermal growth factor family. A structural complement to these soluble molecules is provided by at least three cell–cell adhesion systems endowed with signalling capabilities. One of them uses the neuronal cell adhesion molecule (NCAM), another employs the synaptic cell adhesion molecule (SynCAM), and the third one consists of neuronal contactin interacting with glial receptor-like protein tyrosine phosphatase-β. It is envisioned that, within the median eminence, soluble factors and adhesion molecules work coordinately to control delivery of GnRH to the portal vasculature. [ABSTRACT FROM AUTHOR]

Abstract: Neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator are localized within the postero-lateral hypothalamus and zona incerta. These neurons project diffusely throughout the central nervous system and have been implicated in critical physiological processes such as energy homeostasis and sleep. In the present report, we examined the distribution of MCH immunoreactivity in the brainstem of the cat. In addition to MCH+ axons, we found MCH-immunoreactive cells that have not been previously described either in the midbrain raphe nuclei or in the periaqueductal and periventricular areas. These MCH+ cells constituted: 1. ependymal cells that lined the fourth ventricle and aqueduct, 2. ependymal cells with long basal processes that projected deeply into the subventricular (subaqueductal) parenchyma, and, 3. cells in subventricular regions and the midbrain raphe nuclei. The MCH+ cells in the midbrain raphe nuclei were closely related to neuronal processes of serotonergic neurons. Utilizing Neu-N and GFAP immunohistochemistry we determined that the preceding MCH+ cells were neither neurons nor astrocytes. However, we found that vimentin, an intermediate-filament protein that is used as a marker for tanycytes, was specifically co-localized with MCH in these cells. We conclude that MCH is present in tanycytes whose processes innervate the midbrain raphe nuclei and adjacent subependymal regions. Because tanycytes are specialized cells that transport substances from the cerebrospinal fluid (CSF) to neural parenchyma, we suggest that MCH is absorbed from the CSF by tanycytes and subsequently liberate to act upon neurons of brainstem nuclei. [Copyright &y& Elsevier]

This paper supplements former studies on elasmobranch species with an immunohistochemical investigation into glutamine synthetase and S-100 protein, in addition to GFAP, and extends its scope to the representatives of almost every group of Elasmobranchii: squalomorph sharks, galeomorph sharks, skates (Rajiformes) and rays (Torpediniformes and Myliobatifomes). More glial elements were labeled by S-100 protein, and even more so by using glutamine synthetase immunostaining than by GFAP: more astrocytes (mainly non-perivascular ones) were detected in the telencephalon of sharks, skates and rays. Only the markers S-100 and glutamine synthetase, but not GFAP, characterized the Bergmann-glia of skates and rays and astrocyte-like non-ependymal cells in Squalus acanthias. Another squalomorph shark species, Pristiophorus cirratus, however, had GFAP immunopositive astrocytes. Of all the species studied, the greatest number of GFAP positive astrocytes could be observed in Mobula japanica (order Myliobatiformes), in each major brain part. According to anatomical location, perivascular glia comprised varied types, including even a location in Mobula, which can also be found in mammals. Remnants of radial glia were found in confined areas of skates, less so in rays. In the rhombencephalon and in the spinal cord modified ependymoglia predominated in every group. In conclusion, there was no meaningful difference between the astroglial architectures of squalomorph and galeomorph sharks. The difference in the astroglial structure between sharks and batoids, however, was confined to the telencephalon and mesencephalon, and did not take place in the rhombencephalon, the latter structure being quite similar in all the species studied. The appearance of astrocytes in the relatively thin-walled shark telencephalon, however, indicates that the brain thickening promoted the preponderance of astrocytes rather than their appearance itself. Although the evolutionary changes of astroglia had some similarities in Elasmobranchii and Amniota, there was one meaningful difference: in Elasmobranchii astrocytes did not prevail in conservative brain regions as they did in the progressive brain regions. Copyright © 2008 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Tanycytes are glial cells in the hypothalamus that are functionally part of the blood–brain barrier. They can sense nutrients and metabolites in the circulation such as glucose, then signal to neuronal systems to influence ingestive behaviour and energy storage, and ultimately affect body weight. The complex structure of tanycytes underpins this function, and communication is dependent upon connexin‐43 gap junctions between tanycytes. This Editorial highlights studies by Recabal and coworkers (Recabal et al., 2020) in the current issue that shed some light on how this happens, and on how FGF2 might induce plasticity in hypothalamic structure through changes in tanycyte function that are dependent on connexin‐43 hemichannels. This Editorial Highlights the article https://doi.org/10.1111/jnc.15188 [ABSTRACT FROM AUTHOR]

Abstract: Activation of different brain regions for acute pain-related stress induced by a single subcutaneous injection of 4% formalin was investigated in histidine decarboxylase-deficient mice. Besides pain- and stress-related brain areas and the tuberomamillary neurons, strong Fos activation and c-fos mRNA expression were found in distinct brain regions and cell types, which have not been activated in wild type control mice. These structures include the circumventricular organs (organum vasculosum of the lamina terminalis, subfornical organ, area postrema), some of the ependymal cells along the wall of the ventricles, tanycytes in the third ventricle''s ependyma and the median eminence, as well as in the epithelial cells of the choroid plexus in the lateral, third and fourth ventricles. All of these areas and cell types are known as compartments of the brain-blood-cerebrospinal fluid interface. The present observations provide strong evidence that an acute stressor, formalin-evoked painful stimulus elicits rapid alterations in the activity of neuroglial elements of histidine decarboxylase-deficient mice that are directly involved in the communication between the brain and the cerebrospinal fluid space. [Copyright &y& Elsevier]

Abstract: Morphological studies in rodents have well documented the masculinization of the perinatal brain by estradiol derived from aromatized testosterone, and the resulting irreversible quantitative sex-differences generated in cell numbers or expression of chemical phenotypes. Here, using immunohistochemistry, we explored how this applies to the postnatal development and masculinization of the neurokinin B (NKB)-containing system of the arcuate nucleus/median eminence complex (ARC/ME). In adult rats, NKB-immunoreactive neurons exhibit an unusual, qualitative sexual dimorphism of their ventral axonal projections: to the neuropil in females, to capillary vessels in males. In adults, there was no sex-difference in the numbers of NKB-immunoreactive perikarya or capillary vessels in the ARC/ME, suggesting that this sexual dimorphism cannot be explained by the existence of supernumerary structures. At birth (day 0) the NKB system was immature in both sexes, and while its adult features emerged progressively until puberty in females, they did not develop before puberty (day 40) in males, revealing a sexual dimorphism only late postnatally. When males were orchidectomized at day 30, the masculine distribution of NKB-immunoreactive axons expected at day 40 was not seen, while it was apparent after chronic treatment with testosterone or dihydrotestosterone, suggesting a testicular masculinizing action via androgen receptors at puberty. Moreover in these prepubertal-orchidectomized males, the distribution of NKB-immunoreactive axons was surprisingly feminized by chronic estradiol alone, suggesting that NKB neurons are not irreversibly programmed before puberty. Last, in adult females, the distribution of NKB-immunoreactive axons was feminine 30 days after ovariectomy, and it was masculinized after concurrent chronic dihydrotestosterone, suggesting that NKB neurons remain responsive to androgens late in reproductive life. Thus, the sexual differentiation of the hypothalamus proceeds well beyond the perinatal period and includes the epigenetic action of non-aromatizable androgens upon subsets of neurons that have retained bipotent features. [Copyright &y& Elsevier]

Abstract: The hypothalamic infundibular area is located outside the blood–brain barrier and includes, the ventromedial arcuate nucleus (vmARC) sensing circulating substances, and the median eminence (ME) where neurohormones are released into the hypothalamo-hypophysial vasculature. This integrated functional unit, pivotal in endocrine control, adjusts neuroendocrine output to feedback information. Despite a differing physiology in males and females, this functional unit has not appeared differently organized between sexes. Using immunocytochemistry, we describe here for the first time in adult rats, a conspicuous sex-difference in its axonal wiring by intrinsic glutamatergic neurons containing the neuropeptides neurokinin B (NKB) and dynorphin. In the male, NKB neurons send axons to capillary vessels of the vmARC and of the ME (only where gonadotropin-releasing hormone (GnRH) axons terminate). Electron microscopy revealed that NKB axons target the barrier of tanycytes around fenestrated capillary vessels (in addition to GnRH axons), suggesting a control of regional bidirectional permeability. In the female, NKB neurons send axons to the neuropile of the vmARC, suggesting a direct control of its sensor neurons. The other projections of NKB neurons, studied by surgical isolation of the ARC-ME complex and confocal microscopy, are not sexually dimorphic and target both integrative and neuroendocrine centers controlling reproduction and metabolism, suggesting a broad influence over endocrine function. These observations demonstrate that the mechanisms subserving hypothalamic permeability and sensitivity to feedback information are sexually dimorphic, making the infundibular area a privileged site of generation of the male-to-female differences in the adult pattern of pulsatile hormonal secretions. [Copyright &y& Elsevier]

Abstract: Abide its toxicity, 3,3′-diaminobenzidine-tetrahydrochloride-dihydrate (DAB) was the most potent marker for immunochemistry at the light and electron microscopic level in the last decades. Recently, a sensitive substrate for immunohistochemical staining methods and in-situ hybridization, HistoGreen, was developed for the use with peroxidase. In peroxidase reactions, HistoGreen delivers a green staining product which is suitable for permanent embedding without water. In contrast to DAB, HistoGreen is not toxic. To evaluate its usefulness, we performed comparative immunohistochemistry on angiotensin II (AT1)-receptors with DAB- and HistoGreen-staining on paraffin embedded slices of the rat brain at the light microscopic level. This also included counterstaining with Mayer''s Hemalum and Nuclear Fast Red, respectively. We could demonstrate that HistoGreen delivers a coarsely grained label which is fast detectable in light microscopy. HistoGreen equals DAB in the exact localization of the immunoreaction to a large degree but its reaction product is considerably less stable in alcohol and water than DAB. In combination with Nuclear Fast Red, HistoGreen provides excellent imaging properties for the visualization and documentation of immunoreactive structures paired with an adequate demonstration of cellular details. Its tendency towards rapid over-staining as well as its low stability will restrict the use of HistoGreen in some areas of immunohistochemical research, yet the new chromogen represents an interesting alternative to DAB at the light microscopic level. [Copyright &y& Elsevier]

Glial and endothelial cells interact throughout the brain to define specific functional domains. Whether endothelial cells convey signals to glia in the mature brain is unknown but is amenable to examination in circumventricular organs. Here we report that purified endothelial cells of one of these organs, the median eminence of the hypothalamus, induce acute actin cytoskeleton remodeling in isolated ependymoglial cells and show that this plasticity is mediated by nitric oxide (NO), a diffusible factor. We found that both soluble guanylyl cyclase and cyclooxygenase products are involved in this endothelial-mediated control of ependymoglia cytoarchitecture. We also demonstrate by electron microscopy that activation of endogenous NO release in the median eminence induces rapid structural changes, allowing a direct access of neurosecretory axons containing gonadotropin-releasing hormone (GnRH) (the neuropeptide controlling reproductive function) to the portal vasculature. Local in vivo inhibition of NO synthesis disrupts reproductive cyclicity, a process that requires a pulsatile, coordinated delivery of GnRH into the hypothalamic-adenohypophyseal portal system. Our results identify a previously unknown function for endothelial cells in inducing neuroglial plasticity and raise the intriguing possibility that endothelial cells throughout the brain may use a similar signaling mechanism to regulate glial-neuronal interactions. [ABSTRACT FROM AUTHOR]

Abstract The GLUT2 glucose transporter and the K-ATP-sensitive potassium channels have been implicated as an integral part of the glucose-sensing mechanism in the pancreatic islet β cells. The expression of GLUT2 and K-ATP channels in the hypothalamic region suggest that they are also involved in a sensing mechanism in this area. The hypothalamic glial cells, known as tanycytes α and β, are specialized ependymal cells that bridge the cerebrospinal fluid and the portal blood of the median eminence. We used immunocytochemistry, in situ hybridization and transport analyses to demonstrate the glucose transporters expressed in tanycytes. Confocal microscopy using specific antibodies against GLUT1 and GLUT2 indicated that both transporters are expressed in α and β tanycytes. In addition, primary cultures of mouse hypothalamic tanycytes were found to express both GLUT1 and GLUT2 transporters. Transport studies, including 2-deoxy-glucose and fructose uptake in the presence or absence of inhibitors, indicated that these transporters are functional in cultured tanycytes. Finally, our analyses indicated that tanycytes express the K-ATP channel subunit Kir6.1 in vitro. As the expression of GLUT2 and K-ATP channel is linked to glucose-sensing mechanisms in pancreatic β cells, we postulate that tanycytes may be responsible, at least in part, for a mechanism that allows the hypothalamus to detect changes in glucose concentrations. [ABSTRACT FROM AUTHOR]

Tanycytes are specialized ependymal cells lining the infundibular recess of the third ventricle of the cerebrum. Early and recent investigations involve tanycytes in the mechanism of gonadotropin-releasing hormone (GnRH) release to the portal blood. The present investigation was performed to obtain a specific immunological marker of tanycytes and to identify the compound(s) responsible for this labeling. After 30 days of organ culture, explants of bovine median eminence formed spherical structures mostly constituted by tanycytes. These tanycyte spheres were xenotransplanted to rats, and the antibodies raised by the host animals against the transplanted living tanycytes were used for immunochemical studies of the bovine and rat median eminence. This antiserum immunoreacted with two compounds of 60 kDa and 85 kDa present in extracts of bovine and rat median eminence. The individual immunoblotting analysis of rat medial basal hypothalami showed a decrease in the amount of the 85-kDa compound in castrated rats as compared to control rats processed at oestrus and dioestrus. The antiserum, labeled as anti-P85, when used for immunostaining of sections throughout the rat central nervous system, immunoreacted specifically with the hypothalamic tanycytes. Within tanycytes, P-85 immunoreactivity was exclusively present in the basal processes. It is suggested that the 85-kDa and 60-kDa compounds correspond to two novel proteins selectively expressed by tanycytes. The possibility that they are secretory proteins involved in GnRH release is discussed. Anti-P85 appears to be the first specific marker of hypothalamic tanycytes. [ABSTRACT FROM AUTHOR]

Abstract In recent years compelling evidence has been provided that cell–cell interactions involving non-neuronal cells, such as glial and endothelial cells, are important in regulating the secretion of GnRH, the neuropeptide that controls both sexual development and adult reproductive function. Modification of the anatomical relationship that exist between GnRH nerve endings and glial cell processes in the external zone of the median eminence modulates the access of GnRH nerve terminals to the portal vasculature during the oestrous cycle. The establishment of direct neuro-haemal junctions between GnRH neuroendocrine terminals and the portal vasculature on the day of pro-oestrus may be critical for the transfer of GnRH upon its release into the fenestrated capillaries of the median eminence. Notwithstanding the importance of these plastic rearrangements, glial and endothelial cells also regulate GnRH neuronal function via specific cell–cell signalling molecules. While endothelial cells of the median eminence use nitric oxide to effect this regulatory control, astrocytes employ several growth factors, and in particular those of the EGF family and their erbB receptors to facilitate GnRH release during sexual development. Loss of function of each of these erbB receptors involved in the astroglial control of GnRH secretion leads to delayed sexual development. It is clear that regulation of GnRH secretion by cell–cell communication mechanisms other than transsynaptic inputs is an important component of the central neuroendocrine process controlling mammalian reproduction. [ABSTRACT FROM AUTHOR]

The cell bodies of hypothalamic secretory neurons are localized in areas protected by the blood-brain barrier (BBB), whereas their axon terminals are localized in the median eminence, which lacks a BBB. This implies a complex barrier system, allowing neurons of the central nervous system to secrete into the blood stream without making the BBB leaky. In the present study, three experimental protocols were applied to clarify certain relevant aspects of the barriers operating in the medial basal hypothalamus of the rat. We established that the milieu of the arcuate nucleus is exposed to both the ventricular and the subarachnoidal cerebrospinal fluid (CSF).The median eminence milieu, the perivascular space of the portal vessels, and the subarachnoid space appear to be in open communication; also, β2-tanycytes establish an efficient barrier between the median eminence milieu and the ventricular CSF. Similarly, β1-tanycytes establish a lateral barrier, separating the intercellular space of the median eminence from that of the arcuate nucleus. We also found that the glucose transporter I (GLUT I), a BBB marker, is localized throughout the whole plasma membrane of β1-tanycytes, but is missing from β2-tanycytes. Expression of GLUT I by tanycytes progressively develops during the first postnatal weeks; while the degree of damage of the arcuate nucleus by administration of monosodium glutamate, at different postnatal intervals, parallels that of the GLUT I immunoreactivity of β1-tanycytes. An explanation is offered for the selective destruction of the arcuate neurons by the parenteral administration of monosodium glutamate to infant rats. [ABSTRACT FROM AUTHOR]

The median eminence (ME) is part of the neuroendocrine system (NES) that functions as a crucial interface between the hypothalamus and pituitary gland. The ME contains many non-neuronal cell types, including oligodendrocytes, oligodendrocyte precursor cells (OPCs), tanycytes, astrocytes, pericytes, microglia and other immune cells, which may be involved in the regulation of NES function. For example, in mice, ablation of tanycytes (a special class of ependymal glia with stem cell-like functions) results in weight gain, feeding, insulin insensitivity and increased visceral adipose, consistent with the demonstrated ability of these cells to sense and transport both glucose and leptin, and to differentiate into neurons that control feeding and metabolism in the hypothalamus. To give a further example, OPCs in the ME of mice have been shown to rapidly respond to dietary signals, in turn controlling composition of the extracellular matrix in the ME, derived from oligodendrocyte-lineage cells, which may contribute to the previously described role of these cells in actively maintaining leptin-receptor-expressing dendrites in the ME. In this review, we explore and discuss recent advances such as these, that have developed our understanding of how the various cell types of the ME contribute to its function in the NES as the interface between the hypothalamus and pituitary gland. We also highlight avenues of future research which promise to uncover additional functions of the ME and the glia, stem and progenitor cells it contains., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Clayton, Lovell-Badge and Galichet.)

Glucokinase (GCK) is critical for glucosensing. In rats, GCK is expressed in hypothalamic tanycytes and appears to play an essential role in feeding behavior. In this study, we investigated the distribution of GCK-expressing tanycytes in mice and their role in the regulation of energy balance. In situ hybridization, reporter gene assay, and immunohistochemistry were used to assess GCK expression along the third ventricle in mice. To evaluate the impact of GCK-expressing tanycytes on arcuate neuron function and mouse physiology, Gck deletion along the ventricle was achieved using loxP/Cre recombinase technology in adult mice. GCK expression was low along the third ventricle, but detectable in tanycytes facing the ventromedial arcuate nucleus from bregma −1.5 to −2.2. Gck deletion induced the death of this tanycyte subgroup through the activation of the BAD signaling pathway. The ablation of GCK-expressing tanycytes affected different aspects of energy balance, leading to an increase in adiposity in mice. This phenotype was systematically associated with a defect in NPY neuron function. In contrast, the regulation of glucose homeostasis was mostly preserved, except for glucoprivic responses. This study describes the role of GCK in tanycyte biology and highlights the impact of tanycyte loss on the regulation of energy balance. [Display omitted] • vmARH tanycytes express glucokinase. • Glucokinase deletion in tanycytes induces cell death. • Ablation of vmARH tanycytes alters energy balance and adiposity. • Ablation of vmARH tanycytes alters NPY neuron function. [ABSTRACT FROM AUTHOR]

In adult mammals, neural stem cells are localized in three neurogenic regions, the subventricular zone of the lateral ventricle (SVZ), the subgranular zone of the dentate gyrus of the hippocampus (SGZ) and the hypothalamus. In the SVZ and the SGZ, neural stem/progenitor cells (NSPCs) express the glial fibrillary acidic protein (GFAP) and selective depletion of these NSPCs drastically decreases cell proliferation in vitro and in vivo . In the hypothalamus, GFAP is expressed by α-tanycytes, which are specialized radial glia-like cells in the wall of the third ventricle also recognized as NSPCs. To explore the role of these hypothalamic GFAP-positive tanycytes, we used transgenic mice expressing herpes simplex virus thymidine kinase (HSV-Tk) under the control of the mouse Gfap promoter and a 4-week intracerebroventricular infusion of the antiviral agent ganciclovir (GCV) which kills dividing cells expressing Tk. While GCV significantly reduced the number and growth of hypothalamus-derived neurospheres from adult transgenic mice in vitro , it causes hypogonadotropic hypogonadism in vivo. The selective death of dividing tanycytes expressing GFAP indeed results in a marked decrease in testosterone levels and testicular weight, as well as vacuolization of the seminiferous tubules and loss of spermatogenesis. Additionally, GCV-treated GFAP-Tk mice show impaired sexual behavior, but no alteration in food intake or body weight. Our results also show that the selective depletion of GFAP-expressing tanycytes leads to a sharp decrease in the number of gonadotropin-releasing hormone (GnRH)-immunoreactive neurons and a blunted LH secretion. Overall, our data show that GFAP-expressing tanycytes play a central role in the regulation of male reproductive function., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Butruille, Batailler, Cateau, Sharif, Leysen, Prévot, Vaudin, Pillon and Migaud.)

Hypothalamic dysfunction is an initial event following diet-induced obesity, primarily involving areas regulating energy balance such as arcuate nucleus (Arc) and median eminence (ME). To gain insights into the early hypothalamic diet-induced alterations, adult CD1 mice fed a high-fat diet (HFD) for 6 weeks were studied and compared with normo-fed controls. Transmission and scanning electron microscopy and histological staining were employed for morphological studies of the ME, while Raman spectroscopy was applied for the biochemical analysis of the Arc-ME complex. In HFD mice, ME β2-tanycytes, glial cells dedicated to blood-liquor crosstalk, exhibited remarkable ultrastructural anomalies, including altered alignment, reduced junctions, degenerating organelles, and higher content of lipid droplets, lysosomes, and autophagosomes. Degenerating tanycytes also displayed an electron transparent cytoplasm filled with numerous vesicles, and they were surrounded by dilated extracellular spaces extending up to the subependymal layer. Consistently, Raman spectroscopy analysis of the Arc-ME complex revealed higher glycogen, collagen, and lipid bands in HFD mice compared with controls, and there was also a higher band corresponding to the cyanide group in the former compared to the last. Collectively, these data show that ME β2-tanycytes exhibit early structural and chemical alterations due to HFD and reveal for the first-time hypothalamic cyanide presence following high dietary lipids consumption, which is a novel aspect with potential implications in the field of obesity. [ABSTRACT FROM AUTHOR]

The fine structure of arcuate neurons of the arcuate nucleus, the ependymal tanycytes and the contact zone of the median eminence was examined following immobilization, an acute stress which significantly activated the hypothalamo-pituitary-adrenal (HPA) axis. Arcuate neurons of immobilized adult male hamsters displayed morphological indications of heightened activity; the number of lysosomes and dense core vesicles (80-120 nm) was increased. A markedly greater number of dense core vesicles was present in axon terminals of the contact zone of the mid-central median eminence and the ventral proximal stalk. Tanycytes of the median eminence exhibited an augmented number of electron dense bodies in both perikarya and end processes. These results indicate that the arcuate neurons, the axons of the contact zone, and the ependymal tanycytes of the hamster medial basal hypothalamus (MBH) may be involved in the response to immobilization. [ABSTRACT FROM AUTHOR]

An active role of the ependymal cells (tanycytes) of the median eminence in the transport of hypothalamic hormones has been recently suggested. In order to investigate the fate of material present in the cerebrospinal fluid, a protein tracer, horse-radish peroxidase (HRP) was injected into the left lateral ventricle of rats. Two minutes after the injection, HRP had largely diffused between tanycytes and hypendymal cells. As soon as 5 min after the injection, HRP had completely penetrated all the layers of the median eminence. A few labelled vesicles and lysosomes were occasionally seen in ependymal and glial cells. At longer time intervals (20 min, 1 and 4 hrs), a reaction was observed in the lumen of fenestrated capillaries of the pituitary portal plexus. In many nerve endings of the external zone, vesicles and lysosomes were seen to contain HRP. An interesting observation was the localization of HRP between nerve endings and cells in both the pars nervosa and the pars intermedia of the pituitary gland. No reaction was recorded in the anterior pituitary and the kidney. Seventeen hours after the injection, the extracellular space was free of reaction but a few positive intracellular structures were still found. These results clearly indicate that some material from the third ventricle can rapidly diffuse between cells and axons of the median eminence to reach the fenestrated capillaries of the pituitary portal plexus and the posterior pituitary without involving an active transport by tanycytes. [ABSTRACT FROM AUTHOR]

Tanycytes with foot processes contacting capillary basal membranes were identified in the rat medial habenular nucleus. They constitute a relatively small but constant population of cells among the conventional ependymal cells. In contrast to tanycytes in most circumventricular organs, habenular tanycytes possess cilia. Superimposed upon the cells are nerve fibers belonging to the serotonergic supraependymal axon plexus. Their ultrastructure differs in many respects from that of hypothalamic tanycytes. [ABSTRACT FROM AUTHOR]

An electron microscopic analysis was made of the small-celled part of the mediodorsal cortex of the lizard Agama agama. This cortex consists of four layers: Superficial plexiform layer, cellular layer, deep plexiform layer and fiber layer. In the superficial plexiform layer one type of solitary neuron with smooth dendrites is present. Three types of axon terminals can be observed: terminals with a moderately electron dense matrix packed with spherical vesicles (S1 type), axon terminals with an electron lucent matrix containing fewer spherical synaptic vesicles than the S1 type (S2 type) and axon terminals with an electron lucent matrix and scattered pleomorphic synaptic vesicles (F type). F type axon terminals are larger than S terminals. At the pial surface endfeet of tanycytic processes form a limiting glial layer, contacting one another by means of gap junctions. In the cellular layer perikarya of pyramidal neurons are densely packed. The karyoplasm of these neurons shows either evenly dispersed or discretely clumped chromatin. Spiny dendrites arise from the perikarya and extend into both the superficial and deep plexiform layers. The structure of the deep plexiform layer is roughly similar to that of the superficial plexiform layer. The fiber layer contains the majority of the afferent and efferent axons of the mediodorsal cortex. The axons are myelinated and unmyelinated. Between the fibers, scattered solitary neurons are present, often accompanied by glial cells. The lateral ventricle beneath the fiber layer is lined by a single row of ependymal tanycytes. Tanycytic processes traverse the cortical layers and may form endfeet at the pial surface. Protoplasmic excresenses from some ependymal cells protrude into the ventricle. [ABSTRACT FROM AUTHOR]

Immunohistochemical techniques were used to stain for the astrocytespecific glial fibrillary acidic protein (GFAP) in the cells lining the third ventricle of the developing and mature mouse brain. Before birth immunoreactive tanycytes were only observed in the infundibular recess of the median eminence, where they could first be seen at embryonic day 17. They possessed long processes running towards the ventral surface of the brain. During the early postnatal period GFAP-positive tanycytes gradually appeared throughout the third ventricle, although the ependymal cells themselves remained unstained. The tanycytes retained their immunoreactivity for anti-GFAP serum in the adult, and were also evident in the adult rat third ventricle. It is suggested that the presence of GFAP in these specialised cells of the third ventricle indicates that they, the transient radial glia of the developing cerebral cortex, the persistent Bergmann glia of the cerebellum, similar astrocytes with radial processes in the hippocampal dentate gyrus and conventional astroglia are all closely related cell types. [ABSTRACT FROM AUTHOR]

The ventricular lining of the infundibular recess of monkey brains of both sexes was investigated with SEM. Based on the distribution patterns of surface profiles regional differences in the tanycytic ependyma are described. Variations in the number of surface profiles were observed in females during the ovarian cycle. These variations may be an indication not only of absorptive and/or secretory activities, but may also be related to cell membrane redistribution and membrane turnover in dependence of endocrine factors. In the males such variations were not registered. An essential sexual dimorphism does not seem to exist in the tanycytic ependyma of the monkey, only the reaction pattern is different. Supraependymal macrophages (SEC) were more numerous in the ovulating females than in the males or menstruating females. In addition to these SEC some monkeys had a second SEC type which was arranged in sheets. These are interpreted to be glioses. A variable number of axons can also be found in the lower regions of the infundibular recess. Their morphology suggests the presence of several types of axons, but they do not appear to constitute an organized network. No significant sexual differences were registered in relation to the axons. [ABSTRACT FROM AUTHOR]

Light-and electron-microscopic immunocytochemistry (LM-ICC and EM-ICC) were used to visualize luteinizing hormone-releasing hormone (LHRH) in fibres associated with ventricular ependyma and tanycytes of the median eminence. LM-ICC suggests that LHRH fibers appear to enter the third ventricle. However, with EM-ICC, LHRH fibers are in fact found within ependymal canaliculi formed by adjacent ependymal cells. The canaliculi contain other myelinated and unmyelinated axons in addition to immunoreactive LHRH fibers. Thin slips of ependymal and tanycyte processes project into the canaliculi and enclose axons to varying degrees. At the median eminence many LHRH fibers bend sharply downwards from their ventricular course and travel with tanycytic processes towards their common destination - the perivascular space of the hypophysial-portal vascular system. Here, EM-ICC reveals that LHRH fibers closely contact basal processes of tanycytes. Lateral processes from tanycytes form glioplasmic sheaths which surround some individual LHRH fibers. A few LHRH terminals contact the perivascular space directly but more often are separated from the perivascular space by intervening glia. It is hypothesized that: (1) glia of this region responds to the physiological state of the animal and may determine the degree of LHRH secretion by varying the extent of glial investment of LHRH terminals; and (2) may play a role during development by providing direction and support for LHRH fibers similar to that described for radial and other glial cells. [ABSTRACT FROM AUTHOR]

The afferent connections of the paraventricular nucleus (PVN) of the domestic mallard (Pekin duck), Anas platyrhynchos, were demonstrated by means of microiontophoretic injection of horseradish peroxidase (HRP). To place the HRP injection exactly into the PVN, its location was identified prior to the injection by observing antidiuretic reactions to electrostimulations within the rostral hypothalamus of conscious, hydrated animals. Antidiuresis was induced only when electrostimulation was applied to a distinct hypothalamic area. Two different patterns of antidiuresis were observed: (i) an immediate reduction in rate of production of urine, and (ii) antidiuresis preceded by a period of increase in production of urine. Repeated stimulation of the same site with the same parameters resulted in decreasing antidiuretic effects. At the site where stimulation had elicited the most pronounced antidiuresis of either response type, HRP was injected microiontophoretically. Histological examination after 3-8 days of survival revealed delicate injection sites located exclusively in the periventricular portion of the PVN. Adjacent to the dorsal portion of the PVN retrogradely labeled tanycytes and intraependymal neurons were scattered in the ventricular wall. As demonstrated in neurohistological and electron-microscopic investigations, this ependymal region exhibits a particular arrangement of tanycytes and small neurons (10-15 μm in diameter), some of which belong to the neurosecretory type. Additional HRP-labeled neuronal perikarya afferent to the PVN were demonstrated in the contralateral PVN, and on the ipsilateral side in the lateral septum, lateral hypothalamic area and locus coeruleus. Within the nuclei of the solitary tract, stained nerve cells were found ipsilateral as well as contralateral to the injection site. Several of the neurons demonstrated may be considered as candidates for the transmission of signals originating from various receptive structures relevant for the control of avian salt- and water-balance. The physiological results conform to the concept that neurons of the PVN influence urine formation by controlling the release of arginine-vasotocin (AVT). Evidence that suggests additional modes of control exerted by these neurons in salt- and water-balance is presented. [ABSTRACT FROM AUTHOR]

Tanycytes in the median eminence (ME) of the rat exhibit morphological features suggesting their possible participation in transport phenomena. After adrenalectomy, which modifies the hypothalamo-hypophyseal axis, they undergo morphological changes characterized by an accumulation of lipid droplets, an increased number of bleb-like protrusions at their apex, as well as an increased pinocytosis of intraventricularly injected horseradish peroxidase (HRP). In addition, after adrenalectomy an increased number of vacuoles appears at the level of the tubero-infundibular sulci. Their intracellular location in the tanycytes is demonstrated by an intraventricular injection of HRP. The significance of these vacuoles is discussed in relation to the hydroelectrolytic modifications associated with the state of adrenalectomy. [ABSTRACT FROM AUTHOR]

Transport of ferritin and horseradish peroxidase from the 3rd ventricle to the median eminence was examined in rats during the perinatal life, the time when functional interrelations between hypothalamus and hypophysis are established. Protein tracers injected into the lateral ventricle are adsorbed on the apical surface of the tanycyte, mainly on its protrusions or in indentations. On the 18th day of prenatal life a few small bleblike protrusions are observed. After birth microvilli appear. In time their concentration increases to result in an increase of adsorbed substances. They are taken up by smooth and coated pinocytotic vesicles and transported to the basal portion of the cell or to the intercellular space bypassing junctional complexes. In addition to pinocytotic vesicles protein tracers fill channels of smooth ER or Golgi complex and multivesicular bodies illustrating a process probably involved in metabolic or secretory processes. [ABSTRACT FROM AUTHOR]

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 the 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. [ABSTRACT FROM AUTHOR]

The transport of hormones from the cerebrospinal fluid to the adenohypophysis by the tanycytes of the median eminence was examined in male rats. Electron microscopy revealed that all ependymal cells including the tanycytes disappear or degenerate in rats subjected to electric cauterization of the ependymal layer lining the third ventricle. However, the granular axons in the palisade layer of the median eminence remain intact. In rats subjected to electric lesion, no significant change was found in either the serum-LH level or in the weight of the adenohypophysis, testes, adrenal and thyroid glands. It is concluded that the tanycytes do not participate in the hypothalamic regulation of hypophysial function. [ABSTRACT FROM AUTHOR]

The structural organization of the rostral, lateral and postinfundibular regions of the median eminence (ME) of 5-day cyclic diestrous rats was studied with light and electron microscopic methods. The ependymal cells lining (i) the floor of the infundibular recess (IR) at rostral levels, (ii) the lateral extensions of the IR, and (iii) the floor of the premammillary recess appear to represent the same type of tanycyte ependyma (β1 tanycytes). In the entire width of the rostral and postinfundibular palisade regions, as well as in the lateral palisade region of the preinfundibular ME, the processes of the β1 tanycytes form a continuous cuff. This cuff separates the nerve endings from the blood vessels and the pars tuberalis. At this level, synaptoid contacts between neurosecretory axons and the ependymal cuff can be observed. The ultrastructural characteristics of the β1 tanycytes are described and their ependymal endings tentatively classified into three types. In the lateral regions of the ME, the Golgi study revealed the presence of two fiber systems: (i) one possessing a latero-medial trajectory and distributed in the subependymal region; (ii) the other formed by a loose longitudinal tract originating from neurons of the arcuate nucleus. Some functional implications of the cellular organization of the rat ME are discussed. [ABSTRACT FROM AUTHOR]

The differentiation of tanycytes was studied light and electron microscopically during the perinatal period in rats, the time when functional connections between hypothalamus and hypophysis are established. The 3rd ventricle is slit-like between 16 and 18 days of the prenatal period. Its wall is formed by intensively proliferating matrix cells with apical processes, ovoid perikarya and a basal process. The ventral region of the 3rd ventricle becomes funnel-shaped on the 20th day of the prenatal period. As the cells differentiate, the apical process becomes shorter and broader. Moreover, on day 20 of prenatal life cells without apical processes appear. Their number increases during the postnatal period. The concentration of endoplasmic reticulum, mitochondria, polysomes, lipid droplets, dense bodies (lysosomes), lamellated and multivesicular bodies increases. Initially the cells are similar but from the 3rd day of postnatal life differentiation occurs in different regions of the infundibular recess. After the 5th day, there are no marked changes in the structure and distribution of these cells. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The distribution of luteinizing hormone-releasing hormone (LHRH) was studied in the rat and mouse brain by means of light and electron microscopic immunohistochemistry using the peroxidase-antiperoxidase method. An immunoreactive product to LHRH antiserum was found near the blood vessels of the vascular organ of the lamina terminalis. In the arcuate nucleus-median eminence region, an immunoreactive material occurred bilaterally in the hypothalamic tissue around the tuberoinfundibular sulci. Electron microscopy revealed that immunoreactive fibers observed light microscopically contain numerous granules 100-130 nm in diameter. No immunoreactive product was located in the tanycytes of the median eminence, the perikarya of hypothalamic neurons, and the parenchyma of several circumventricular organs (subfornical organ, subcommissural organ, pineal organ, area postrema). [ABSTRACT FROM AUTHOR]

The SFO of the chicken is divided in half by a large central blood sinus; ventrally it is covered by a thin layer of ependyma (including tanycytes, dendrites, and axons) which connects the two lateral halves and protrudes as a midsagittal crest into the lumen of the third ventricle. The ependyma consists predominantly of tanycytes with long basal processes which terminate upon perivascular spaces. These cells have an extensive Golgi apparatus and abundant lysosomes; their cellular apices containing polyribosomes and a few vesicles frequently protrude into the ventricle. In addition to astrocytes, oligodendrocytes, and microglial cells, there is another glial cell population that is distinguished by the presence of parallel stacks or spherical to ovoid conglomerates of rough ER and their unique location, i.e., limited to areas ventral and ventral-lateral to the large blood sinus. Two types of neurons are present: neurons in which there is a paucity of granulated vesicles and occasional vacuoles in both the cytoplasm and nuclei, the second type of neuron elaborates many granulated vesicles. Numerous puncta adhaerentia are observed between adjacent neuronal perikarya and between glial processes and neuronal perikarya. Diverse axon types are found within the chicken SFO. Axo-dendritic and axo-somatic axon terminals and presynaptic axon dilations contain assorted combinations of electron-lucent and granulated vesicles of different maximal diameters. Based on the morphology of these axons, cholinergic, peptidergic, and serotoninergic fibers are described. There are two additional groups of axons whose classification awaits further investigation. The chicken SFO differs from the mammalian SFO in several respects: it possesses an ependyma with secretory and/or absorptive tanycytes predominating; it is divided midsagittally by a central blood sinus; its lateral and dorsal limits are nebulous; a previously undescribed peculiar type of glial cell is found in a limited portion of the organ; supraependymal neurons are lacking. [ABSTRACT FROM AUTHOR]

The present study continues a previous investigation on the median eminence (EM) (Krisch et al., 1978). In rats with high levels of neurohormones (LHRH, vasopressin) a limited immunohistochemical labeling of perivascular tanycyte processes can be observed surrounding capillaries in the marginal region of the organum vasculosum laminae terminalis (OVLT) and in the inner part of the subfornical organ (SFO). This labeling extends from the perivascular space a short distance along the tanycyte processes. By conventional electron microscopy and by freeze-etching, tight junctions are demonstrated at a distance from the capillary lumen which corresponds to the borderline of the immunohistochemical labeling of perivascular tanycyte processes in light microscopic preparations. The tight junctions are arranged in several parallel and helical rows and correspond to those found in the median eminence. Consequently, the immunohistochemical labeling in the OVLT and in the SFO marks the intercellular cleft. In the circumventricular organs the immunostaining labels the extension of the perivascular space characterized by the hemal milieu. The perivascular space is separated off by tight junctions from the CSF-milieu of the adjacent neuropil. Furthermore, the present study demonstrates tight junctions in the marginal region of the area postrema (AP) between the perivascular processes of the tanycytes. [ABSTRACT FROM AUTHOR]

In stressed rats the tanycytes of the ventrolateral wall of the third ventricle exhibit by light microscopic immunohistochemistry a positive staining for neurohormones which is distinctly limited to the distal perivascular end of the tanycyte process. Since by electron microscopic immuncytochemistry the tanycyte cytoplasm does not show any reaction product, the light microscopic reaction most likely results from a labeling of the intercellular space in the direct vicinity of the subendothelial cleft. Whether this subendothelial space is permeable to neurohormones was tested by injection of HRP. In the region of the arcuate nucleus 30 min after intravenous application, the marker is affixed to the membranes of the perivascular tanycyte processes in the subendothelial cleft of capillaries possessing non-fenestrated endothelia. Occasionally, HRP penetrates for a short distance between the tanycytes. Then the labeling of the intercellular cleft ends abruptly. Here, several parallel ridges of tight junctions between the perivascular distal tanycyte processes are found by the freezeetching technique. Since HRP cannot reach the subendothelial clefts of this region by passing through capillary walls due to the presence of a blood-brain barrier, it is suggested that the marker penetrates from the median eminence this far via the subendothelial extracellular space. It is prevented from spreading further by the tight junctions of the perivascular tanycyte endings. The same way may be taken by the neurohormones. Hence, a border area exists adjacent to the dorsolateral aspect of the neurohemal region of the median eminence where the tanycytes isolate the neuropil from the cerebrospinal fluid not only by their apical tight junctions, but also by basal tight junctions from the subendothelial cleft. This communicates with the perivascular space of the portal vessels. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]