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Neuroendocrine Interactions in the Control of Glucose- and Energy Homeostasis
Published in André Kleinridders, Physiological Consequences of Brain Insulin Action, 2023
Canonical WNT signalling has been widely implicated in the expansion of progenitor poos or neural stem cells, suggesting that this pathway is a required factor for neural stem cell maintenance. However, at least ten distinct regions have been identified as proliferation zones within the adult zebrafish brain (99). This suggests that the WNT-responsive cells found in the hypothalamus of adult zebrafish serve another purpose. The change in function of WNT signalling during development and in adult organisms is far from fully understood. In this regard, it is important to note that the key loci for WNT signalling in the adult hypothalamus represent a stem cell population that consists of tanycytes lining the third ventricle and arcuate nucleus neurons (100, 101). Tanycytes are a subtype of ependymal cells with long processes that reach the fenestrated capillary network, while their tight junctions prevent the diffusion of blood-borne molecules into the adjacent cerebrospinal fluid. Studies from divergent fields have demonstrated the significance of tanycytes and ependymal cells in the neuroendocrine control of feeding and energy balance (102). However, a study by Wang et al. shows that WNT pathway activation decreases the number of hypothalamic tanycytes (96). This excludes WNT signalling from being a necessary requirement for tanycyte formation and maintenance, thereby complicating things further.
Distribution and Characteristics of Brain Dopamine
Published in Nira Ben-Jonathan, Dopamine, 2020
The pituitary gland is suspended from the median eminence by the infundibulum (pituitary stalk) and is nested within the sella turcica of the sphenoid bone of the skull. The median eminence is one of the seven areas of the brain called circumventricular organs, which have fenestrated capillaries and are therefore devoid of the BBB [37]. As illustrated in Figure 3.9, the median eminence is composed of three distinct zones: ependymal, internal, and external. The ependymal zone forms the floor of the third ventricle and has several specialized features, including tight junctions between adjacent cells and highly specialized cells, the tanycytes. Tanycytes are glial-like cells that extend protrusions and microvilli into the cerebrospinal fluid (CSF) at their ventricular surface and long cytoplasmic processes into the body of the median eminence. Tanycytes are exposed to CSF from the third ventricle and also have an access to circulating hormones and various metabolites through the fenestrated capillaries [38]. Some of the tanycytes act as conduits for trafficking certain molecules into the brain parenchyma, while others act as neural stem/progenitor cells that supply the postnatal and adult hypothalamus with new neurons. During embryonic development, tanycytes also serve as a scaffolding for axons that enter the median eminence, guiding them to their ultimate destination in the external zone.
Functional Neural Connections of the Area Postrema
Published in I. Robin A. Barraco, Nucleus of the Solitary Tract, 2019
Alastair V. Ferguson, Vicki L. Lowes
The capillaries in the vasculature of the AP are morphologically quite different to those which form the normal blood brain barrier (BBB). Unlike the capillaries of regions within the BBB where endothelial cells are joined by tight junctions, the endothelial cells of the AP are fenestrated3 and are thus permeable to proteins and other substances that are unable to cross the BBB. The AP thereby provides a route by which circulating substances can gain access to the central nervous system. On the ventricular surface, the epithelium of the AP has a delicate, flattened epithelium4 with short extensions called tanycytes that form a matting on the surface.2 These tanycytes probably provide a system for the transport of substances between the cerebrospinal fluid (CSF) and the intercellular spaces of the brain.5 The apical portion of the ependymal cells in the AP are joined by zonulae adherentes, junctional complexes which do not allow passage of the protein tracer horseradish peroxidase from the CSF.6 Only in the rat does there seem to be slight permeability of AP ependyma to macromolecules in the CSF, although not nearly as much as in adjacent mural ependyma.3 It appears, therefore, that there may be diffusion barriers between the AP and the ventricular CSF, suggesting that the primary function of the AP is to sense circulating rather than CSF substances.
Early life stress decreases cell proliferation and the number of putative adult neural stem cells in the adult hypothalamus
Published in Stress, 2021
Pascal Bielefeld, Maralinde R. Abbink, Anna R. Davidson, Niels Reijner, Oihane Abiega, Paul J. Lucassen, Aniko Korosi, Carlos P. Fitzsimons
In addition to the hippocampus, several ‘non-canonical’ neurogenic niches have been recently described in the adult mammalian brain, including the rodent hypothalamus (Yoo and Blackshaw 2018; Feliciano et al., 2015; Kokoeva et al., 2007; Xu et al., 2005). This novel hypothalamic neurogenic niche has been observed in mouse, sheep and human brains (Batailler et al., 2014; Pellegrino et al., 2018). The identity of putative NSPC populations generating new neurons in the hypothalamus is still being investigated, but most studies point toward populations of tanycytes that express NSPC markers such as Nestin, Sox2, or vimentin (Haan et al., 2013; Lee et al., 2012; Robins et al., 2013a). In mice, hypothalamic tanycytes are divided in four types based on their cell type marker expression and localization: α1, α2, β1 and β2. While all α- and β-tanycytes co-express putative NSPC markers such as Sox2 and Nestin, they differ in their localization relative to the 3rd ventricle wall. α-Tanycytes are located more dorsally, while β-tanycytes occupy more ventral parts of the 3rd ventricle ependyma (Goodman and Hajihosseini, 2015). In addition, while the processes of α-tanycytes project horizontally to terminate in close proximity to the dorsomedial and ventromedial hypothalamic nucleus (α1) as well as the dorsomedial part of the arcuate nucleus (α2), the processes from β-tanycytes curve to contact the hypothalamic parenchymal capillaries in the arcuate nucleus (β1) or the portal blood vessels of the median eminence (ME) (β2) (Prevot et al., 2018; Rizzoti and Lovell-Badge, 2017; Rodriguez et al., 2005).
One-week exposure to a free-choice high-fat high-sugar diet does not disrupt blood–brain barrier permeability in fed or overnight fasted rats
Published in Nutritional Neuroscience, 2019
M. Rijnsburger, U. A. Unmehopa, L. Eggels, M. J. Serlie, S. E. la Fleur
The delivery of blood-borne molecules like hormones and nutrients to the hypothalamus is restricted by the blood–brain barrier (BBB).1,2 The arcuate nucleus (Arc) of the hypothalamus lies adjacent to the third ventricle and is in close proximity with the median eminence (ME), a circumventricular organ with an incomplete BBB. Thus, the hypothalamus is in (in)direct contact with circulating molecules through the systemic circulation and through the cerebrospinal fluid (CSF). The brain–CSF barrier of the third ventricle is composed of tanycytes, specialized hypothalamic glia cells.3 The tanycytes at the floor of the third ventricle (β2 type) are unique, because they have direct access to the circulation via fenestrations of the BBB found at the ME. β1 tanycytes, present on the border between the ME and the Arc, form the ME-Arc barrier and thus allow the actual entrance of substances into the hypothalamus.4 Tight junction (TJ) complexes between adjacent tanycytes act as a physical barrier controlling the paracellular passage of substrates over the brain–CSF or ME–Arc barrier.5 These complexes consist of occludin, claudins, and junctional adhesion molecules, groups of transmembrane proteins important for intercellular adhesion. In addition, zonula occludens (ZO) proteins are intracellular TJ proteins and proposed to be scaffolding proteins that link the extracellular TJs to the actin cytoskeleton.5
The autoimmune basis of hypopituitarism in traumatic brain injury: fiction or reality?
Published in British Journal of Neurosurgery, 2019
Viraat Harsh, Sukriti Jha, Hitesh Kumar, Anil Kumar
Animal models have permitted controlled infliction of trauma and helped discern the relationship between clinical results of TBI and dysfunction at cellular or molecular levels. A recent study by Osterstock et al described pathological alterations evident in the median eminence, which included disrupted tight junctions in tanycytes (special ependymal cells with processes extending upto hypothalamus), increased concentration of endogenous IgG near the third ventricle and increased permeability of third ventricle. What is important to note is that tanycytes are bipolar cells which provide a kind of passage between the CSF and blood in portal capillaries, transporting antigens and triggering immunological phenomena after TBI. This experimental model though still nascent, attempts to explain autoantibody development after TBI and resultant PTHP when both pituitary and stalk are normal.16–18