The distribution of orthogonal arrays and their relationship to intercellular junctions in neuroglia of the freeze-fractured hypothalamo-neurohypophysial system

Summary Using freeze-fracture techniques, we have investigated membrane specializations of the glia associated with the hypothalamo-neurohypophysial system of the rat. In the paraventricular (PVN) and supraoptic (SON) nuclei, astrocytes in areas of high neuronal density (i.e., magnocellular regions) display orthogonal arrays of 6–7 nm particles soley near gap junctions, while astrocytes in areas of lower neuronal density (i.e., parvocellular regions) contain additional arrays in membranes not displaying gap junctions. Arrays are especially numerous on astrocytic perivascular end-feet in both nuclei and in the laminations of the pial-glial limitans ventral to the SON. Ependymal cells near the PVN show arrays both on their lateral surfaces (displaying gap junctions) and on their apical surfaces (facing the CSF). Tight junctions are not noted on astrocytes or ependymal cells, but are noted on both the somas and myelin lamellae of oligodendroglia. Both of these latter membranes occasionally contain gap junctions as well; however, orthogonal arrays are never noted on oligodendroglia.The plasma membranes of pituicytes in the neurohypophysis display gap junctions, complex junctions, and tight junctions. Orthogonal arrays are noted near the first two of these, but not near the last. Arrays in the neural lobe appear most dense on membranes adjacent to subpial or perivascular spaces. Pituicyte membranes containing orthogonal arrays appear infrequently near the neural stalk, increasing towards the distal end of the neural lobe. The distribution of orthogonal arrays in this system, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.     

The Neuroscience Research Institute at Peking University: A Place for the Solution of Pain and Drug Abuse

Neuroscience research in China has undergone rapid expansion since 1980. The Neuroscience Research Institute of Peking University, one of the most active neuroscience research groups in China, was founded in 1987. Currently, the institute is overseeing four research areas, i.e., (1) pain and analgesia, (2) drug abuse and acupuncture treatment for drug addiction, (3) the mechanism of neurological degenerative disorders, and (4) the role of neuroglia in central nervous system injury. The institute is simultaneously investigating both theoretical and clinical studies. Acupuncture remains the core of research, while pain and drug abuse form the two disciplines.     

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood–brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.     

Intracellular Concentrations of Major Ions in Rat Myelinated Axons and Glia: Calculations Based on Electron Probe X-Ray Microanalyses

Abstract: Electron probe x-ray microanalysis (EPMA) was used to measure water content (percent water) and dry weight elemental concentrations (in millimoles per kilogram) of Na, K, Cl, and Ca in axoplasm and mitochondria of rat optic and tibial nerve myelinated axons. Myelin and cytoplasm of glial cells were also analyzed. Each anatomical compartment exhibited characteristic water contents and distributions of dry weight elements, which were used to calculate respective ionized concentrations. Free axoplasmic [K⁺] ranged from ≈155 mM in large PNS and CNS axons to ≈120–130 mM in smaller fibers. Free [Na⁺] was ≈15–17 mM in larger fibers compared with 20–25 mM in smaller axons, whereas free [Cl^?] was found to be 30–55 mM in all axons. Because intracellular Ca is largely bound, ionized concentrations were not estimated. However, calculations of total (free plus bound) aqueous concentrations of this element showed that axoplasm of large CNS and PNS axons contained ≈0.7 mM Ca, whereas small fibers contained 0.1–0.2 mM. Calculated ionic equilibrium potentials were as follows (in mV): in large CNS and PNS axons, E_K = ?105, E_Na = 60, and E_Cl = ?28; in Schwann cells, E_K = ?107, E_Na = 33, and E_Cl = ?33; and in CNS glia, E_K = ?99, E_Na = 36, and E_Cl = ?44. Calculated resting membrane potentials were as follows (in mV, including the contribution of the Na⁺,K⁺-ATPase): large axons, about ?80; small axons, about ?72 to ?78; and CNS glia, ?91. E_Cl is more positive than resting membrane potential in PNS and CNS axons and glia, indicating active accumulation. Direct EPMA measurement of elemental concentrations and subsequent calculation of ionized fractions in axons and glia offer fundamental neurophysiological information that has been previously unattainable.     

The fine structure of neuroglia in the central nervous system of nereid polychaetes

Summary The principal supportive elements of the nereid central nervous system are non-neuronal cells that are referred to as supportive glia. Supportive glial cells form a conspicuous cortex in the nerve cord. The inner region of this cortex consists of closely packed processes and cell bodies of fibrous supportive glial cells that are arranged in concentric layers around the perimeter of the neuropile. The fibrous appearance of the glial cells results from dense bundles of cytoplasmic filaments. Many fibrous glial processes penetrate the neuropile and ramify among the neuronal elements. Larger, irregularly shaped cells are the chief supportive glial elements of the peripheral region of the cortex where they line the stromal sheath (neural lamella) and invest the neuronal perikarya with extensive concentric systems of lamellate processes. These glial cells usually possess a relatively undifferentiated cytoplasm with scattered glycogen granules, but occasionally have a well developed Golgi apparatus, endoplasmic reticulum and densely packed particulate glycogen. The supportive glia exhibits numerous desmosomes as well as 5-layered (tight) and 7-layered (gap) junctions. Interspersed among the supportive glial cells are non-neuronal cells referred to as granulocytes. These cells have abundant large, granular inclusions, electron lucent vesicles, plasmalemmal infoldings and microtubules. The granulocytes may be derived from undifferentiated glial cells or may represent coelomocytes that have invaded the nervous tissue.Supported by USPHS Grants No. NIH 5P01 NS-07512, NIH 2T01 GM-00102, and NB-00840.The author acknowledges the excellent technical assistance of Sarah Wurzelmann and Stanley Brown, and thanks Dr. Berta Scharrer for many stimulating discussions.     

On the fine structure of the external glial layer in the isocortex of man

Summary The surface of the external glial layer of the isocortex in the human temporal lobe is generally slightly undulated, with a few protrusions and indentations. The surface is formed by an uninterrupted basement membrane which is continuous over the surface no matter how tortuous it becomes. The overall thickness of the glial layer is generally 15 to 25 m, but diminishes to about 5 m immediately beneath blood vessels. It consists mainly of a variable number of stacked glial cell processes.Two groups of cell bodies are encountered particularly in the middle and lower levels of the glial layer. Most of the cells are specialized fibrous astrocytes. They are characterized by eccentrically placed, rounded nuclei with homogeneously dispersed chromatin, and electron-lucent cytoplasm rich in filaments. Lipofuscin pigment granules occupy large areas of the perikaryon. The astroglial cells give rise to four types of processes: foot-processes, tangential and radial processes, and processes irregular in outline.The foot-processes ascend towards the cortical surface and terminate as flat expansions spreading out immediately beneath the basement membrane. Contiguous terminal expansions are connected by gap junctions. The individual profiles are irregular in form and fit together like in a jig-saw puzzle. The plasmalemma beneath the basement membrane is underlined by a fuzzy material, which is penetrated by glial filaments. In the terminal expansions individual or groups of mitochondria are abundant.The tangential processes are straight and slender and form a lattice within the middle and deep level of the external glial layer. They contain numerous filaments, evenly distributed or fasciculated. The remainder of the lattice is filled up by a considerable number of processes irregular in outline and varying greatly in size. They contain fewer filaments than the tangential processes, coursing in all directions, and glycogen particles. In both types of processes only a few mitochondria are present. These processes are also connected by gap junctions and desmosomes, too.Large cytoplasmic areas of astroglial cells localized in the deepest portion of the glial layer protrude into the neuropil of the molecular layer, giving rise to several radiate processes, which extend deeper into the cortex.The second, heterogeneous group of cell bodies is characterized by elongated nuclei, ovoid or irregular in outline, which are smaller than those of astroglial cells, and contain blocks of condensed chromatin; a thin cytoplasmic rim generating a few appendages surrounds the nucleus. The first sub-type is characterized by a nucleus with large chromatin blocks bordering the inner nuclear membrane and a medium-dense cytoplasmic matrix. The second sub-type displays smaller chromatin condensations at the inner nuclear membrane and many microtubules are scattered throughout an electron-lucent cytoplasm.     

Ionotropic ATP receptors in neuronal-glial communication

In the central nervous system ATP is released from both neurones and astroglial cells acting as a homo- and heterocellular neurotransmitter. Glial cells express numerous purinoceptors of both ionotropic (P2X) and metabotropic (P2Y) varieties. Astroglial P2X receptors can be activated by ongoing synaptic transmission and can mediate fast local signalling through elevation in cytoplasmic Ca(2+) and Na(+) concentrations. These ionic signals can be translated into various physiological messages by numerous pathways, including release of gliotransmitters, metabolic support of neurones and regulation of activity of postsynaptic glutamate and GABA receptors. Ionotropic purinoceptors represent a novel pathway of glia-driven modulation of synaptic signalling that involves the release of ATP from neurones and astrocytes followed by activation of P2X receptors which can regulate synaptic activity by variety of mechanisms expressed in both neuronal and glial compartments.     

Oligodendrocytes in the pons and middle cerebellar peduncle of the cat

Summary Morphology, microtopography and numerical density of oligodendrocytes were analyzed by light microscopy in the pontine gray and middle cerebellar peduncle of adult cats. The cells were selectively stained by use of the dicyanoargentate technique (Ogawa et al. 1975) that visualizes the entire cell population including perikarya and characteristic features of processes. On the basis of different microtopographical relations to neuronal perikarya and/or transversely oriented axon bundles, six groups of oligodendrocytes were separately analyzed: interfascicular, intrafascicular, perifascicular, perineuronal satellite, perifascicular-perineuronal, and neuropil cells. The cell morphology did not co-vary with any of these groups, but the shape of oligodendrocytes was on an average more elongated in the peduncle than in the pontine gray. The average cell density was similar in the gray and white matter (55000–56000 cells/mm³). However, 76% of the cells were concentrated near neuronal perikarya and axon bundles in a volume fraction of only 34%. Between adjacent neurons and axon bundles the cell density was even higher suggesting an additive behavior of these two topographical groups of oligodendrocytes. Axon bundles within the pontine gray contained only very few oligodendrocytes (density 6% that of the peduncle). These observations and quantitative data suggest that the perifascicular cells belong to the group of oligodendrocytes that are topographically related to axons (similar to interfascicular glia of the white matter) rather than to neuronal perikarya or neuropil.Dedicated to Prof. Paul Glees on the occasion of his 75th birthday     

A Golgi study of third ventricle tanycytes in the adult rodent brain

Summary Tanycytes along the third ventricle have been studied in adult rat and mouse brains with the rapid Golgi method. A tanycyte can be divided into three portions: somatic, neck, tail. The somatic portion is in the ependymal layer and frequently has thin cytoplasmic extensions. The neck portion originates from the soma and sticks into the periventricular layer. It, too, has numerous fine lamellar processes radiating from it. The neck contacts a blood vessel. Distal to this contact, the neck becomes thin and devoid of its cytoplasmic processes. This is the tail portion, which courses through hypothalamic nuclei to terminate as small bulbous swellings either on a vessel or at the pial surface.Although tanycytes occur throughout the dorsoventral extent of the ventricle, they are especially numerous ventrally. Midway down the ventricular wall, the neck processes interdigitate and form a moderately loose fabric beneath the ependyma. Proceeding ventrally, this becomes denser and thicker.Because the tails have no apparent associations with cells in the hypothalamic nuclei, the functional interactions of tanycytes with hypothalamic neuropil are probably confined to the periventricular layer.Supported by: NINDS Grants 5 RO1 NS 09001-02 NEUA, 5 TO1 NB 5309, and GM 00958, and by the Eleanor Roosevelt Cancer Foundation Research Institute.It is a pleasure to acknowledge the expert photographic assistance of Mr. Keith Johnson.