The astrocyte as a component of the node of Ranvier

Astrocyte pathology in the ventral prefrontal white matter in depression

Astrocyte functions in white matter are less well understood than in gray matter. Our recent study of white matter in ventral prefrontal cortex (vPFC) revealed alterations in expression of myelin-related genes in major depressive disorder (MDD). Since white matter astrocytes maintain myelin, we hypothesized that morphometry of these cells will be altered in MDD in the same prefrontal white matter region in which myelin-related genes are altered. White matter adjacent to vPFC was examined in 25 MDD and 21 control subjects. Density and size of GFAP-immunoreactive (-ir) astrocyte cell bodies was measured. The area fraction of GFAP-ir astrocytes (cell bodies + processes) was also estimated. GFAP mRNA expression was determined using qRT-PCR. The density of GFAP-ir astrocytes was also measured in vPFC white matter of rats subjected to chronic unpredictable stress (CUS) and control animals. Fibrous and smooth GFAP-ir astrocytes were distinguished in human white matter. The density of both types of astrocytes was significantly decreased in MDD. Area fraction of GFAP immunoreactivity was significantly decreased in MDD, but mean soma size remained unchanged. Expression of GFAP mRNA was significantly decreased in MDD. In CUS rats there was a significant decrease in astrocyte density in prefrontal white matter. The decrease in density and area fraction of white matter astrocytes and GFAP mRNA in MDD may be linked to myelin pathology previously noted in these subjects. Astrocyte pathology may contribute to axon disturbances in axon integrity reported by neuroimaging studies in MDD and interfere with signal conduction in the white matter.

Perinodal glial swelling mitigates axonal degradation in a model of axonal injury

Mild traumatic brain injury (mTBI) has been associated with the damage to myelinated axons in white matter tracts. Animal models and in vitro studies suggest that axonal degradation develops during a latent period following a traumatic event. This delay has been attributed to slowly developing axonal membrane depolarization that is initiated by injury-induced ionic imbalance and in turn, leads to the activation of Ca(2+) proteases via pathological accumulation of Ca(2+). However, the mechanisms mitigating the transition to axonal degradation after injury remain elusive. We addressed this question in a detailed biophysical model of axonal injury that incorporated ion exchange and glial swelling mechanisms. We show that glial swelling, which often co-occurs with mTBI, promotes axonal survival by regulating extracellular K(+) dynamics, extending the range of injury parameters in which axons exhibit stable membrane potential postinjury. In addition, glial swelling was instrumental in reducing axonal sensitivity to repetitive stretch injury that occurred several minutes following the first one. Results of this study suggest that acute post-traumatic swelling of perinodal astrocytes helps prevent or postpone axonal degradation by maintaining physiologically relevant levels of extracellular K(+).

Extracellular matrix organization in various regions of rat brain grey matter

Previous studies revealed the concentration of extracellular matrix proteoglycans in the so-called perineuronal nets on the one hand and in certain zones of the neuropil on the other. This nonhomogeneous distribution suggested a non-random chemical and spatial heterogeneity of the extracellular space. In the present investigation, regions dominated by one of both distribution patterns, i.e. piriform and parietal cortex, reticular thalamic nucleus, medial septum/diagonal band complex and cerebellar nuclei, were selected for correlative light and electron microscopic analysis. The labelling was performed by the use of the N-acetylgalactosamine-binding plant lectin Wisteria floribunda agglutinin visualized by peroxidase staining and additionally by photoconversion of red carbocyanine fluorescence labelling for electron microscopy. The intense labelling of the neuropil of a superficial piriform region, presumably identical with sublayer Ia, was confined to a fine meshwork spreading over the extracellular space between non-myelinated axons, dendrites and glial profiles. In the reticular thalamic nucleus the neuronal cell bodies were embedded in zones of labelled neuropil. In contrast to these patterns, the labelled extracellular matrix in different cortical layers and in the other subcortical regions was concentrated in perineuronal nets as large accumulations at surface areas of the neuronal perikarya and dendrites and the attached presynaptic boutons. Astrocytic processes usually were separated from the neuronal surface by the interposed extracellular material. Despite a great variability, the width of the extracellular space containing the labelled matrix components in all perineuronal nets appeared to be considerably larger than that in the labelled zones of neuropil and the non-labelled microenvironment of other neurons. Our results support the view that differences expressed in topographical and spatial peculiarities of the extracellular matrix constituents are related to neuron-type and system-specific functional properties.

Morphology of horseradish peroxidase (HRP)-injected glial cells in the myenteric plexus of the guinea-pig

Glial cells of the myenteric plexus from guinea pig small intestine were intracellularly filled with horseradish peroxidase (HRP), and histochemically stained. Camera lucida-like drawings of twenty cells were morphologically and morphometrically analyzed. The cells have very small ellipsoid somata (8.5 +/- 0.7 microns equivalent diameter, i.e., about 330 micron3 volume), and send up to 20 thin and short processes (less than 26 to about 110 microns in length). The morphology of the cells appears to depend on their location within the plexus. Glial cells located within the ganglia are similar to CNS protoplasmic astrocytes; they are star-shaped, and their very short processes are irregularly branched. In contrast, glial cells within the interganglionic fiber tracts resemble CNS fibrous astrocytes. They extend longer processes that are parallel to the fiber tracts, and show less tendency to branch. We propose that the morphology of enteric glia is determined by the structure of the microenvironment. Both cell types form several flat endfeet at a basal lamina either surrounding blood vessels or at the ganglionic border. Furthermore, the occurrence of "holes" in the glial cell processes suggests that particular neuronal cell processes may be enwrapped in a specific manner. Fractal analysis of camera lucida-like drawings of the cells showed that the cells have a highly complex surface structure, comparable to that of protoplasmic astrocytes in the brain. These tiny cells may possess a membrane surface area of approximately 2000 micron2, almost 90% of which are contributed by the cell processes.(ABSTRACT TRUNCATED AT 250 WORDS)

Na+ channels of Müller (glial) cells isolated from retinae of various mammalian species including man

Within the last few years, the expression of voltage-dependent, TTX-sensitive Na+ channels has been demonstrated in several types of neuroglial cells such as astrocytes and Schwann cells. Recently, we reported the occurrence of such Na+ currents in retinal Müller (glial) cells from dog and cat. This paper deals with the description of the properties of Na+ currents in Müller cells isolated from retinae of several mammalian species, as well as from human retinae. These Na+ currents were eliminated by TTX (1 microM), and by exposure to sodium-free extracellular solution; typically, they were demonstrable only after blocking most of the K+ conductance by Ba2+ (1 mM). Voltage-dependent activation and inactivation characteristics and time constants of the Na+ currents were similar to those of currents carried by neuronal Na+ channels. The estimated number of sodium channels per cell was low (about 1,500 channels per 7,500 microns 2), and the K+ conductance exceeded the peak Na+ conductance by an average factor of 5. Thus, the cells were incapable of generating action-potential-like responses under current clamp. Modelling estimations show that triggering of glial Na+ currents under physiological conditions, if any, can at best occur by emhaptic transmission at perinodal sites of optic axons. It is speculated that glial Na+ channels might be involved in neuroglial signalling events.

Mapping of perineuronal nets in the rat brain stained by colloidal iron hydroxide histochemistry and lectin cytochemistry

Net-like structures, visualized with the Golgi technique and several histochemical and immunocytochemical methods, have been described to ensheath somata, parts of dendrites and axon initial segments of various types of neurons. The origin and function of these perineuronal nets have been controversially discussed. Recently, it was confirmed that they are glia-associated. In the present study such perineuronal nets were demonstrated by using colloidal iron hydroxide staining for detection of polyanionic components and the plant lectins Vicia villosa agglutinin and Wisteria floribunda agglutinin with affinity for N-acetylgalactosamine. This paper shows their distribution patterns and the occurrence of regional specialization of these nets which might provide a basis to suggest functional implications of these structures. Perineuronal nets were found in more than 100 brain regions, such as neocortex, hippocampus, piriform cortex, basal forebrain complex, dorsal lateral septal nucleus, lateral hypothalamic area, reticular thalamic nucleus, zona incerta, deep parts of superior and inferior colliculus, red nucleus, substantia nigra, some tegmental nuclei, cerebellar nuclei, dorsal raphe and cuneiform nuclei, central gray, trochlear nucleus, pontine and medullar reticular nuclei, superior olivary nucleus and vestibular nuclei. Neurons enwrapped by perineuronal nets not only differ in morphology but also in transmitter content. In neocortical and hippocampal regions there occurs a much higher number of perineuronal nets ensheathing non-pyramidal cells than in paleocortical structures. Most subcortical regions containing perineuronal nets were found to be integrated in motor functions. The findings are discussed with respect to known electrophysiological data of cell types described in our investigation as net-associated. There are some indications that such cells may represent fast firing types.

Expression of janusin (J1-160/180) in the retina and optic nerve of the developing and adult mouse

We have analyzed the expression of the oligodendrocyte-derived extra-cellular matrix molecule janusin (previously termed J1-160/180) in the retina and optic nerve of developing and adult mice using indirect light and electron microscopic immunocytochemistry, immunoblot analysis, and enzyme-linked immunosorbent assay. In the optic nerve, janusin is not detectable in neonatal and only weakly detectable in 7-day-old animals. Expression is at a peak in 2- or 3-week-old animals and subsequently decreases with increasing age. In the retina, expression increases until the third postnatal week and then remains at a constant level. In immunocytochemical investigations at the light microscopic level, janusin was found in the myelinated regions of the nerve with spots of increased immunoreactivity possibly corresponding to an accumulation of the molecule at the nodes of Ranvier. At the electron microscopic level, contact sites between unmyelinated axons, between axons and glial cells, and between axons and processes of myelinating oligodendrocytes were immunoreactive. Cell surfaces of astrocytes at the periphery of the nerve and forming the glial-limiting membrane, in contrast, were only weakly immunopositive or negative. In cell cultures of young postnatal mouse or rat optic nerves, oligodendrocytes and type-2 astrocytes, but not type-1 astrocytes were stained by janusin antibodies. In the oligodendrocyte-free retina, janusin was detectable in association with neuronal cell surfaces, but not with cell surfaces of Müller cells or retinal astrocytes. Our observations indicate that expression of janusin in the optic nerve and in the retina is developmentally differentially regulated and that other cell types, in addition to oligodendrocytes, express the molecule. Since the time course of janusin expression in the optic nerve coincides with the appearance of oligodendrocytes and myelin and since janusin is associated with cell surfaces of oligodendrocytes and outer aspects of myelin sheaths and is concentrated at nodes of Ranvier, we suggest that janusin is functionally involved in the process of myelination.

Perineuronal nets provide a polyanionic, glia-associated form of microenvironment around certain neurons in many parts of the rat brain

The nature and function of previously described perineuronal nets are still obscure. In the present study their polyanionic components were demonstrated in the rat brain using colloidal iron hydroxide (CIH) staining. In subcortical regions, such as the red nucleus, cerebellar, and vestibular nuclei, most neurons were ensheathed by CIH-binding material. In the cerebral cortex perineuronal nets were seen around numerous nonpyramidal neurons. Biotinylated hyaluronectin revealed that hyaluronan occurs in perineuronal nets. Two plant lectins [Wisteria floribunda agglutinin (WFA) and Vicia villosa agglutinin (VVA)] with affinity for N-acetylgalactosamine visualized perineuronal nets similar to those rich in anionic components. Glutamic acid decarboxylase (GAD)-immunoreactive synaptic boutons were shown to occupy numerous meshes of perineuronal VVA-positive nets. Electron microscopically, VVA binding sites were scattered throughout perisynaptic profiles, but accumulated at membranes and in the extracellular space except not in synaptic clefts. To investigate the spatial relationship between glial cell processes and perineuronal nets, two astrocytic markers (S100-protein and glutamine synthetase) were visualized at the light and electron microscopic level. Two methods to detect microglia by the use of Griffonia simplicifolia agglutinin (GSA I-B4) and the monoclonal antibody, OX-42, were also applied. Labelled structures forming perineuronal nets were observed with both astrocytic, but not with microglial, markers. It is concluded that perineuronal nets are composed of a specialized type of glia-associated extracellular matrix rich in polyanionic groups and N-acetylgalactosamine. The net-like appearance is due to perisynaptic arrangement of the astrocytic processes and these extracellular components. Similar to the ensheathment of nodes of Ranvier, perineuronal nets may provide a special ion buffering capacity required around various, perhaps highly active, types of neurons.

Localization of janusin mRNA in the central nervous system of the developing and adult mouse

Janusin (formerly termed J1-160/180) is an oligodendrocyte-derived extracellular matrix molecule which is restricted to the central nervous system and which is expressed late during development (Pesheva et al., J. Cell Biol., 1765-1778, 1989). To gain insights into the molecule's morphogenetic functions and to identify its cellular source in vivo, we have studied the localization of janusin messenger RNA in the optic nerve, retina and spinal cord and the expression of janusin protein in the spinal cord of developing and adult mice. Moreover, we have analysed optic nerve cell cultures and retinal cell suspensions in double-labelling experiments using a janusin-specific anti-sense complementary RNA probe and cell type-specific antibodies to identify the cell types containing janusin transcripts. In developing animals, oligodendrocytes were strongly labelled with the janusin anti-sense cRNA probe during the period of myelination. The number of labelled cells and intensity of the hybridization signal decreased significantly with increasing age. Interestingly, expression of janusin was not confined to oligodendrocytes. Some neuronal cell types and type-2 astrocytes present in optic nerve cell cultures also contained janusin transcripts. In contrast to oligodendrocytes, the number and labelling intensity of neurons containing janusin transcripts remained constant during postnatal development and into adulthood. Expression of janusin protein in the spinal cord was developmentally regulated, with a peak of expression in 2- or 3-week-old animals. The molecule was visible in the white and grey matter. In myelinated regions, it was associated with myelinated fibres and accumulated at nodes of Ranvier. These observations suggest that janusin may be of functional relevance for myelination.

Molecular dissection of the myelinated axon

The membrane of the myelinated axon expresses a rich repertoire of physiologically active molecules: (1) Voltage-sensitive NA+ channels are clustered at high density (approximately 1,000/microns 2) in the nodal axon membrane and are present at lower density (< 25/microns 2) in the internodal axon membrane under the myelin. Na+ channels are also present within Schwann cell processes (in peripheral nerve) and perinodal astrocyte processes (in the central nervous system) which contact the Na+ channel-rich axon membrane at the node. In some demyelinated fibers, the bared (formerly internodal) axon membrane reorganizes and expresses a higher-than-normal Na+ channel density, providing a basis for restoration of conduction. The presence of glial cell processes, adjacent to foci of Na+ channels in immature and demyelinated axons, suggests that glial cells participate in the clustering of Na+ channels in the axon membrane. (2) "Fast" K+ channels, sensitive to 4-aminopyridine, are present in the paranodal or internodal axon membrane under the myelin; these channels may function to prevent reexcitation following action potentials, or participate in the generation of an internodal resting potential. (3) "Slow" K+ channels, sensitive to tetraethylammonium, are present in the nodal axon membrane and, in lower densities, in the internodal axon membrane; their activation produces a hyperpolarizing afterpotential which modulates repetitive firing. (4) The "inward rectifier" is activated by hyperpolarization. This channel is permeable to both Na+ and K+ ions and may modulate axonal excitability or participate in ionic reuptake following activity. (5) Na+/K(+)-ATPase and (6) Ca(2+)-ATPase are also present in the axon membrane and function to maintain transmembrane gradients of Na+, K+, and Ca2+. (7) A specialized antiporter molecule, the Na+/Ca2+ exchanger, is present in myelinated axons within central nervous system white matter. Following anoxia, the Na+/Ca2+ exchanger mediates an influx of Ca2+ which damages the axon. The molecular organization of the myelinated axon has important pathophysiological implications. Blockade of fast K+ channels and Na+/K(+)-ATPase improves action potential conduction in some demyelinated axons, and block of the Na+/Ca2+ exchanger protects white matter axons from anoxic injury. Modification of ion channels, pumps, and exchangers in myelinated fibers may thus provide an important therapeutic approach for a number of neurological disorders.

The CNS-PNS transitional zone of the rat. Morphometric studies at cranial and spinal levels

The transitional zone is that length of rootlet containing both central and peripheral nervous tissue. The CNS-PNS interface may be defined as the basal lamina covering the intricately interwoven layer of astrocyte processes which forms the CNS surface and which is pierced by axons passing between the CNS and PNS. Study of transitional zone development defines morphologically the growth, relative movement and interaction of central and peripheral nervous tissues as they establish their mutually exclusive territories on either side of the CNS-PNS boundary, and helps to explain the wide variations in the form of the mature transitional zone. Nerve rootlets at first consist of bundles of bare axons. These become segregated by matrices of fine Schwann cell processes peripherally and of astrocyte processes centrally. The latter may prevent Schwann cell invasion of the CNS. Astrocyte processes branch profusely and come to form the principal central nervous tissue component of the transitional zone. Developmental changes in the transitional zone vary markedly between nerves, reflecting differences in its final morphology. Widespread relative movements and migration of CNS and PNS tissues take place during development, so that the central-peripheral interface changes shape and position, commonly oscillating along the proximodistal axis of the rootlet. For example, developing cervical ventral rootlets contain a transient central tissue projection, while that of lumbar ventral rootlets and to a lesser extent that of cervical dorsal rootlets alternately increase and decrease in length. In the developing cochlear nerve, a central tissue projection is present before birth, but regresses somewhat before a marked outgrowth of central nervous tissue along the nerve takes place, which reaches into the modiolus during the first week postnatum. During development, some astrocytic tissue may even break off and migrate distally into the root, giving rise to one or more glial islands within it. During the period immediately preceding birth, Schwann cells come to be present in very large numbers in that part of the rootlet immediately distal to the CNS-PNS interface, the proximal rootlet segment. Here they form prominent sleeves or clusters of closely packed cells which intertwine with and encapsulate one another on the rootlet surface. Such Schwann cell overcrowding in the proximal rootlet segment could result in part from distal overgrowth of the rapidly expanding CNS around axon bundles, which might strip the Schwann cells distally off the bundle segments so engulfed.(ABSTRACT TRUNCATED AT 400 WORDS)

Lysosomal activity at nodes of Ranvier during retrograde axonal transport of horseradish peroxidase in alpha-motor neurons of the cat

Lysosomal activity at nodes of Ranvier of feline hindlimb alpha-motor neurons was examined by light and electron microscopical acid phosphatase (AcPase) histochemistry during retrograde axonal transport of intramuscularly injected horseradish peroxidase (HRP). Several nodes along the PNS parts of the alpha-motor axons of the HRP-injected side showed accumulations of AcPase-positive bodies in the constricted nodal axon segment and the adjacent paranodal axoplasm. Such lysosomal accumulations were most prominent in the ventral root and differed in number and intensity depending on survival time after the HRP injection. At nodes showing high AcPase activity the axoplasm proximal to the nodal midlevel was occupied by many small, AcPase-positive, vesiculotubular profiles. Larger AcPase-positive bodies were mainly situated distal to the nodal midlevel. Double incubation for demonstration of both HRP and AcPase activity showed similar accumulations of AcPase-positive bodies at some of the HRP-transporting nodes. The AcPase activity differed considerably between nodes exhibiting comparable levels of HRP-positivity. Many of the AcPase-positive bodies also contained HRP reaction product. At some HRP-positive nodes the number of AcPase-positive bodies situated in the paranodal axon-Schwann cell network was elevated when compared to nodes of the contralateral, control side. In contrast to the PNS nodes, the nodal occurrence and distribution of lysosomes in the CNS part of alpha-motor axons seemed not to be affected by HRP transport. These observations support our previous proposal that nodes of Ranvier in the PNS parts of alpha-motor axons, in contrast to their CNS nodes, possess an ability to control passage of and initiate lysosomal degradation of axonally transported substances. Such an ability may provide a protective function to the motor neuron by restricting the intraneuronal transport of materials imbibed by the axon terminals outside the CNS.

Chondroitin 4-sulfate proteoglycan forms an extracellular network in human and rat central nervous system

Chondroitin 4-sulfate proteoglycan (C4S-PG) was localized both in rat and human central nervous system (CNS) by monoclonal and polyclonal antisera recognizing the 4-sulfate disaccharide (C4S). In the rat the whole CNS was studied in serial coronal sections. A positive extracellular meshwork was observed both in white and grey matters. In the white matter (WM) C4S-PG formed a network around myelinated axons, sparing myelin sheaths and axoplasms. The neuropil of the grey matter (GM) showed a positive meshwork constituted by delicate intermingling filaments. The cytoplasms of neuronal, glial and endothelial cells were negative. Stronger straining than in the neuropil was observed around the soma and the proximal part of the cell processes of some neurons located in the cortex, in the deep cerebellar nuclei and in some other CNS nuclei. A similar pattern was also observed in human CNS, the only difference being a smaller amount of cortical neurons surrounded by a rim of C4S-PG. This study shows that a PG bearing C4S disaccharide is located extracellularly in the rodent and human CNS and that C4S disaccharides can be present in different types of CNS proteoglycans (PGs).

Anoxic injury of mammalian central white matter: decreased susceptibility in myelin-deficient optic nerve

The rat optic nerve, a typical central nervous system white matter tract, rapidly loses excitability when it is exposed to anoxia and is irreversibly damaged by prolonged anoxia. Neonatal optic nerve is extremely resistant to anoxia-induced dysfunction and injury; the adult pattern of response to anoxia appears between 10 and 20 days postnatal, that is, during the period of oligodendroglial proliferation and myelination. To test the hypothesis that myelination, or associated events, confer anoxic susceptibility on developing white matter, we analyzed the effects of anoxia on the myelin-deficient (md) strain of rat. Acutely isolated optic nerves from 19- to 21-day-old md rats and control optic nerves from unaffected male littermates were maintained in vitro at 37 degrees C, and exposed to a standard 60-minute period of anoxia. The supramaximal compound action potential was recorded and amplitude of the compound action potential, expressed as % of amplitude before anoxic exposure, was determined. The compound action potential was nearly abolished within 3 to 6 minutes after onset of anoxia in control optic nerves, while optic nerves from md rats displayed a slower decrease in compound action potential amplitude during anoxia, with a distinct action potential present even after 60 minutes of anoxia. Optic nerves from md rats showed significantly greater recovery of compound action potential (71 +/- 25%) than did control optic nerves (33 +/- 21%; p less than 0.02) after 60 minutes of anoxia. These findings support the hypothesis that myelination, or changes associated with it, may be important in the development of anoxic susceptibility in central white matter.

Immuno-ultrastructural localization of sodium channels at nodes of Ranvier and perinodal astrocytes in rat optic nerve

Immuno-electron microscopic localization of sodium channels at nodes of Ranvier within adult optic nerve was demonstrated with polyclonal antibody 7493. The 7493 antisera, which is directed against purified sodium channels from rat brain, recognizes a 260 kDa protein in immunoblots of the crude glycoprotein fraction from adult rat optic nerve. Intense immunoreactivity with 7493 antisera was observed at nodes of Ranvier. Axon membrane at the node was densely stained, whereas paranodal and internodal axon membrane did not exhibit immunoreactivity. The axoplasm beneath the nodal membrane displayed variable immunostaining. Neither terminal paranodal oligodendroglial loops nor oligodendrocyte plasmalemma were immunoreactive with 7493 antisera. However, perinodal astrocyte processes exhibited intense immunoreactivity with the anti-sodium channel antisera. Optic nerves incubated with pre-immune sera, or with 7493 antisera that had been pre-adsorbed with purified sodium channel protein, displayed no immunoreactivity. These results demonstrate localization of sodium channels at high density at mammalian nodes of Ranvier and in some perinodal astrocyte processes. The latter observation offers support for an active role for perinodal astrocyte processes in the aggregation of sodium channels within the axon membrane at the node of Ranvier.

A Novel Type of Glial Cell Associated with Nodes of Ranvier in Rat Optic Nerve

Using Golgi impregnation and intracellular injection of horseradish peroxidase, we show that the adult rat optic nerve contains two distinct types of astrocyte-like glial cells: one has mainly radially oriented processes that terminate on blood vessels or on the pial surface; the other has mainly longitudinally oriented processes that associate with, and often terminate at, nodes of Ranvier, but do not end on blood vessels or the pial surface. The sequence of appearance of the two types of glial cells in the developing nerve, taken together with previous immunocytochemical findings, suggests that these cells may correspond to the two types of astrocytes previously described in cultures of perinatal optic nerve cells-those with mainly radially oriented processes corresponding to type-1 astrocytes and those with mainly longitudinally oriented processes corresponding to type-2 astrocytes. To our knowledge, this is the first description of a class of central nervous system (CNS) glial cell whose processes are primarily associated with nodes of Ranvier.

Attempt to classify glial cells by means of their process specialization using the rabbit retinal Müller cell as an example of cytotopographic specialization of glial cells

The rabbit retinal Müller cell is one of the most widely studied glial cell types, and it has all forms of contacts that a glial cell can express, viz. 1) to a (ventricular) fluid space, 2) to a mesenchymal borderline (basal lamina), and 3) to neuronal compartments. This cell demonstrates the local adaptation of cell processes to the microenvironment with which they are in contact. Summarizing available data on Müller cells and other glial cell types, it is concluded that the structure with which the process is in contact determines the type of glial cell process that develops. The type I process has microvilli, desmosome-like junctions, and high Na+,K+-ATPase activity; this type of process is in direct contact with a fluid such as cerebrospinal fluid. The type II endfoot-bearing process contains gliofilaments and has a high K+ conductivity; this type of process is covered by a basal lamina and is in contact with mesenchyme. The type III sheath-bearing process insulates neuronal compartments and expresses suitable membrane properties for glia-neuronal communication. Since structurally similar processes have been shown to have similar physiological properties, a new systematic classification of glial cells is proposed, based on the presence or absence of defined types of cell processes. This approach is believed to provide new insights into the function of neuroglia in both the central and peripheral nervous systems, in vertebrates and invertebrates, and even during ontogenetic development.

Demyelination in canine distemper encephalomyelitis: an ultrastructural analysis

A morphological study of selected white matter lesions was carried out in three dogs with canine distemper encephalomyelitis. Two dogs had experimental infections while the third was a spontaneous case. Two stages were identified in the process of demyelination. The earliest evidence of myelin injury was a ballooning change in myelin sheaths involving single or multiple axons. This was followed by a progressive stripping of compact sheaths by the cytoplasmic fingers of phagocytic cells which infiltrated and removed myelin lamellae. Some axonal necrosis also accompanied these changes. Where demyelination occurred, canine distemper viral nucleocapsids were found in astrocytes, macrophages, ependymal cells and infiltrating lymphocytes. In contrast, oligodendrocytes were conspicuous by their apparent lack of infection. Thus it seems that myelin loss cannot be ascribed to oligodendrocyte infection. Perturbed astrocyte function following canine distemper viral infection may cause oedema of myelin sheaths, leading to ballooning and primary demyelination. Cells which phagocytosed myelin were mainly identified as microglial cells with lesser involvement by astrocytes. Rarely, oligodendrocytes also acted as macrophages. Myelin debris was engulfed in bulk or as small droplets into coated pits. Remyelination was present in established plaques although not in great abundance, perhaps due to the diminished oligodendrocyte numbers and a relative increase in immature forms of these cells. These observations are compared to similar changes observed in other demyelinating diseases of animals and man.

The oligodendrocyte-type-2 astrocyte cell lineage is specialized for myelination

Astrocytes are one of the most numerous cell types in the vertebrate central nervous system (CNS) and yet their functions are largely unknown. In the rat optic nerve there are two distinct types of astrocyte: type-1 astrocytes develop from one type of precursor cell, and type-2 astrocytes develop from bipotential, oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells, that initially give rise to oligodendrocytes (which make myelin in the CNS), and then to type-2 astrocytes. Type-1 astrocytes form the glial limiting membrane at the periphery of the optic nerve and are probably responsible for glial scar formation following nerve transection. The functions of type-2 astrocytes, which, like oligodendrocytes, are found mainly in tracts of myelinated axons throughout the CNS, are unknown. In this report we provide evidence that processes from type-2 astrocytes contribute to the structure of nodes of Ranvier, suggesting that the O-2A cell lineage is specialized for constructing myelin sheaths and nodes in the mammalian CNS.