Cell length to diameter relation of rat fetal radial glia--does impaired K+ transport capacity of long thin cells cause their perinatal transformation into multipolar astrocytes?

Evolutionary Modifications Are Moderate in the Astroglial System of Actinopterygii as Revealed by GFAP Immunohistochemistry

The present paper is the first comparative study on the astroglia of several actinopterygian species at different phylogenetical positions, teleosts (16 species), and non-teleosts (3 species), based on the immunohistochemical staining of GFAP (glial fibrillary acidic protein), the characteristic cytoskeletal intermediary filament protein, and immunohistochemical marker of astroglia. The question was, how the astroglial architecture reflexes the high diversity of this largest vertebrate group. The actinopterygian telencephalon has a so-called ‘eversive’ development in contrast to the ‘evagination’ found in sarcopterygii (including tetrapods). Several brain parts either have no equivalents in tetrapod vertebrates (e.g., torus longitudinalis, lobus inferior, lobus nervi vagi), or have rather different shapes (e.g., the cerebellum). GFAP was visualized applying DAKO polyclonal anti-GFAP serum. The study was focused mainly on the telencephalon (eversion), tectum (visual orientation), and cerebellum (motor coordination) where the evolutionary changes were most expected, but the other areas were also investigated. The predominant astroglial elements were tanycytes (long, thin, fiber-like cells). In the teleost telencephala a ‘fan-shape’ re-arrangement of radial glia reflects the eversion. In bichir, starlet, and gar, in which the eversion is less pronounced, the ‘fan-shape’ re-arrangement did not form. In the tectum the radial glial processes were immunostained, but in Ostariophysi and Euteleostei it did not extend into their deep segments. In the cerebellum Bergmann-like glia was found in each group, including non-teleosts, except for Cyprinidae. The vagal lobe was uniquely enlarged and layered in Cyprininae, and had a corresponding layered astroglial system, which left almost free of GFAP the zones of sensory and motor neurons. In conclusion, despite the diversity and evolutionary alterations of Actinopterygii brains, the diversity of the astroglial architecture is moderate. In contrast to Chondrichthyes and Amniotes; in Actinopterygii true astrocytes (stellate-shaped extraependymal cells) did not appear during evolution, and the expansion of GFAP-free areas was limited.

Distribution of GFAP in Squamata: Extended Immunonegative Areas, Astrocytes, High Diversity, and Their Bearing on Evolution

Squamata is one of the richest and most diverse extant groups. The present study investigates the glial fibrillary acidic protein (GFAP)-immunopositive elements of five lizard and three snake species; each represents a different family. The study continues our former studies on bird, turtle, and caiman brains. Although several studies have been published on lizards, they usually only investigated one species. Almost no data are available on snakes. The animals were transcardially perfused. Immunoperoxidase reactions were performed with a mouse monoclonal anti-GFAP (Novocastra). The original radial ependymoglia is enmeshed by secondary, non-radial processes almost beyond recognition in several brain areas like in other reptiles. Astrocytes occur but only as complementary elements like in caiman but unlike in turtles, where astrocytes are absent. In most species, extended areas are free of GFAP—a meaningful difference from other reptiles. The predominance of astrocytes and the presence of areas free of GFAP immunopositivity are characteristic of birds and mammals; therefore, they must be apomorphic features of Squamata, which appeared independently from the evolution of avian glia. However, these features show a high diversity; in some lizards, they are even absent. There was no principal difference between the glial structures of snakes and lizards. In conclusion, the glial structure of Squamata seems to be the most apomorphic one among reptiles. The high diversity suggests that its evolution is still intense. The comparison of identical brain areas with different GFAP contents in different species may promote understanding the role of GFAP in neuronal networks. Our findings are in accordance with the supposal based on our previous studies that the GFAP-free areas expand during evolution.

Evolution of Neuroglia

As the nervous system evolved from the diffused to centralised form, the neurones were joined by the appearance of the supportive cells, the neuroglia. Arguably, these non-neuronal cells evolve into a more diversified cell family than the neurones are. The first ancestral neuroglia appeared in flatworms being mesenchymal in origin. In the nematode C. elegans proto-astrocytes/supportive glia of ectodermal origin emerged, albeit the ensheathment of axons by glial cells occurred later in prawns. The multilayered myelin occurred by convergent evolution of oligodendrocytes and Schwann cells in vertebrates above the jawless fishes. Nutritive partitioning of the brain from the rest of the body appeared in insects when the hemolymph-brain barrier, a predecessor of the blood-brain barrier was formed. The defensive cellular mechanism required specialisation of bona fide immune cells, microglia, a process that occurred in the nervous system of leeches, bivalves, snails, insects and above. In ascending phylogeny, new type of glial cells, such as scaffolding radial glia, appeared and as the bran sizes enlarged, the glia to neurone ratio increased. Humans possess some unique glial cells not seen in other animals.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

The homeostatic astroglia emerges from evolutionary specialization of neural cells

Evolution of the nervous system progressed through cellular diversification and specialization of functions. Conceptually, the nervous system is composed from electrically excitable neuronal networks connected with chemical synapses and non-excitable glial cells that provide for homeostasis and defence. Astrocytes are integrated into neural networks through multipartite synapses; astroglial perisynaptic processes closely enwrap synaptic contacts and control homeostasis of the synaptic cleft, supply neurons with glutamate and GABA obligatory precursor glutamine and contribute to synaptic plasticity, learning and memory. In neuropathology, astrocytes may undergo reactive remodelling or degeneration; to a large extent, astroglial reactions define progression of the pathology and neurological outcome.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Stratification of astrocytes in healthy and diseased brain

Astrocytes, a subtype of glial cells, come in variety of forms and functions. However, overarching role of these cell is in the homeostasis of the brain, be that regulation of ions, neurotransmitters, metabolism or neuronal synaptic networks. Loss of homeostasis represents the underlying cause of all brain disorders. Thus, astrocytes are likely involved in most if not all of the brain pathologies. We tabulate astroglial homeostatic functions along with pathological condition that arise from dysfunction of these glial cells. Classification of astrocytes is presented with the emphasis on evolutionary trails, morphological appearance and numerical preponderance. We note that, even though astrocytes from a variety of mammalian species share some common features, human astrocytes appear to be the largest and most complex of all astrocytes studied thus far. It is then an imperative to develop humanized models to study the role of astrocytes in brain pathologies, which is perhaps most abundantly clear in the case of glioblastoma multiforme.

Surface topography during neural stem cell differentiation regulates cell migration and cell morphology

We sought to determine the contribution of scaffold topography to the migration and morphology of neural stem cells by mimicking anatomical features of scaffolds found in vivo. We mimicked two types of central nervous system scaffolds encountered by neural stem cells during development in vitro by constructing different diameter electrospun polycaprolactone (PCL) fiber mats, a substrate that we have shown to be topographically similar to brain scaffolds. We compared the effects of large fibers (made to mimic blood vessel topography) with those of small-diameter fibers (made to mimic radial glial process topography) on the migration and differentiation of neural stem cells. Neural stem cells showed differential migratory and morphological reactions with laminin in different topographical contexts. We demonstrate, for the first time, that neural stem cell biological responses to laminin are dependent on topographical context. Large-fiber topography without laminin prevented cell migration, which was partially reversed by treatment with rock inhibitor. Cell morphology complexity assayed by fractal dimension was inhibited in nocodazole- and cytochalasin-D-treated neural precursor cells in large-fiber topography, but was not changed in small-fiber topography with these inhibitors. These data indicate that cell morphology has different requirements on cytoskeletal proteins dependent on the topographical environment encountered by the cell. We propose that the physical structure of distinct scaffolds induces unique signaling cascades that regulate migration and morphology in embryonic neural precursor cells. J. Comp. Neurol. 524:3485-3502, 2016. © 2016 Wiley Periodicals, Inc.

Glial Asthenia and Functional Paralysis: A New Perspective on Neurodegeneration and Alzheimer's Disease

Neuroglia are represented by several population of cells heterogeneous in structure and function that provide for the homeostasis of the brain and the spinal cord. Neuroglial cells are also central for neuroprotection and defence of the central nervous system against exo- and endogenous insults. At the early stages of neurodegenerative diseases including Alzheimer's disease neuroglial cells become asthenic and lose some of their homeostatic, neuroprotective, and defensive capabilities. Astroglial reactivity, for example, correlates with preservation of cognitive function in patients with mild cognitive impairment and prodromal Alzheimer's disease. Here, we overview the experimental data indicating glial paralysis in neurodegeneration and argue that loss of glial function is fundamental for defining the progression of neurodegenerative diseases.

Caenorhabditis elegans glia modulate neuronal activity and behavior

Glial cells of Caenorhabditis elegans can modulate neuronal activity and behavior, which is the focus of this review. Initially, we provide an overview of neuroglial evolution, making a comparison between C. elegans glia and their genealogical counterparts. What follows is a brief discussion on C. elegans glia characteristics in terms of their exact numbers, germ layers origin, their necessity for proper development of sensory organs, and lack of their need for neuronal survival. The more specific roles that various glial cells have on neuron-based activity/behavior are succinctly presented. The cephalic sheath glia are important for development, maintenance and activity of central synapses, whereas the amphid glia seem to set the tone of sensory synapses; these glial cell types are ectoderm-derived. Mesoderm-derived Glial-Like cells in the nerve Ring (GLRs) appear to be a part of the circuit for production of motor movement of the worm anterior. Finally, we discuss tools and approaches utilized in studying C. elegans glia, which are assets available for this animal, making it an appealing model, not only in neurosciences, but in biology in general.

Development of interlaminar astroglial processes in the cerebral cortex of control and Down's syndrome human cases

Glial cytoarchitecture in human cerebral cortex is constituted by two overlapping layouts: the (general mammalian) "glial syncytium" and the (primate-specific) "interlaminar glial palisade" (IGP) composed by astroglial cells, with long, radial processes that traverse several supragranular layers. In this study, the emergence and early organization of the IGP was analyzed using immunocytochemical procedures in postmortem infantile human control and age matched, Down's syndrome (DS) cases. In control cases, first signs of a radial array of unbranched astroglial processes were apparent at the end of the period of "physiological astrocytosis" (20-40 days of postnatal life), and its general profile (except perhaps the density of cell processes) reached the adult-like configuration by the second month of life. The initial organization of the IGP was similar in control and DS cases, although a breakdown in DS became manifest by the first year of age, or earlier, albeit with individual variations. These changes tended to evolve in a "mosaic" fashion and included partial disruption of the palisade, or persistence of the "physiological astrocytosis". These observations were compared against samples from elder DS cases with an Alzheimer's type of dementia (AtD). Collectively, results suggest that DS also involves astroglial alterations during early stages of brain development, and that those changes progress with age, until an AtD ensues during adult life.

Postnatal development of interlaminar astroglial processes in the cerebral cortex of primates

Long astroglial processes traversing several cortical laminae appear to be characteristic of primate brains. Whether interlaminar processes develop as a modification of radial glia or are truly postnatal elements stemming from stellate astroglia, could be assessed by analyzing their early developmental stages. A survey of glial fibrillar acidic protein immunoreactive (GFAP-IR) astroglial interlaminar processes in the cerebral cortex of Ceboidea monkeys at various postnatal developmental ages, and in human cortical samples of a ten day and a seven year old child disclosed that such processes develop postnatally. At one month of age GFAP-IR interlaminar processes in monkeys were scarce and short in most frontal, parietal or occipital (striate) cortical areas, except for sulcal (principal and orbital sulci) and temporal cortical areas. Some processes were weakly positive for vimentin, and these were most abundant in ventral temporal cortical areas. At two months of age processes were present in all these areas, albeit in restricted patches and significantly shorter than in adults. The expression of this pattern was increased at seven months of age. At three years of age almost every area showed abundant processes and with lengths comparable to the adult Ceboidea individuals. In humans, at 10 days of age long interlaminar processes were readily apparent in a frontal cortex sample, becoming most apparent at the age of seven years although not reaching yet the adult characteristics as described previously.

CONCLUSIONS

(1) GFAP-IR interlaminar processes develop postnatally, thus typifying a subtype of the classical stellate forms; (2) they bear no obvious direct relationship with radial glia; (3) their development is not contemporary among the various cortical regions. These long cellular processes represent an addition to those already described for other astroglial cell types in the adult mammalian brain (Golgi-Bergmann glia, tanicytes, Muller cells).

Comparative morphometry of Bergmann glial (Golgi epithelial) cells. A Golgi study

Bergmann glial (Golgi epithelial) cells were Golgi-impregnated in the cerebella of species with great differences in the thickness of the molecular layer, in small African native mouse, rat, rhesus monkey, and man. The thickness of the molecular layer determines the length of the radial Bergmann cell processes. Whereas the overall morphology of the cells was found to be strikingly similar in all species studied, there were great quantitative differences in length and diameter of the stem processes. Species with thick molecular layers (man, monkey) have thicker stem processes than species with short distances between Bergmann glial cell soma and pial surface (rat, mouse). This could mean that larger animals with longer gestation periods allow for prolonged growth of cell volumes. On the other hand, an increase in the diameter of long processes should reduce the cytoplasmic resistance against ionic currents; this would be important when Bergmann glial cells--like retinal Müller cells--would act as "cables" for spatial buffering of potassium ions released by electrically active neurons. By contrast, the fractal dimension--i.e., a quantitative measure of the complexity of the cell's border--of the cell processes was lower in species with long processes. In an age series of rat cells, the fractal dimension is shown to increase slightly up to a very old age.

Radial glia give rise to perinodal processes

Nodes of Ranvier in the central nervous system in mammals are characterized by the presence of perinodal astrocytic processes. This study examines the association between processes of radial glia and the axolemma at nodes of Ranvier in the spinal cord of the mature axolotl, an animal in which radial glia represent a large portion of the total glial population. The radial glial cells have their cell bodies located close to the central canal. Those situated dorsal to the canal send long processes to the dorsal surface of the spinal cord. Along this trajectory these processes coalesce into large fascicles in the midline and form the dorsal median septum. Slender branches rise from the processes in these fascicles and extend into the adjacent white matter to terminate in close apposition to the axolemma at nodes of Ranvier. This arrangement provides an intracellular pathway extending from the perinodal region to the surface of the spinal cord. Radial glia ventral to the central canal give rise to processes that project to the glia limitans adjacent to the ventral spinal artery. These ventrally projecting processes appear to be more irregular in their branching pattern than their dorsal counterparts. Multiple slender processes are seen in close apposition to the nodal axolemma of myelinated axons in the ventral white commissure, again providing an intracellular pathway that runs from the perinodal region to the cord surface. In one instance a radial glial process was observed to occupy a pocket formed by the invagination of the nodal axolemma. The axonal cytoplasm adjacent to the invagination contained a variety of organelles, e.g. multivesicular bodies, vesicles and endoplasmic reticulum, suggesting that this relationship between the radial glial process and the axon is more than a passive interaction. These observations are consistent with the view that processes of radial glial cells may regulate the extracellular environment adjacent to the nodal axolemma, and/or play an active role in the maintenance of the nodal membrane. The existence of perinodally-directed processes of radial glial cells in the salamander indicates that this axo-glial specialization reflects an important functional interaction preserved across a large segment of the phylogenetic scale.

Ontogeny of radial and other astroglial cells in murine cerebral cortex

Three cell forms of astroglial lineage populate the prenatal and early postnatal murine cerebral wall. In the present review we consider the ontogeny of these cell forms with respect to histogenetic events of the perinatal period. Classic bipolar radial glial cells predominate prior to E17. The bipolar coexist with monopolar radial forms in the perinatal period. Both bipolar and monopolar radial forms coexist with multipolar astrocytes in the course of the first postnatal week and are ultimately succeeded by the multipolar cells. The shift from bipolar to monopolar radial forms is initially coincident with translocation of somata of bipolar cells from the ventricular zone to the upper intermediate zone and cortical strata. Arborization appears to occur both at the growing tips and along the shaft of the processes of both bipolar and monopolar radial cell types. As arborization continues, the processes of the monopolar radial cells come to resemble those of the multipolar astrocytes. Eventually the radial cells are fully transformed into the multipolar astrocytic forms. During this period of transition, radial processes in the cortex appear to be degenerating, suggesting that regressive processes contribute to the cytologic transformation. This sequence of transformations begins late in the period of neuronal migration and continues through the early stages of growth and differentiation in the murine cerebral cortex. The signals that induce these changes may arise from differentiating neurons within the cortex. These transformations occur at a time when radial glial fibers are no longer required as guides for neuronal migration, and the glial population assumes new roles related to the development and operation of cortical neuronal circuits.

Potassium as a signal for both proliferation and differentiation of rabbit retinal (Müller) glia growing in cell culture

Retinal glial (Müller) cells were grown from explants of early postnatal rabbit retinae. The resulting monolayers of flat cells were exposed to control media (containing 5.85 mM K+), and to media with enhanced K+ concentrations (10 and 20 mM) or arginine-vasopressin (AVP, 20 micrograms/ml) or epithelial growth factor (EGF, 10 ng/ml). Autoradiographically, protein synthesis was quantified as L-[3H]-lysine incorporation, and DNA synthesis as [3H]-thymidine incorporation. Furthermore, the activity of Na+,K(+)-ATPase was measured radiochemically. Short exposure to either moderately enhanced K+ concentrations (10 mM) or to AVP, stimulated L-[3H]-lysine incorporation into the cells. Long-lasting exposure to either high K+ concentrations (20 mM) or to EGF stimulated [3H]-uptake. The Na+,K(+)-ATPase activity of cell cultures increased with increasing K+ concentration of the media. It is suggested that release of K+ by active neuronal compartments stimulates local protein synthesis of glial cells, resulting in the formation of glial sheaths with active K+ uptake capacity. Strong K+ release may even induce glial proliferation.

Size and density of glial and neuronal cells within the cerebral neocortex of various insectivorian species

Morphometric measurements were done on frontal sections through the somatosensory neocortex of various insectivorian species. All measured parameters varied with the size of animals; there was a better correlation with the ventriculartopial brain wall thickness than with the brain weight. The following rules were evaluated: with increasing brain wall thickness, 1) lamina I becomes thinner; 2) the nuclei of both neuronal and glial cells become larger; 3) the volume density of neuronal cells decreases greatly; 4) the volume density of glial cells increases slightly; and 5) as a result, the glia:neuron index increases markedly. There was no equal number of neurons under a unit surface area in the cortices of any species studied. Developmental processes that might account for the above-mentioned rules are discussed in this report.

Glia:neuron index: review and hypothesis to account for different values in various mammals

The present paper proposes a hypothesis to account for different values of the glia:neuron index in comparable central nervous system tissues of various mammals. This hypothesis assumes that K+ ions released by active neurons are a mitogenic signal for glial cells. The thicker the tissue (for example, the brain wall), the more difficult is efficient K+ clearance, and more perinatal glial cell proliferation should occur. Thus, this hypothesis accounts for higher glia:neuron indices in mammals with thicker brain walls.

Müller (glial) cells but not astrocytes in the retina of the goldfish possess orthogonal arrays of particles

Müller cells as the main glial component of the retina were investigated in the goldfish by means of ultrathin sections and freeze-fracture replicas. In the optic nerve head, they were directly compared with astrocytes. Whereas astrocytic endfeet bordering the vitreous body can easily be identified by their dense bundles of intermediate filaments, scarce membranous organelles, paravitreous caveolae, and lateral desmosomes, Müller cell endfeet reveal a looser arrangement of intermediate filaments, a characteristic pattern of triangularly shaped endoplasmic reticulum, large and pale mitochondria, and, if at all, very few desmosome-like junctions. The paravitreous membranes at the cytoplasmic face are covered by a fuzzy coat, which is less marked in astrocytic endfeet. Caveolae are lacking. Considering the freeze-fracture architecture of the membranes of both glial cell types, the Müller cells reveal orthogonal arrays of particles (OAP), which were predominantly located opposite to the inner limiting membrane; their density (109 +/- 33 OAP/microns 2) decreases abruptly with the loss of the contact between membrane and vitreous body. In contrast, astrocytes of the optic nerve head in the retina do not show any OAP in their membranes at all and are interconnected by tight junctions and desmosomes. The hypothesis suggesting that OAP might be correlated with K+ channels involved in the spatial buffering of the extracellular space is reconsidered with comparative reference to recent electrophysiological data. Further, the heterogeneity of Müller cell and astrocyte membrane equipment with OAP in the goldfish is briefly discussed.

Heterogeneity of Müller cell endfeet in the rabbit retina as revealed by freeze-fracturing

The Müller cell membranes of the rabbit retina were investigated by means of freeze-fracturing. Orthogonal arrays of particles (OAP) were found almost exclusively where the Müller cell endfoot had direct contact with the vitreous body. In the center of the retina where the Müller cells are long and thin, the density of OAP was about 170/microns2. In the periphery where the Müller cells are shorter and thicker, the density of OAP was about 20/microns2. In this area, only half of the Müller cell endfeet were equipped with OAP. The results are discussed with respect to electrophysiological studies on the role of various K+ channels in glial cells. It is proposed that OAP might possibly represent specialized 'maxi'-K+ channels which are involved in K+ spatial buffering of the extracellular space.

Quantitative-morphometric aspects of Bergmann glial (Golgi epithelial) cell development in rats. A Golgi study

Bergmann glial (Golgi epithelial) cells in the cerebella of rats of various ages were stained by the rapid Golgi technique, and their radial stem processes were measured for length and diameter. Additionally, the average number of such processes per cell was counted, and the development of bushy lateral protrusions was quantified. The length of radial processes--depending on the thickness of the molecular layer--was found to increase up to the end of the 2nd year of life. This elongation was accompanied by a reduction of the mean process diameter which was, however, not sufficient to prevent an increase in the cytoplasmic volume of the elongating cells. A marked outgrowth of lateral protrusions was observed up to at least the 5th month of life. These data are compared with earlier findings on the development of rat brain stem fetal radial glia, and of rabbit retinal Müller cells. Common mechanisms of glial cell development are discussed.