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Cajal's contributions to glia research
Abstract
In 1906, Santiago Ramón y Cajal was awarded the Nobel Prize in Physiology or Medicine in recognition of his work on the structure of neurons and their connections. What is less well known is that he also had a keen interest in glia and developed specific staining methods for their study. In addition to describing their morphology, he speculated on a role for glia in sleep and wakefulness and even in executive brain functions such as attention. In this article, we focus on Cajal's histological research into glial cells; this research includes original drawings of astrocytes, oligodendrocytes, microglia and radial glia, as well as his scientific writings. We aim to show that, concerning glia as well as neurons, Cajal was far ahead of his time.
... There are many modifications of Golgi staining and, in skilful hands, this technique delivers detailed images of astrocytes ( Fig. 3.5), with primary, secondary and even some distal processes, especially when used in combination with electron microscopy. 34,36,37 The sublimated gold-chloride staining of Cajal, 38 which labels astroglial filaments and endfeet, is used but rarely nowadays. 39,40 The same applies to Hortega's silver impregnation method, which, with some modifications, has been occasionally used to visualise astrocytes for light and electron microscopy. ...
... 38 Metabotropic receptors. Metabotropic G-protein coupled receptors (GPCRs) are seven-transmembrane polypeptide chains. ...
... Nervous systems throughout the animal kingdom vary in structure and complexity and are made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glial cells, historically considered to only provide support functions to neurons. Glial cells were first described by Virchow in the 1850s as "nervenkitt" or nerve glue, implying a homogenous population of support cells holding them together (García-Marín et al., 2007). However, several different types of glia can be differentiated based on their different functions and morphology. ...
... The term astrocyte was coined by Michael von Lenhossek to describe star-shaped cells observed in histological brain specimens . Subsequently, Camillo Golgi and Ramon y Cajal with the development of novel histological stains illustrated several astrocytes with their elaborated processes (García-Marín et al., 2007;Navarrete and Araque, 2014). Conventionally, two major classes of astrocytes have been distinguished in histological sections of the central nervous system (CNS) based on their morphology and distribution, the fibrous and protoplasmic astrocytes (Miller and Raff, 1984). ...
Glia are as numerous in the brain as neurons and widely known to serve supportive roles such as structural scaffolding, extracellular ionic and neurotransmitter homeostasis, and metabolic support. However, over the past two decades, several lines of evidence indicate that astrocytes, which are a type of glia, play active roles in neural information processing. Astrocytes, although not electrically active, can exhibit a form of excitability by dynamic changes in intracellular calcium levels. They sense synaptic activity and release neuroactive substances, named gliotransmitters, that modulate neuronal activity and synaptic transmission in several brain areas, thus impacting animal behavior. This “dialogue” between astrocytes and neurons is embodied in the concept of the tripartite synapse that includes astrocytes as integral elements of synaptic function. Here, we review the recent work and discuss how astrocytes via calcium-mediated excitability modulate synaptic information processing at various spatial and time scales.
... Many of the functions now ascribed to astrocytes were postulated nearly 100 years ago by Santiago Ramón y Cajal (1852e1934), who used metal impregnation staining methods to label astrocytes [1]. His hand-drawn images revealed an intimate relationship shared between astrocytes, neuronal cell bodies and their processes, and the vasculature (Figure 1a). ...
... (a-i) Hand drawn illustrations by Santiago Ramón y Cajal depicting astrocytes in the brain contacting neurons and (a-ii) the vasculature. Ramón y Cajal's illustrations revealed the stellate nature of astrocyte morphology [1]. Nearly a century after Ramón y Cajal's illustrations, (b) dye filling of single astrocytes in rat hippocampus revealed that astrocytes are densely ramified cells composed of numerous leaflet-like processes. ...
Astrocytes represent an abundant type of glial cell involved in nearly every aspect of central nervous system (CNS) function, including synapse formation and maturation, ion and neurotransmitter homeostasis, blood–brain barrier maintenance, as well as neuronal metabolic support. These various functions are enabled by the morphological complexity that astrocytes adopt. Recent experimental advances in genetic and viral labeling, lineage tracing, and live- and ultrastructural imaging of miniscule astrocytic sub-compartments reveal a complex morphological heterogeneity that is based on the origin, local function, and environmental context in which astrocytes reside. In this minireview, we highlight recent findings that reveal the plastic nature of astrocytes in the healthy brain, particularly at the synapse, and emerging technologies that have advanced our understanding of these morphologically complex cells.
... In 1846 Virchow defined glial cells as 'the glue of the brain', attributing them a connective function, a view which remained for many years. However, already in the late XIX century Ramon y Cajal had foreseen the importance of these cells for a plethora of brain functions (Ramon y Cajal, 1909, reviewed by Garcia-Marin et al., 2007). Glial cells are subdivided into macroglia (i.e. ...
... astrocytes and oligodendrocytes) and microglia. Macroglial cells derive from neuroepithelial cells and thus from the same progenitors Garcia-Marin et al., 2007. responsible for neuron production. ...
Cerebral cortical development is a finely regulated process, depending on diverse progenitor cells. Abnormal behavior of the latter can give rise to cortical malformations. Mutations in Eml1/EML1 were identified in the HeCo mouse, as well as in three families presenting severe subcortical heterotopia (SH). SH is characterized by the presence of mislocalized neurons in the white matter. At early stages of corticogenesis, abnormally positioned apical radial glia progenitors (aRG) were found cycling outside the proliferative ventricular zone (VZ) in the HeCo cortical wall. I focused my research on characterizing aRG in the VZ to assess why some cells leave this region and thus to further understand SH mechanisms. Combining confocal and electron microscopy (EM), I uncovered abnormalities of centrosomes and primary cilia in Eml1-mutant aRGs: primary cilia are shorter, and often remain basally oriented within vesicles. Searching for Eml1-interacting partners using mass spectrometry (MS), combined with exome sequencing of SH patient DNAs, allowed us to identify a ciliary Eml1-interacting partner, RPGRIP1L, showing mutations in a SH patient. Gene ontology analyses of MS data pointed to Golgi apparatus and protein transport as enriched categories. Indeed, Golgi abnormalities were identified in HeCo aRGs. Altogether, these data indicate that the Golgi-to-primary cilium axis is perturbed in Eml1mutant conditions, pointing to new intracellular pathways involved in severe neurodevelopmental disorders.
... Astroglial endfeet envelope brain microvessels as well as synapses (Figure 9) [30][31][32]. This anatomical location of astroglia seems to be suitable for the direct uptake of glucose from the microvessels [3]. ...
... suggesting the occurrence of active aerobic glycolysis in astroglia. Although the glucose consumption of astroglia seems to be comparable to that of neurons, the in vivo location of astroglia in the brain may make glucose uptake more suitable [30][31][32]. In contrast, neurons are not in direct contact with microvessels. ...
Astroglia or astrocytes, the most abundant cells in the brain, are interposed between neuronal synapses and the microvasculature in the brain’s gray matter. This unique anatomical location allows astroglia to play pivotal roles in brain metabolism as well as in the regulation of cerebral blood flow. In particular, astroglial cellular metabolic compartmentation exerts supportive roles in dedicating neurons to the generation of action potentials and protects neurons against the oxidative stress associated with their high energy consumption. Key products of astroglia include lactate and ketone bodies (beta-hydroxybutyrate and acetoacetate), which can also be produced avidly by muscle and liver, respectively. Therefore, brain cells, skeletal muscles, and hepatocytes constitute a metabolic compartmentation in the whole body. In this chapter, I will focus on brain cells, especially astroglia, since the impairment of normal astroglial function can lead to numerous neurological disorders including stroke, neurodegenerative diseases, and neuro-immunological diseases. I will also discuss the metabolic responses of brain cells in terms of food consumption and exercise. A better understanding of the astroglial metabolic response is expected to lead to the development of novel therapeutic strategies for diverse neurological diseases.
... Astrocytes were first described in the 19th century as a homogenous cell population and believed to have a passive and supportive role for neuronal function [1]. Golgi and Cajal performed a more extensive morphological characterization of astrocytes and discovered the heterogeneity of these cells [2]. Among mammals, human astrocytes exhibit distinct features, including notably larger volumes, increased synaptic interactions, enhanced calcium propagation, and double the number of morphological subtypes compared to astrocytes found in rodents [3][4][5][6]. ...
Astrocytes, the most abundant glial cell type in the brain, play crucial roles in maintaining homeostasis within the central nervous system (CNS). Impairment or abnormalities of typical astrocyte functions in the CNS serve as a causative or contributing factor in numerous neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. Currently, disease-modeling and drug-screening approaches, primarily focused on human astrocytes, rely on human pluripotent stem cell (hPSC)-derived astrocytes. However, it is important to acknowledge that these hPSC-derived astrocytes exhibit notable differences across studies and when compared to their in vivo counterparts. These differences may potentially compromise translational outcomes if not carefully accounted for. This review aims to explore state-of-the-art in vitro models of human astrocyte development, focusing on the developmental processes, functional maturity, and technical aspects of various hPSC-derived astrocyte differentiation protocols. Additionally, it summarizes their successful application in modeling neurological disorders. The discussion extends to recent advancements in the large-scale production of human astrocytes and their application in developing high-throughput assays conducive to therapeutic drug discovery.
... Although the mechanisms orchestrating this dance between sleep stages have remained elusive, emerging from this complexity is the role of astrocytes, star-shaped glial cells now recognized as pivotal in the sleep-wake cycle. Astrocytes emerged as early as 1895, when Cajal proposed that astrocytes acted as physical barriers that interrupted synaptic transmission during sleep [17]. Since then, a growing body of evidence has suggested that astrocytes are involved in regulating sleep-wake cycles and maintaining homeostasis [18][19][20][21]. ...
Astrocytes, the most abundant cells in the brain, are integral to sleep regulation. In the context of a healthy neural environment, these glial cells exert a profound influence on the sleep-wake cycle, modulating both rapid eye movement (REM) and non-REM sleep phases. However, emerging literature underscores perturbations in astrocytic function as potential etiological factors in sleep disorders, either as protopathy or comorbidity. As known, sleep disorders significantly increase the risk of neurodegenerative, cardiovascular, metabolic, or psychiatric diseases. Meanwhile, sleep disorders are commonly screened as comorbidities in various neurodegenerative diseases, epilepsy, and others. Building on existing research that examines the role of astrocytes in sleep disorders, this review aims to elucidate the potential mechanisms by which astrocytes influence sleep regulation and contribute to sleep disorders in the varied settings of brain diseases. The review emphasizes the significance of astrocyte-mediated mechanisms in sleep disorders and their associated comorbidities, highlighting the need for further research.
... Astrocytes are also involved in the control of cerebral blood flow through neurovascular coupling, regulating synaptic activity and plasticity, encircling the synapses of other neurons, helping to form the blood-brain barrier (BBB), which ensures brain homeostasis, and releasing vasoactive substances that cause arteriole dilation in highly active neural regions. In 1895, Cajal hypothesized that astrocytes control sleep by extending their dendrites into synapses during sleep and retracting them during wakefulness (García-Marín et al., 2007). Although this hypothesis was later proven inaccurate (Bellesi et al., 2015), recent studies have highlighted the importance of astrocytes in modulating sleep (Halassa et al., 2009;Jackson et al., 2020). ...
Sleep plays an essential role in all studied animals with a nervous system. However, sleep deprivation leads to various pathological changes and neurobehavioral problems. Astrocytes are the most abundant cells in the brain and are involved in various important functions, including neurotransmitter and ion homeostasis, synaptic and neuronal modulation, and blood-brain barrier maintenance; furthermore, they are associated with numerous neurodegenerative diseases, pain, and mood disorders. Moreover, astrocytes are increasingly being recognized as vital contributors to the regulation of sleep-wake cycles, both locally and in specific neural circuits. In this review, we begin by describing the role of astrocytes in regulating sleep and circadian rhythms, focusing on: (i) neuronal activity; (ii) metabolism; (iii) the glymphatic system; (iv) neuroinflammation; and (v) astrocyte-microglia cross-talk. Moreover, we review the role of astrocytes in sleep deprivation comorbidities and sleep deprivation-related brain disorders. Finally, we discuss potential interventions targeting astrocytes to prevent or treat sleep deprivation-related brain disorders. Pursuing these questions would pave the way for a deeper understanding of the cellular and neural mechanisms underlying sleep deprivation-comorbid brain disorders.
... Using this technique, Ramon y Cajal confirmed earlier ideas of the origin of astrocytes from radial glia and also demonstrated that astrocytes can divide in the adult brain, thus laying the basis for much later discoveries of the stem properties of astroglia. 196,197 He also contemplated the proliferative capacity of astrocytes after observing pairs of astrocytes joined by their soma; these pairs were defined as twin astrocytes or 'astrocitos gemelos' (see, 196 and for more details 198 ). Retzius. ...
... As originally suggested by Ramony Cajal (Garcia-Marin et al., 2007), growing evidence shows that microglia dynamics are coupled to and intertwined with the sleep-wake cycle. Microglia are resident innate immune cells ubiquitously distributed in the central nervous system and account for 10-15% of all brain cells (Wolf et al., 2017). ...
Microglia surveillance manifests itself as dynamic changes in cell morphology and functional remodeling in response to fluctuations in the neural environment. Whether and how microglia surveillance is coupled to brain state switches during natural sleep-wake cycles, as well as under sleep deprivation, remain unclear. To address this question, we used miniature two-photon microscopy (mTPM) to acquire time-lapse high-resolution microglia images of the somatosensory cortex, along with EEG/EMG recordings and behavioral video, in freely-behaving mice. We uncovered fast and robust brain state-dependent changes in microglia surveillance, occurring in parallel with sleep dynamics and early-onset phagocytic microglial contraction during sleep deprivation stress. With the aid of the biosensor GRABNE2m, we also detected local norepinephrine fluctuation occurring in a sleep state-dependent manner. We showed that the locus coeruleus-norepinephrine system, which is crucial to sleep homeostasis, is required for both sleep state-dependent and stress-induced microglial responses and involves β2-adrenergic receptor signaling. These results provide direct evidence that microglial surveillance is exquisitely tuned to signals and stressors that regulate sleep dynamics and homeostasis so as to adjust its varied roles to complement those of neurons in the brain. In vivo imaging with mTPM in freely behaving animals, as demonstrated here, opens a new avenue for future investigation of microglia dynamics and sleep biology in freely behaving animals.
... The term "glial cells", or the concept of "neuroglia," is based on the collective function that these types of cells play in nervous system homeostasis, regardless of their distinct structure, specific functions, and embryonic origins . Glial cells were previously mischaracterized as a mere sort of "glue" for adjoining neurons without further, more complex functions (García-Marín et al., 2007;Jäkel & Dimou, 2017). From this concept arose the name glia (from the ancient Greek word "glía" and the German word "Nervenkitt", both meaning "glia" in English). ...
Zika virus (ZIKV) is a strongly neurotropic flavivirus whose infection has been associated with microcephaly in neonates. However, clinical and experimental evidence indicate that ZIKV also affects the adult nervous system. In this regard, in vitro and in vivo studies have shown the ability of ZIKV to infect glial cells. In the central nervous system (CNS), glial cells are represented by astrocytes, microglia, and oligodendrocytes. In contrast, the peripheral nervous system (PNS) constitutes a highly heterogeneous group of cells (Schwann cells, satellite glial cells, and enteric glial cells) spread through the body. These cells are critical in both physiological and pathological conditions; as such, ZIKV-induced glial dysfunctions can be associated with the development and progression of neurological complications, including those related to the adult and aging brain. This review will address the effects of ZIKV infection on CNS and PNS glial cells, focusing on cellular and molecular mechanisms, including changes in the inflammatory response, oxidative stress, mitochondrial dysfunction, Ca2+ and glutamate homeostasis, neural metabolism, and neuron-glia communication. Of note, preventive and therapeutic strategies that focus on glial cells may emerge to delay and/or prevent the development of ZIKV-induced neurodegeneration and its consequences.
... Studies in the nineteenth century revealed that astrocytes extend large endfoot processes to cover the vasculature, leading Camillo Golgi to hypothesize that they deliver nutrients to neurons and Ernesto Lugaro to propose that they remove the toxic waste products of neuronal metabolism. Santiago Ramón y Cajal further suggested that astrocyte endfoot contraction may dilate blood vessels and that this regulation may underlie attention and sleep-wake cycles (Antonakou & Triarhou 2019, García-Marín et al. 2007, Ramón y Cajal 1895. In the twentieth century, astrocytes were largely ignored due to technical challenges in studying their physiology. ...
Astrocyte endfeet enwrap the entire vascular tree within the central nervous system, where they perform important functions in regulating the blood-brain barrier (BBB), cerebral blood flow, nutrient uptake, and waste clearance. Accordingly, astrocyte endfeet contain specialized organelles and proteins, including local protein translation machinery and highly organized scaffold proteins, which anchor channels, transporters, receptors, and enzymes critical for astrocyte-vascular interactions. Many neurological diseases are characterized by the loss of polarization of specific endfoot proteins, vascular dysregulation, BBB disruption, altered waste clearance, or, in extreme cases, loss of endfoot coverage. A role for astrocyte endfeet has been demonstrated or postulated in many of these conditions. This review provides an overview of the development, composition, function, and pathological changes of astrocyte endfeet and highlights the gaps in our knowledge that future research should address.
Expected final online publication date for the Annual Review of Neuroscience, Volume 46 is July 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... As originally suggested by Ramony Cajal (Garcia-Marin et al., 2007), growing evidence shows that microglia dynamics are coupled to and intertwined with the sleep-wake cycle. Microglia are resident innate immune cells ubiquitously distributed in the central nervous system and account for 10-15% of all brain cells (Wolf et al., 2017). ...
Microglia surveillance manifests itself as dynamic changes in cell morphology and functional remodeling in response to fluctuations in the neural environment. Whether and how microglia surveillance is coupled to brain state switches during natural sleep-wake cycles, as well as under sleep deprivation, remain unclear. To address this question, we used miniature two-photon microscopy (mTPM) to acquire time-lapse high-resolution microglia images of the somatosensory cortex, along with EEG/EMG recordings and behavioral video, in freely-behaving mice. We uncovered fast and robust brain state-dependent changes in microglia surveillance, occurring in parallel with sleep dynamics and early-onset phagocytic microglial contraction during sleep deprivation stress. With the aid of the biosensor GRAB NE2m , we also detected local norepinephrine fluctuation occurring in a sleep state-dependent manner. We showed that the locus coeruleus-norepinephrine system, which is crucial to sleep homeostasis, is required for both sleep state-dependent and stress-induced microglial responses and involves β 2 -adrenergic receptor signaling. These results provide direct evidence that microglial surveillance is exquisitely tuned to signals and stressors that regulate sleep dynamics and homeostasis so as to adjust its varied roles to complement those of neurons in the brain. In vivo imaging with mTPM in freely behaving animals, as demonstrated here, opens a new avenue for future investigation of microglia dynamics and sleep biology in freely behaving animals.
... Different glial cells have corresponding, specific staining methods. Cajal staining is a selective technique for astrocytes (Garcia-Marin et al., 2007), and immunohistochemistry or immunofluorescence analysis for glial fibrillary acidic protein (GFAP) can show the activation of astrocytes (Lu et al., 2022). Silver carbonate staining shows cell bodies and microglial processes (McCarter, 1940;Wang and Wei, 2012). ...
Neurologic injury after cardiopulmonary resuscitation is the main cause of the low survival rate and poor quality of life among patients who have experienced cardiac arrest. In the United States, as the American Heart Association reported, emergency medical services respond to more than 347,000 adults and more than 7,000 children with out-of-hospital cardiac arrest each year. In-hospital cardiac arrest is estimated to occur in 9.7 per 1,000 adult cardiac arrests and 2.7 pediatric events per 1,000 hospitalizations. Yet the pathophysiological mechanisms of this injury remain unclear. Experimental animal models are valuable for exploring the etiologies and mechanisms of diseases and their interventions. In this review, we summarize how to establish a standardized rat model of asphyxia-induced cardiac arrest. There are four key focal areas: (1) selection of animal species; (2) factors to consider during modeling; (3) intervention management after return of spontaneous circulation; and (4) evaluation of neurologic function. The aim was to simplify a complex animal model, toward clarifying cardiac arrest pathophysiological processes. It also aimed to help standardize model establishment, toward facilitating experiment homogenization, convenient interexperimental comparisons, and translation of experimental results to clinical application.
... To investigate whether cortical astrocytes in VWM show abnormalities, we performed a morphological analysis [26]. Typically, astrocytes in the cortex are highly branched with bushy processes [28], consistent with our control findings ( Figure 7A). In the control, cortical astrocytes were only visible in layer I close to the glia limitans and around blood vessels. ...
Vanishing white matter (VWM) is classified as a leukodystrophy with astrocytes as primary drivers in its pathogenesis. Magnetic resonance imaging has documented the progressive thinning of cortices in long-surviving patients. Routine histopathological analyses, however, have not yet pointed to cortical involvement in VWM. Here, we provide a comprehensive analysis of the VWM cortex. We employed high-resolution-mass-spectrometry-based proteomics and immunohistochemistry to gain insight into possible molecular disease mechanisms in the cortices of VWM patients. The proteome analysis revealed 268 differentially expressed proteins in the VWM cortices compared to the controls. A majority of these proteins formed a major protein interaction network. A subsequent gene ontology analysis identified enrichment for terms such as cellular metabolism, particularly mitochondrial activity. Importantly, some of the proteins with the most prominent changes in expression were found in astrocytes, indicating cortical astrocytic involvement. Indeed, we confirmed that VWM cortical astrocytes exhibit morphological changes and are less complex in structure than control cells. Our findings also suggest that these astrocytes are immature and not reactive. Taken together, we provide insights into cortical involvement in VWM, which has to be taken into account when developing therapeutic strategies.
... forêt (Fahim, 2022), deux chercheurs imminents en neuroscience, Cajal et Golgi en 1906, ont tenté d'y répondre (Changeux, 2001;DeFelipe, 2002DeFelipe, , 2006DeFelipe & Jones, 1992;Garcia-Lopez et al., 2007;Garcia-Marin et al., 2007;Ghosh, 2020;Shepherd et al., 2011). De leurs observations, ils ont conclu : même si les connexions anatomiques entre les neurones se développent selon un plan prédéfini, leur force et leur efficacité ne sont pas prédéterminées et peuvent être modifiées par l'expérience pour le meilleur et pour le pire (Albright et al., 2000;Bullock, 2006;Bullock et al., 2005;de Castro, 2019;Delgado-Garcia, 2015). ...
L’objectif du modèle PRESENCE est de mettre en lumière les connaissances sur le développement du cerveau au service des neurosciences de l’éducation. PRESENCE combine une exploration approfondie de l'organisation du cerveau avec une revue de la littérature et perspective théorico-pratique sur la façon dont il permet l'émergence d'états mentaux complexes. Habilement tissé ensemble, le résultat est une image unique du cerveau qui est enracinée dans la morphologie et le fonctionnement cellulaire par la prédisposition génétique/épigénétique, l’élagage synaptique, la neuroplasticité et la neurogenèse puis mise en mouvement par la dynamique des réseaux de neurones et leur synchronisation en passant par la conscience et le libre arbitre. PRESENCE est un modèle sur lequel le CAS en neuroscience de l’éducation s’est basé. Le premier épisode de cette série de huit épisodes nous fait voyager au cœur de la prédisposition génétique et épigénétique. Les connexions sont établies selon un plan génétiquement programmé mais leur maintien et leur qualité peuvent être largement régulés par l’activité neuronale et donc l’expérience. En intervenant auprès du jeune il faut garder à l’esprit le fait qu’on intervient auprès d’une structure cérébrale génétiquement et épigénétiquement complexe et imprévisible ; que nos interventions ont les capacités d’interagir avec cette structure et la modifier pour le meilleur ou pour le pire. Les études dans le domaine ainsi que les réflexions de l’auteure présentées dans cette mini-revue de la littérature mettent en lumière le rôle de l’environnent dans le tissage des réseaux de neurones de l’enfant. Tout particulièrement, les recherches en neurosciences soulignent l’impact de l’environnement dans le développement de l’Être en construction.
... Cortical astrocytes contact neuronal cell bodies, dendritic spines, nodes of Ranvier, blood vessels, and synapses within their arborization domain (Serwanski et al., 2017;Cohen-Salmon et al., 2021). Cajal already observed a complex arborization using Golgi's method (Ramón y Cajal, 1909;García-Marín et al., 2007). Recent works relying on endogenous sparse labeling techniques (Hösli et al., 2022) and digital reconstructions (Zisis et al., 2021) have revealed the complex three-dimensional structural details of astroglia processes at the vascular but also at the synaptic interfaces (Torres-Ceja and Olsen, 2022). ...
In the mammalian brain, astrocytes form a heterogeneous population at the morphological, molecular, functional, intra-, and inter-region levels. In the past, a few types of astrocytes have been first described based on their morphology and, thereafter, according to limited key molecular markers. With the advent of bulk and single-cell transcriptomics, the diversity of astrocytes is now progressively deciphered and its extent better appreciated. However, the origin of this diversity remains unresolved, even though many recent studies unraveled the specificities of astroglial development at both population and individual cell levels, particularly in the cerebral cortex. Despite the lack of specific markers for each astrocyte subtype, a better understanding of the cellular and molecular events underlying cortical astrocyte diversity is nevertheless within our reach thanks to the development of intersectional lineage tracing, microdissection, spatial mapping, and single-cell transcriptomic tools. Here we present a brief overview describing recent findings on the genesis and maturation of astrocytes and their key regulators during cerebral cortex development. All these studies have considerably advanced our knowledge of cortical astrogliogenesis, which relies on a more complex mode of development than their neuronal counterparts, that undeniably impact astrocyte diversity in the cerebral cortex.
... Astrocytes, also known as astroglia, are the most abundant and diverse population of glial cells in the central nervous system (CNS). This term was popularized by Santiago Ramón y Cajal who also developed the gold sublimate-method which labeled GFAP (glial fibrillary acidic protein), with this technique it was demonstrated that astrocytes originated from radial glia (Ramóny Cajal, 1916;García-Marín et al., 2007). Astrocytes develop in the embryonic stage and in the late postnatal period (Miale and Sidman, 1961). ...
The roof of the fourth ventricle (4V) is located on the ventral part of the cerebellum, a region with abundant vascularization and cell heterogeneity that includes tanycyte-like cells that define a peculiar glial niche known as ventromedial cord. This cord is composed of a group of biciliated cells that run along the midline, contacting the ventricular lumen and the subventricular zone. Although the complex morphology of the glial cells composing the cord resembles to tanycytes, cells which are known for its proliferative capacity, scarce or non-proliferative activity has been evidenced in this area. The subventricular zone of the cerebellum includes astrocytes, oligodendrocytes, and neurons whose function has not been extensively studied. This review describes to some extent the phenotypic, morphological, and functional characteristics of the cells that integrate the roof of the 4V, primarily from rodent brains.
... Ramoìn y Cajal provided one of the founding nervous system descriptions and the first extensive neuron classification based on morphology, the principal criteria available at the time (Ramón Y Cajal, 1911). Early drawings by Virchow and Deiter described the cells known today as (macro)glia, which were grouped morphologically by Lenhossek, Andriezen, and Koelliker in a classification largely adopted and developed by Cajal (García-Marín et al., 2007). Neurons and glia are now recognized cell components of all bilaterian nervous systems, composing peripheral sensory structures and centralized ganglia. ...
Nervous system cells, the building blocks of circuits, have been studied with ever-progressing resolution, yet neural circuits appear still resistant to schemes of reductionist classification. Due to their sheer numbers, complexity and diversity, their systematic study requires concrete classifications that can serve reduced dimensionality, reproducibility, and information integration. Conventional hierarchical schemes transformed through the history of neuroscience by prioritizing criteria of morphology, (electro)physiological activity, molecular content, and circuit function, influenced by prevailing methodologies of the time. Since the molecular biology revolution and the recent advents in transcriptomics, molecular profiling gains ground toward the classification of neurons and glial cell types. Yet, transcriptomics entails technical challenges and more importantly uncovers unforeseen spatiotemporal heterogeneity, in complex and simpler nervous systems. Cells change states dynamically in space and time, in response to stimuli or throughout their developmental trajectory. Mapping cell type and state heterogeneity uncovers uncharted terrains in neurons and especially in glial cell biology, that remains understudied in many aspects. Examining neurons and glial cells from the perspectives of molecular neuroscience, physiology, development and evolution highlights the advantage of multifaceted classification schemes. Among the amalgam of models contributing to neuroscience research, Caenorhabditis elegans combines nervous system anatomy, lineage, connectivity and molecular content, all mapped at single-cell resolution, and can provide valuable insights for the workflow and challenges of the multimodal integration of cell type features. This review reflects on concepts and practices of neuron and glial cells classification and how research, in C. elegans and beyond, guides nervous system experimentation through integrated multidimensional schemes. It highlights underlying principles, emerging themes, and open frontiers in the study of nervous system development, regulatory logic and evolution. It proposes unified platforms to allow integrated annotation of large-scale datasets, gene-function studies, published or unpublished findings and community feedback. Neuroscience is moving fast toward interdisciplinary, high-throughput approaches for combined mapping of the morphology, physiology, connectivity, molecular function, and the integration of information in multifaceted schemes. A closer look in mapped neural circuits and understudied terrains offers insights for the best implementation of these approaches.
... Based upon his histological and morphological studies, Ramon y Cajal hypothesized that astrocytes control the switch from wakefulness to sleep (Garcia-Marin et al., 2007). Modern tools for astrocyte-specific manipulation have recently allowed researchers to probe neuron-astrocyte interactions in the regulation of sleep and wake in vivo. ...
Astrocytes are non-neuronal cells that regulate synapses, neuronal circuits, and behavior. Astrocytes ensheath neuronal synapses to form the tripartite synapse where astrocytes influence synapse formation, function, and plasticity. Beyond the synapse, recent research has revealed that astrocyte influences on the nervous system extend to the modulation of neuronal circuitry and behavior. Here we review recent findings on the active role of astrocytes in behavioral modulation with a focus on in vivo studies, primarily in mice. Using tools to acutely manipulate astrocytes, such as optogenetics or chemogenetics, studies reviewed here have demonstrated a causal role for astrocytes in sleep, memory, sensorimotor behaviors, feeding, fear, anxiety, and cognitive processes like attention and behavioral flexibility. Current tools and future directions for astrocyte-specific manipulation, including methods for probing astrocyte heterogeneity, are discussed. Understanding the contribution of astrocytes to neuronal circuit activity and organismal behavior will be critical toward understanding how nervous system function gives rise to behavior.
... From a very old point of view, the role of astrocytes is to create the perfect microenvironment for the neurons to fulfill their functions in the brain. Many features and roles attributed today to astrocytes, were already proposed by the Spanish neuroscientist Santiago Ramon y Cajal more than a century ago (De Carlos & Borrell, 2007;Figueres-Oñate et al., 2014;García-Marín et al., 2007). His scientific handwritten notebooks (1882) contain sketches of astrocytes displaying a large number of different morphologies with specific spatial arrangements ( Figure 1). ...
Since the early observations made by Santiago Ramon y Cajal more than a century ago till now, astrocytes have gradually gained protagonism as essential partners of neurons in building brain circuits that regulate complex behavior. In mammals, processes such as sleep–wake cycle, locomotor activity, cognition and memory consolidation, homeostatic and hedonic appetite and stress response (among others), are synchronized in 24-h rhythms by the circadian system. In such a way, physiology efficiently anticipates and adapts to daily recurring changes in the environment. The hypothalamic suprachiasmatic nucleus (SCN) is considered the central pacemaker, it has been traditionally described as a nucleus of around 10,000 neurons nearly all GABAergic able to be entrained by light and to convey time information through multiple neuronal and hormonal pathways. Only recently, this neuro-centered view was challenged by breakthrough discoveries implicating astrocytes as essential time-keepers. In the present review, we will describe the current view on the SCN circuit and discuss whether astrocytic functions described in other brain regions and state-of-the-art experimental approaches, could help explaining better those well- and not so well-known features of the central pacemaker.
... Astroglia are further subdivided into astrocytes, radial glia, Bergmann cells, M€ uller cells, pituicytes, and tanycytes (Table 1). While all these cells derive from the ectoderm and develop locally in the central nervous system (CNS), microglia colonize the developing brain and are of mesodermal cell origin (García-Marín, García-López, & Freire, 2007;Sierra et al., 2016). Microglia belong to the macrophage lineage and make up the largest part of the brain-resident immune cell population. ...
In the past, brain function and the onset and progression of neurological diseases have been studied in a neuron-centric manner. However, in recent years the focus of many neuroscientists has shifted to other cell types that promote neurodevelopment and contribute to the functionality of neuronal networks in health and disease. Particularly microglia and astrocytes have been implicated in actively contributing to and controlling neuronal development, neuroinflammation, and neurodegeneration. Here, we summarize the development of brain-resident macrophages and astrocytes and their core functions in the developing brain. We discuss their contribution and intercellular crosstalk during tissue homeostasis and pathophysiology. We argue that in-depth knowledge of non-neuronal cells in the brain could provide novel therapeutic targets to reverse or contain neurological diseases.
... Despite this discovery, glia received little attention among researchers until Santiago Ramón y Cajal's histological study in the late 19th century, when he developed specific staining methods for glial cells and described their morphology. Based on his studies, in 1909, he raised the question: "What is the function of glia ?" in 1909(Garcia-Marin et al., 2007. Since then, significant attention has been brought upon discovering glia's diverse physiological roles. ...
Astrocytes are the key component of the central nervous system (CNS), serving as pivotal regulators of neuronal synapse formation and maturation through their ability to dynamically and bidirectionally communicate with synapses throughout life. In the past 20 years, numerous astrocyte-derived molecules promoting synaptogenesis have been discovered. However, our understanding of the cell biological basis underlying intra-neuron processes and astrocyte-mediated synaptogenesis is still in its infancy. Here, we provide a comprehensive overview of the various ways astrocytes talk to neurons, and highlight astrocytes’ heterogeneity that allow them to displays regional-specific capabilities in boosting synaptogenesis. Finally, we conclude with promises and future directions on how organoids generated from human induced pluripotent stem cells (hiPSCs) effectively address the signaling pathways astrocytes employ in synaptic development.
... Many of the posited roles for astrocytes have since proven true. The history of the discovery of astrocytes is comprehensively reviewed elsewhere [9,10]. It was long believed that astrocytes were many times more numerous than neurons in the brain [11][12][13]. ...
Astrocytes are the primary homeostatic cells of the central nervous system, essential for normal neuronal development and function, metabolism and response to injury and inflammation. Here, we review postmortem studies examining changes in astrocytes in subjects diagnosed with the neuropsychiatric disorders schizophrenia (SCZ), major depressive disorder (MDD), and bipolar disorder (BPD). We discuss the astrocyte-related changes described in the brain in these disorders and the potential effects of psychotropic medication on these findings. Finally, we describe emerging tools that can be used to study the role of astrocytes in neuropsychiatric illness.
... Although sleep-wake cycle-dependent synaptic strength is very important for brain function, the precise mechanism(s) underlying the regulation of the cycle and how it affects brain functions remain unclear. In 1895, Cajal proposed a possible role of astrocytes in the sleep-wake cycle (García-Marín et al., 2007). During sleep, endfeet of astrocytes invade the synaptic cleft, and disconnect synapses as a circuit breaker. ...
Cortical microglia exhibit a ramified shape during sleep, while they have a hyper-ramified shape during wakefulness, which is characterized by their longer processes with increased branching points. The microglial molecular circadian clock regulates expressions of both cathepsin S (CatS) and P2Y12 receptors in the brain with a peak at zeitgeber time 14 (2 h after beginning of the dark phase). We postulated that these two microglia-specific molecules contribute to diurnal alterations of microglial shapes and neuronal activities in the cerebral cortex. During wakefulness, CatS secreted from cortical microglia may be involved in P2Y12 receptor-dependent process extension. Secreted CatS subsequently degrades the perineuronal nets, initiating the downscaling of both spine density and synaptic strength of cortical neurons toward the beginning of sleep. The downscaling of both spine density and synaptic strength of cortical neurons during sleep could improve signal-to-noise, which would benefit memory consolidation, or allow for new learning to occur during subsequent waking. Furthermore, disruption of CatS induces the sleep disturbance and impaired social interaction in mice. Moreover, the microglial clock system disruption may also play a role in the early pathogenesis of Alzheimer's disease. The reduced expression of BMAL1 in cortical microglia caused by oligomeric amyloid β may induce the increased presence of inflammatory phenotype through a reduction in RORα, which in turn reduced IκBα and enhanced NF-κB activation.
These observations suggest that the microglial clock system disruption contribute to pathogeneses of sleep disturbance, impaired social interaction and cognitive impairment. Therefore, the growing understanding of the microglial circadian molecular clock might aid in the development of novel pharmacological interventions against both neuropsychiatric and neurodegenerative disorders.
... [4][5][6] More than a century ago, Cajal proposed that astrocytes, a subtype of glial cells, modulate neural connectivity across the sleep/wake cycle. 7 Since that time, emerging data have suggested that astrocytes play a key role in the regulation of sleep. [8][9][10][11] However, whether and how astrocytes sense and discharge sleep need is enigmatic. ...
Sleep is under homeostatic control, whereby increasing wakefulness generates sleep need and triggers sleep drive. However, the molecular and cellular pathways by which sleep need is encoded are poorly understood. In addition, the mechanisms underlying both how and when sleep need is transformed to sleep drive are unknown. Here, using ex vivo and in vivo imaging, we show in Drosophila that astroglial Ca²⁺ signaling increases with sleep need. We demonstrate that this signaling is dependent on a specific L-type Ca²⁺ channel and is necessary for homeostatic sleep rebound. Thermogenetically increasing Ca²⁺ in astrocytes induces persistent sleep behavior, and we exploit this phenotype to conduct a genetic screen for genes required for the homeostatic regulation of sleep. From this large-scale screen, we identify TyrRII, a monoaminergic receptor required in astrocytes for sleep homeostasis. TyrRII levels rise following sleep deprivation in a Ca²⁺-dependent manner, promoting further increases in astrocytic Ca²⁺ and resulting in a positive-feedback loop. Moreover, our findings suggest that astrocytes then transmit this sleep need to a sleep drive circuit by upregulating and releasing the interleukin-1 analog Spätzle, which then acts on Toll receptors on R5 neurons. These findings define astroglial Ca²⁺ signaling mechanisms encoding sleep need and reveal dynamic properties of the sleep homeostatic control system.
... Une étude utilisant des souris Nestin-CreER montre également la présence de progéniteurs gliaux Nestin+, non rattachés à la surface piale et proliférant, au niveau de la zone sous ventriculaire après injections de tamoxifène à la fin de l'embryogenèse . La gliogenèse périnatale se produit donc à la fois au niveau de la zone ventriculaire où se trouvent les cellules de la glie radiaire et au niveau de la zone sous ventriculaire où se trouvent des progéniteurs intermédiaires (Levison et al., 1999) qui donnent naissance à des cellules migrant vers la substance blanche et le cortex cérébral et se différenciant en astrocytes ou oligodendrocytes Parnavelas, 1999;Kakita and Goldman, 1999;Marshall and Goldman, 2002 and Rakic, 2011)) en microscopie électronique (Smart and Leblond, 1961;Mori and Leblond, 1969;Ichikawa et al., 1983;García-Marín et al., 2007;Fan and Agid, 2018). ...
Les astrocytes représentent une des populations cellulaires les plus nombreuses du cerveau. Ces cellules gliales extrêmement ramifiées y jouent un rôle essentiel, notamment dans le cortex cérébral, où elles forment un réseau tridimensionnel continu tout en présentant une hétérogénéité importante au niveau morphologique, moléculaire et fonctionnel. Afin de déterminer comment le réseau astrocytaire est établi au cours du développement cortical murin, des analyses clonales ont été effectuées grâce à une stratégie de marquage multicolore permettant d’étudier simultanément la descendance de nombreux progéniteurs. Les résultats de ces travaux montrent que les clones d’astrocytes corticaux s’imbriquent avec leurs voisins et présentent une variabilité importante au niveau de leur composition en termes de nombre et sous-types cellulaires, leur organisation et leur dispersion spatiale. Le réseau astrocytaire se développe au cours d’une première phase dynamique de prolifération et de dispersion pendant la première semaine postnatale, suivie par une phase de maturation à l’échelle de la cellule avec augmentation du volume des astrocytes et de la complexité de leur arborisation. Ces travaux montrent par ailleurs la contribution non négligeable de progéniteurs postnataux au réseau astrocytaire, qui s’ajoute à celle des cellules souches neurales corticales embryonnaires. La grande variabilité du réseau astrocytaire à l’échelle clonale suggère que son développement repose sur des unités clonales plastiques composées de cellules dont l’organisation spatiale et les caractéristiques finales dépendent probablement de leurs interactions avec leur environnement neuronal via des acteurs moléculaires qui restent à caractériser.
... After the advancement of microscopy and dyeing techniques, Otto Dieters was the first to visualize astrocytes and provide key descriptions, while Camillo Golgi was the first to prove that astrocytes differ from neurons; however, he also believed that astrocytes and neurons could be transformed from one to the other. Lenhossék introduced the term "astrocyte" in 1895 [18] and developed astrocytespecific gold and mercury chloride-sublimation staining techniques that label glial fibrillary acidic protein (GFAP); it was this staining that allowed Cajal to discriminate astrocytes from radial glial cells [19]. GFAP and vimentin constitute intermediate filaments, and these components form the basic skeleton of an astrocyte [1]. ...
Astrocytes are the most populous glial cells in the central nervous system (CNS). They are essential to CNS physiology and play important roles in the maintenance of homeostasis, development of synaptic plasticity, and neuroprotection. Nevertheless, under the influence of certain factors, astrocytes may also exert detrimental effects through a process of reactive astrogliosis. Previous studies have shown that astrocytes have more than one type of polarization. Two types have been extensively researched. One is a damaging change that occurs under inflammation and has been termed A1 astrocyte, while the other is a restorative change that occurs under ischemic induction and was termed A2 astrocyte. Researchers are now increasingly paying attention to the role of astrocytes in spinal cord injury (SCI), degenerative diseases, chronic pain, neurological tumors, and other CNS disorders. In this review, we discuss (a) the characteristics of polarized astrocytes, (b) the relationship between astrocyte polarization and SCI, and (c) new implications of reactive astrogliosis for future SCI therapies.
... [4][5][6] More than a century ago, Cajal proposed that astrocytes, a subtype of glial cells, modulate neural connectivity across the sleep/wake cycle. 7 Since that time, emerging data have suggested that astrocytes play a key role in the regulation of sleep. [8][9][10][11] However, whether and how astrocytes sense and discharge sleep need is enigmatic. ...
Sleep is under homeostatic control, whereby increasing wakefulness generates sleep need and triggers sleep drive. However, the molecular and cellular pathways by which sleep need is encoded are poorly understood. In addition, the mechanisms underlying both how and when sleep need is transformed to sleep drive are unknown. Here, using ex vivo and in vivo imaging, we show in Drosophila that astroglial Ca ²⁺ signaling increases with sleep need. We demonstrate that this signaling is dependent on a specific L-type Ca ²⁺ channel and is required for homeostatic sleep rebound. Thermogenetically increasing Ca ²⁺ in astrocytes induces persistent sleep behavior, and we exploit this phenotype to conduct a genetic screen for genes required for the homeostatic regulation of sleep. From this large-scale screen, we identify TyrRII, a monoaminergic receptor required in astrocytes for sleep homeostasis. TyrRII levels rise following sleep deprivation in a Ca ²⁺ -dependent manner, promoting further increases in astrocytic Ca ²⁺ and resulting in a positive-feedback loop. These data suggest that TyrRII acts as a gate to enable the transformation of sleep need to sleep drive at the appropriate time. Moreover, our findings suggest that astrocytes then transmit this sleep need to the R5 sleep drive circuit, by upregulation and release of the interleukin-1 analog Spatzle. These findings define astroglial Ca2+ signaling mechanisms encoding sleep need and reveal dynamic properties of the sleep homeostatic control system.
... The idea that astrocytes can participate in sleep is quite ancient as already suggested by Ramon y Cajal in 1895 and reviewed in (114). In a theoretical article, Cajal had proposed that astrocytes can act as a physical barrier that would interrupt the synaptic transmission during sleep, by extending their processes into the synaptic cleft, thus preventing communication between the pre-and postsynaptic elements. ...
Sleep is a naturally occurring physiological state that is required to sustain physical and mental health. Traditionally viewed as strictly regulated by top-down control mechanisms, sleep is now known to also originate locally. Glial cells are emerging as important contributors to the regulation of sleep-wake cycles, locally and among dedicated neural circuits. A few pioneering studies revealed that astrocytes and microglia may influence sleep pressure, duration as well as intensity, but the precise involvement of these two glial cells in the regulation of sleep remains to be fully addressed, across contexts of health and disease. In this overview article, we will first summarize the literature pertaining to the role of astrocytes and microglia in the regulation of sleep under normal physiological conditions. Afterward, we will discuss the beneficial and deleterious consequences of glia-mediated neuroinflammation, whether it is acute, or chronic and associated with brain diseases, on the regulation of sleep. Sleep disturbances are a main comorbidity in neurodegenerative diseases, and in several brain diseases that include pain, epilepsy, and cancer. Identifying the relationships between glia-mediated neuroinflammation, sleep-wake rhythm disruption and brain diseases may have important implications for the treatment of several disorders.
... The idea that astrocytes can participate in sleep is quite ancient as already suggested by Ramon y Cajal in 1895 and reviewed in (114). In a theoretical article, Cajal had proposed that astrocytes can act as a physical barrier that would interrupt the synaptic transmission during sleep, by extending their processes into the synaptic cleft, thus preventing communication between the pre-and postsynaptic elements. ...
Sleep is a naturally occurring physiological state that is required to sustain physical and mental health. Traditionally viewed as strictly regulated by top-down control mechanisms, sleep is now known to also originate locally. Glial cells are emerging as important contributors to the regulation of sleep-wake cycles, locally and among dedicated neural circuits. A few pioneering studies revealed that astrocytes and microglia may influence sleep pressure, duration as well as intensity, but the precise involvement of these two glial cells in the regulation of sleep remains to be fully addressed, across contexts of health and disease. In this overview article, we will first summarize the literature pertaining to the role of astrocytes and microglia in the regulation of sleep under normal physiological conditions. Afterward, we will discuss the beneficial and deleterious consequences of glia-mediated neuroinflammation, whether it is acute, or chronic and associated with brain diseases, on the regulation of sleep. Sleep disturbances are a main comorbidity in neurodegenerative diseases, and in several brain diseases that include pain, epilepsy, and cancer. Identifying the relationships between glia-mediated neuroinflammation, sleep-wake rhythm disruption and brain diseases may have important implications for the treatment of several disorders. © 2020 American Physiological Society. Compr Physiol 10:687-712, 2020.
... 4,5 In addition, their unique anatomical location, which is interposed between neurons and cerebral microvessels and was depicted more than 100 years ago in a sketch by a legendary neuropathologist, Santiago Ramón y Cajal, has been attracting the attention of many neuroscientists. 6 In fact, neurons do not have any direct contact with microvessels despite their strict dependence on a continuous supply of glucose and oxygen from outside the brain through the cerebral blood flow. In contrast, 99% of the surfaces of brain capillaries are covered by astroglial foot processes (end-feet), indicating that all essential materials supplied from the cerebral circulation must interact with astroglia before reaching the neurons. ...
... 4,5 In addition, their unique anatomical location, which is interposed between neurons and cerebral microvessels and was depicted more than 100 years ago in a sketch by a legendary neuropathologist, Santiago Ramón y Cajal, has been attracting the attention of many neuroscientists. 6 In fact, neurons do not have any direct contact with microvessels despite their strict dependence on a continuous supply of glucose and oxygen from outside the brain through the cerebral blood flow. In contrast, 99% of the surfaces of brain capillaries are covered by astroglial foot processes (end-feet), indicating that all essential materials supplied from the cerebral circulation must interact with astroglia before reaching the neurons. ...
Astroglia or astrocytes, the most abundant cells in the brain, are interposed between neuronal synapses and microvasculature in the brain gray matter. They play a pivotal role in brain metabolism as well as in the regulation of cerebral blood flow, taking advantage of their unique anatomical location. In particular, the astroglial cellular metabolic compartment exerts supportive roles in dedicating neurons to the generation of action potentials and protects them against oxidative stress associated with their high energy consumption. An impairment of normal astroglial function, therefore, can lead to numerous neurological disorders including stroke, neurodegenerative diseases, and neuroimmunological diseases, in which metabolic derangements accelerate neuronal damage. The neurovascular unit (NVU), the major components of which include neurons, microvessels, and astroglia, is a conceptual framework that was originally used to better understand the pathophysiology of cerebral ischemia. At present, the NVU is a tool for understanding normal brain physiology as well as the pathophysiology of numerous neurological disorders. The metabolic responses of astroglia in the NVU can be either protective or deleterious. This review focuses on three major metabolic compartments: (i) glucose and lactate; (ii) fatty acid and ketone bodies; and (iii) D‐ and L‐serine. Both the beneficial and the detrimental roles of compartmentalization between neurons and astroglia will be discussed. A better understanding of the astroglial metabolic response in the NVU is expected to lead to the development of novel therapeutic strategies for diverse neurological diseases.
Glia comprise a heterogeneous group of cells involved in the structure and function of the central and peripheral nervous system. Glial cells are found from invertebrates to humans with morphological specializations related to the neural circuits in which they are embedded. Glial cells modulate neuronal functions, brain wiring and myelination, and information processing. For example, astrocytes send processes to the synaptic cleft, actively participate in the metabolism of neurotransmitters, and release gliotransmitters, whose multiple effects depend on the targeting cells. Human astrocytes are larger and more complex than their mice and rats counterparts. Astrocytes and microglia participate in the development and plasticity of neural circuits by modulating dendritic spines. Spines enhance neuronal connectivity, integrate most postsynaptic excitatory potentials, and balance the strength of each input. Not all central synapses are engulfed by astrocytic processes. When that relationship occurs, a different pattern for thin and large spines reflects an activity-dependent remodeling of motile astrocytic processes around presynaptic and postsynaptic elements. Microglia are equally relevant for synaptic processing, and both glial cells modulate the switch of neuroendocrine secretion and behavioral display needed for reproduction. In this chapter, we provide an overview of the structure, function, and plasticity of glial cells and relate them to synaptic maturation and modulation, also involving neurotrophic factors. Together, neurons and glia coordinate synaptic transmission in both normal and abnormal conditions. Neglected over decades, this exciting research field can unravel the complexity of species-specific neural cytoarchitecture as well as the dynamic region-specific functional interactions between diverse neurons and glial subtypes.
Glia and neurons are intimately associated throughout bilaterian nervous systems, and were early proposed to interact for patterning circuit assembly. The investigations of circuit formation progressed from early hypotheses of intermediate guideposts and a “glia blueprint”, to recent genetic and cell manipulations, and visualizations in vivo . An array of molecular factors are implicated in axon pathfinding but their number appears small relatively to circuit complexity. Comprehending this circuit complexity requires to identify unknown factors and dissect molecular topographies. Glia contribute to both aspects and certain studies provide molecular and functional insights into these contributions. Here, I survey glial roles in guiding axon navigation in vivo , emphasizing analogies, differences and open questions across major genetic models. I highlight studies pioneering the topic, and dissect recent findings that further advance our current molecular understanding. Circuits of the vertebrate forebrain, visual system and neural tube in zebrafish, mouse and chick, the Drosophila ventral cord and the C. elegans brain-like neuropil emerge as major contexts to study glial cell functions in axon navigation. I present astroglial cell types in these models, and their molecular and cellular interactions that drive axon guidance. I underline shared principles across models, conceptual or technical complications, and open questions that await investigation. Glia of the radial-astrocyte lineage, emerge as regulators of axon pathfinding, often employing common molecular factors across models. Yet this survey also highlights different involvements of glia in embryonic navigation or pioneer axon pathfinding, and unknowns in the molecular underpinnings of glial cell functions. Future cellular and molecular investigations should complete the comprehensive view of glial roles in circuit assembly.
Human gliogenesis remains poorly understood, and derivation of astrocytes from human pluripotent stem cells (hPSCs) is inefficient and cumbersome. Here, we report controlled glial differentiation from hPSCs that bypasses neurogenesis, which otherwise precedes astrogliogenesis during brain development and in vitro differentiation. hPSCs were first differentiated into radial glial cells (RGCs) resembling resident RGCs of the fetal telencephalon, and modulation of specific cell signaling pathways resulted in direct and stepwise induction of key astroglial markers (NFIA, NFIB, SOX9, CD44, S100B, glial fibrillary acidic protein [GFAP]). Transcriptomic and genome-wide epigenetic mapping and single-cell analysis confirmed RGC-to-astrocyte differentiation, obviating neurogenesis and the gliogenic switch. Detailed molecular and cellular characterization experiments uncovered new mechanisms and markers for human RGCs and astrocytes. In summary, establishment of a glia-exclusive neural lineage progression model serves as a unique serum-free platform of manufacturing large numbers of RGCs and astrocytes for neuroscience, disease modeling (e.g., Alexander disease), and regenerative medicine.
Our awareness of the number of synapse regulatory functions performed by astroglia is rapidly expanding, raising interesting questions regarding astrocyte heterogeneity and specialization across brain regions. Whether all astrocytes are poised to signal in a multitude of ways, or are instead tuned to surrounding synapses and how astroglial signaling is altered in psychiatric and cognitive disorders are fundamental questions for the field. In recent years, molecular and morphological characterization of astroglial types has broadened our ability to design studies to better analyze and manipulate specific functions of astroglia. Recent data emerging from these studies will be discussed in depth in this review. I also highlight remaining questions emerging from new techniques recently applied toward understanding the roles of astrocytes in synapse regulation in the adult brain.
Neurons and glia are the two main cell classes in the nervous systems of most animals. Although functionally distinct, neurons and glia are both characterized by multiple branching arbors stemming from the cell bodies. Glial processes are generally known to form smaller trees than neuronal dendrites. However, the full extent of morphological differences between neurons and glia in multiple species and brain regions has not yet been characterized, nor is it known whether these cells can be reliably distinguished based on geometric features alone. Here, we show that multiple supervised learning algorithms deployed on a large database of morphological reconstructions can systematically classify neuronal and glial arbors with nearly perfect accuracy and precision. Moreover, we report multiple morphometric properties, both size related and size independent, that differ substantially between these cell types. In particular, we newly identify an individual morphometric measurement, Average Branch Euclidean Length that can robustly separate neurons from glia across multiple animal models, a broad diversity of experimental conditions, and anatomical areas, with the notable exception of the cerebellum. We discuss the practical utility and physiological interpretation of this discovery.
Over the last 15 years, the field of neuroscience has evolved toward recognizing the critical role of astroglia in shaping neuronal synaptic activity and along with the pre- and postsynapse is now considered an equal partner in tripartite synaptic transmission and plasticity. The relative youth of this recognition and a corresponding deficit in reagents and technologies for quantifying and manipulating astroglia relative to neurons continues to hamper advances in understanding tripartite synaptic physiology. Nonetheless, substantial advances have been made and are reviewed herein. We review the role of astroglia in synaptic function and regulation of behavior with an eye on how tripartite synapses figure into brain pathologies underlying behavioral impairments in psychiatric disorders, both from the perspective of measures in postmortem human brains and more subtle influences on tripartite synaptic regulation of behavior in animal models of psychiatric symptoms. Our goal is to provide the reader a well-referenced state-of-the-art understanding of current knowledge and predict what we may discover with deeper investigation of tripartite synapses using reagents and technologies not yet available.
Initially, glial cells were believed to function as “glue or packaging” cells of the brain; however, the last two decades of research from basic and clinical scientists have duly recognized them as one of the most important cell types in the mammalian brain. It has been now well established that glial cells, and, in particular, astrocytes, play immensely important roles that enable neurons to function optimally. This chapter describes the historical aspects of research on astrocytes, their role during brain development and synaptogenesis and other physiological functions of the brain, and finally how they contribute to disease pathogenesis or CNS disorders.KeywordsAstrocytesBlood-brain barrierGlial cellsGFAP
Astrocytes are abundant glial cells in the central nervous system (CNS) that perform diverse functions in health and disease. Astrocyte dysfunction is found in numerous diseases, including multiple sclerosis, Alzheimer disease, Parkinson disease, Huntington disease and neuropsychiatric disorders. Astrocytes regulate glutamate and ion homeostasis, cholesterol and sphingolipid metabolism and respond to environmental factors, all of which have been implicated in neurological diseases. Astrocytes also exhibit significant heterogeneity, driven by developmental programmes and stimulus-specific cellular responses controlled by CNS location, cell–cell interactions and other mechanisms. In this Review, we highlight general mechanisms of astrocyte regulation and their potential as therapeutic targets, including drugs that alter astrocyte metabolism, and therapies that target transporters and receptors on astrocytes. Emerging ideas, such as engineered probiotics and glia-to-neuron conversion therapies, are also discussed. We further propose a concise nomenclature for astrocyte subsets that we use to highlight the roles of astrocytes and specific subsets in neurological diseases. In this Review, Quintana and colleagues discuss astrocytes, a type of glial cell that could be manipulated to treat neurological conditions. Potential astrocyte targets, and the progess made towards developing astrocyte-directed therapies, are highlighted, along with their potential pitfalls. They also propose a novel nomenclature for astrocyte subsets.
Le cerveau est richement vascularisé et sensible aux pathogènes et aux molécules circulant dans le sang. L’unité fonctionnelle appelée unité gliovasculaire composée des vaisseaux sanguins (cellules endothéliales et murales) et des pieds astrocytaires périvasculaires (PAPVs) maintient l’équilibre homéostatique entre le sang et le cerveau. Au cours de ma thèse, j’ai étudié le développement postnatal de l’unité gliovasculaire chez la souris en situation physiologique et pathologique. J’ai mis en évidence (1) une maturation des cellules endothéliales et de leur phénotype de barrière hémato encéphalique, (2) une maturation des cellules musculaires lisses vasculaires et de leur capacité contractile. Enfin, j’ai montré que cette dernière maturation coïncide avec à la formation et la maturation des PAPVs. Dans un modèle pathologique de leucoencéphalopathie mégalencéphalique à kystes sous-corticaux (MLC), j’ai montré que les maturations postnatales des cellules musculaires lisses vasculaires et des PAPVs sont altérées. J’ai également observé un défaut de couplage neurovasculaire et de drainage des fluides intraparenchymateux. Mes résultats suggèrent que la MLC a pour origine un défaut développemental de l’unité gliovasculaire.
Derivation of astrocytes from human pluripotent stem cells (hPSCs) is inefficient and cumbersome, impeding their use in biomedical research. Here, we developed a highly efficient chemically defined astrocyte differentiation strategy that overcomes current limitations. This approach largely bypasses neurogenesis, which otherwise precedes astrogliogenesis during brain development and in vitro experiments. hPSCs were first differentiated into radial glial cells (RGCs) exhibiting in vivo-like radial glia signatures. Activation of NOTCH and JAK/STAT pathways in bona fide RGCs resulted in direct astrogliogenesis confirmed by expression of various glial markers (NFIA, NFIB, SOX9, CD44, S100B, GFAP). Transcriptomic and genome-wide epigenetic analyses confirmed RGC-to-astrocyte differentiation and absence of neurogenesis. The morphological and functional identity of hPSC-derived astrocytes was confirmed by using an array of methods (e.g. electron microscopy, calcium imaging, co-culture with neurons, grafting into mouse brains). Lastly, the scalable protocol was adapted to a robotic platform and used to model Alexander disease. In conclusion, our findings uncover remarkable plasticity in neural lineage progression that can be exploited to manufacture large numbers of human hPSC-derived astrocytes for drug development and regenerative medicine.
La maladie d'Alzheimer (MA) est une maladie neurodégénérative qui affecte progressivement les fonctions cognitives et la mémoire. Le cerveau des personnes atteintes de la MA est caractérisé par le dépôt extracellulaire d'amyloïde-β (Aβ), un peptide qui s'agrège au sein de structures appelées "plaques séniles". Cependant, il a été reconnu que les formes solubles oligomériques d’Aβ (Aβo) sont les formes du peptide qui déclenchent la pathologie. Elles sont impliquées dans des dysfonctionnements synaptiques qui sont considérés comme l'un des premiers événements de la MA. Des études récentes suggèrent que les astrocytes pourraient jouer un rôle majeur dans les dysfonctionnements synaptiques, mais leur implication dans les premiers stades de la MA reste peu documentée. En utilisant l'imagerie calcique, nous avons montré que l'application brève d’Aβo sur des tranches aiguës de cerveau de souris induit une hyperexcitabilité calcique astrocytaire dans l'hippocampe. Cette hyperexcitabilité est indépendante de l'activité neuronale et se produit dans les microdomaines des prolongements astrocytaires impliqués dans la formation des synapses tripartites. Dans la même échelle de temps, nous avons observé une hyperactivité au sein des neurones voisins, en utilisant des enregistrements par patch-clamp en configuration cellule entière. Cette hyperactivité dépend de la signalisation calcique dans le réseau astrocytaire. De manière intéressante, l'inhibition du canal calcique TRPA1, exprimé dans les astrocytes, bloque l'effet d’Aβo et restaure l'activité des astrocytes et des neurones à un niveau basal. Par ailleurs, l'inhibition chronique de TRPA1 dans le modèle de souris APP/PS1-21 de la MA bloque les perturbations neuronales et astrocytaires précliniques et prévient les troubles de l'apprentissage. En somme, ce travail de thèse suggère un rôle essentiel pour l'hyperexcitabilité précoce astrocytaire dans la pathogenèse de la MA, et souligne que TRPA1 est une cible thérapeutique prometteuse avec un effet neuroprotecteur.
In the brain, mitochondrial metabolism has been largely associated with energy production, and its dysfunction is linked to neuronal cell loss. However, the functional role of mitochondria in glial cells has been poorly studied. Recent reports have demonstrated unequivocally that astrocytes do not require mitochondria to meet their bioenergetics demands. Then, the question remaining is, what is the functional role of mitochondria in astrocytes? In this work, we review current evidence demonstrating that mitochondrial central carbon metabolism in astrocytes regulates overall brain bioenergetics, neurotransmitter homeostasis and redox balance. Emphasis is placed in detailing carbon source utilization (glucose and fatty acids), anaplerotic inputs and cataplerotic outputs, as well as carbon shuttles to neurons, which highlight the metabolic specialization of astrocytic mitochondria and its relevance to brain function.
Axons in the adult mammalian central nervous system (CNS) fail to regenerate inside out due to intrinsic and extrinsic neuronal determinants. During CNS development, axon growth, synapse formation and function are tightly regulated processes allowing immature neurons to effectively grow an axon, navigate towards target areas, form synaptic contacts and become part of information processing networks that control behavior in adulthood. Not only immature neurons are able to precisely control the expression of a plethora of genes necessary for axon extension and pathfinding, synapse formation and function, but also non-neuronal cells such as astrocytes and microglia actively participate in sculpting the nervous system through refinement, consolidation and elimination of synaptic contacts. Recent evidence indicates that a balancing act between axon regeneration and synaptic function may be crucial for rebuilding functional neuronal circuits after CNS trauma and disease in adulthood. Here we review the role of classical and new intrinsic and extrinsic neuronal determinants in the context of CNS development, injury and disease. Moreover, we discuss strategies targeting neuronal and non-neuronal cell behaviors, either alone or in combination, to promote axon regeneration and neuronal circuit formation in adulthood.
Inflammasome activation is a developing process differently from classical inflammatory responses. It has been known that inflammasome activation occurs by both microbial agents and the effects of some metabolic stress products (Adenosine triphosphate (ATP) and external agents like asbestos, Inflammasome activation is a process that is initiated by Interleukin-1beta (IL-1beta), Interleukin-18 (IL-18) cytokines production from activated cells and terminated by caspase-1 activation. In this process, inflammasome activation may cause cell death via pyroptotic pathway. Therefore, inflammasome activation is needed to be inhibited.
Dimethyl fumarate (DMF) is anti-inflammatory molecule that has pro-metabolic effects. DMF, known as also methyl ester of fumaric acid, has been shown that it provided improvement for multiple sclerosis in clinical parameters and decreased frequency of attacks. And also, it has been established that DMF inhibited inflammasome activation in THP-1 monocyte cells which came from same ancestor of microglial cells.
In our work, determining effects of DMF on inflammasome activation in N9 microglial cells has been aimed. In this regard, NOD-like receptor family, pyrin domain containing protein 3 (NLRP3) inflammasome activation in N9 microglial cells was induced with lipopolysaccharide (LPS) and ATP model. Firstly, the effective and non-toxic dose of DMF was determined. After that, inflammasome activation parameters were analyzed with real-time PCR, ELISA and Western-blotting. Additionally, mitochondrial Reactive Oxygen Species (mtROS) production and mitochondrial membrane integrity were analyzed via flow cytometry, immunofluorescence (IF) and fluorometric analyses. Consequently, DMF significantly suppressed LPS + ATP induced NLRP3 inflammasome, decreased Caspase-1 activity, secretion of IL-1beta and IL-18 cytokines, and reduced the rate of pyroptotic cell death resulting from inflammasome activation. Furthermore, the level of mitochondrial ROS was declined and mitochondrial membrane integrity was restored. This study is the first to demonstrate that DMF is effective to suppress the NLRP3 inflammasome activation in microglial cells.
Astrocytes play numerous vital roles in the central nervous system. Accordingly, it is of merit to identify structural and functional properties of astrocytes in both health and disease. The majority of studies examining the morphology of astrocytes have employed immunoassays for markers such as glial fibrillary acidic protein, which are insufficient to encapsulate the considerable structural complexity of these cells. Herein, we describe a method utilizing a commercially available and validated, genetically encoded membrane‐associated fluorescent marker of astrocytes, AAV5‐GfaABC1D‐Lck‐GFP. This tool and approach allow for visualization of a single isolated astrocyte in its entirety, including fine peripheral processes. Astrocytes are imaged using confocal microscopy and reconstructed in three dimensions to obtain detailed morphometric data. We further provide an immunohistochemistry procedure to assess colocalization of isolated astrocytes with synaptic markers throughout the z‐plane. This technique, which can be utilized via a standard laboratory confocal microscope and Imaris software, allows for detailed analysis of the morphology and synaptic colocalization of astrocytes in fixed tissue. © 2020 by John Wiley & Sons, Inc.
Basic Protocol 1 : Microinjection of AAV5‐GfaABC1D‐Lck‐GFP into the nucleus accumbens of rats
Basic Protocol 2 : Tissue processing and immunohistochemistry for post‐synaptic density‐95
Basic Protocol 3 : Single‐cell image acquisition
Basic Protocol 4 : Three‐dimensional reconstruction of single cells
Basic Protocol 5 : Three‐dimensional colocalization analysis
Neurotransmitter released from neurons is known to signal to neighbouring neurons and glia. Here we demonstrate an additional signalling pathway in which glutamate is released from astrocytes and causes an NMDA (N-methyl-D-aspartate) receptor-mediated increase in neuronal calcium. Internal calcium was elevated and glutamate release stimulated by application of the neuroligand bradykinin to cultured astrocytes. Elevation of astrocyte internal calcium was also sufficient to induce glutamate release. To determine whether this released glutamate signals to neurons, we studied astrocyte-neuron co-cultures. Bradykinin significantly increased calcium levels in neurons co-cultured with astrocytes, but not in solitary neurons. The glutamate receptor antagonists D-2-amino-5-phosphonopentanoic acid and D-glutamylglycine prevented bradykinin-induced neuronal calcium elevation. When single astrocytes were directly stimulated to increase internal calcium and release glutamate, calcium levels of adjacent neurons were increased; this increase could be blocked by D-glutamylglycine. Thus, astrocytes regulate neuronal calcium levels through the calcium-dependent release of glutamate.
The spatial-temporal characteristics of intracellular calcium ([Ca2+]i) changes elicited in neurons and astrocytes by various types of stimuli were investigated by means of confocal fluorescent microscopy in acute rat brain slices loaded with the Ca2+ indicator indo-1. Neurons and astrocytes from the visual cortex and CA1 hippocampal region were identified in situ on the basis of their morphological, electrophysiological, and pharmacological features. We show here that stimulation of neuronal afferents triggered periodic [Ca2+]i oscillations in astrocytes. The frequency of these oscillations was under a dynamic control by neuronal activity as it changed according to the pattern of stimulation. After repetitive episodes of neuronal stimulation as well as repetitive stimulation with a metabotropic glutamate receptor agonist, astrocytes displayed a long-lasting increase in [Ca2+]i oscillation frequency. Oscillating astrocytes were accompanied by repetitive [Ca2+]i elevations in adjacent neurons, most likely because of the release of glutamate via a tetanus toxin-resistant process. These results reveal that [Ca2+]i oscillations in astrocytes represent a highly plastic signaling system that underlies the reciprocal communication between neurons and astrocytes.
Radial glial cells are now recognized as a transient population that serves as scaffolding for neuronal migration. The recognition of the existence and role of radial glia has not been smooth, and here we provide a brief historical overview on the pioneering studies on this subject. The histologists and embryologists Albert Kölliker and Wilhelm His performed seminal investigations on cortical morphogenesis in the last decades of the 19th century. However, the introduction of the silver impregnation Golgi technique, and its diffusion in the late 1880s, played a crucial role in the detection of radial glial processes. The radial arrangement of fibers emerging from the neuroepithelium lining the central canal was initially detected in the embryonic spinal cord by Camillo Golgi himself. The first Golgi impregnation of the cerebral cortex of mammalian fetuses was performed by Giuseppe Magini, who detected radial fibers extending from the ventricular neuroepithelium, and observed cells intercalated along these processes. Radial fibers, regarded as epithelial or ependymal processes, were then observed in the developing spinal cord and cerebral cortex by several investigators. Santiago Ramón y Cajal was the first to suggest that radial fibers were modified astrocytic processes functioning as a support during cortical histogenesis. Cajal acknowledged Magini's findings, but he criticized Magini's observations on the existence of neurons along radial fibers. With the advent of electron microscopy, the existence of radially arranged glial processes along which young neurons migrate was finally ascertained in the early 1970s by Pasko Rakic, thus opening a new era in the cellular and molecular biology of radial glia.
Analysis of excitatory synaptic transmission in the rat hypothalamic supraoptic nucleus revealed that glutamate clearance and, as a consequence, glutamate concentration and diffusion in the extracellular space, is associated with the degree of astrocytic coverage of its neurons. Reduction in glutamate clearance, whether induced pharmacologically or associated with a relative decrease of glial coverage in the vicinity of synapses, affected transmitter release through modulation of presynaptic metabotropic glutamate receptors. Astrocytic wrapping of neurons, therefore, contributes to the regulation of synaptic efficacy in the central nervous system.
Neurogenesis in the dentate gyrus of the hippocampus persists throughout life in many vertebrates, including humans. The progenitors of these new neurons reside in the subgranular layer (SGL) of the dentate gyrus. Although stem cells that can self-renew and generate new neurons and glia have been cultured from the adult mammalian hippocampus, the in vivo primary precursors for the formation of new neurons have not been identified. Here we show that SGL cells, which express glial fibrillary acidic protein and have the characteristics of astrocytes, divide and generate new neurons under normal conditions or after the chemical removal of actively dividing cells. We also describe a population of small electron-dense SGL cells, which we call type D cells and are derived from the astrocytes and probably function as a transient precursor in the formation of new neurons. These results reveal the origins of new neurons in the adult hippocampus.
Magnocellular neurons located in the supraoptic nucleus send their principal axons to terminate in the neurohypophysis, where they release vasopressin and oxytocin into the blood circulation. This magnocellular hypothalamo-neurohypophysial system is known to undergo dramatic activity-dependent structural plasticity during chronic physiological stimulation, such as dehydration and lactation. This structural plasticity is accompanied not only by synaptic remodeling, increased direct neuronal membrane apposition, and dendritic bundling in the supraoptic nucleus, but also organization of neurovascular contacts in the neurohypophysis. The adjacent glial cells actively participate in these plastic changes in addition to magnocellular neurons themselves. Many molecules that are possibly concerned with dynamic structural remodeling are highly expressed in the hypothalamo-neurohypophysial system, although they are generally at low expression levels in other regions of adult brains. Interestingly, some of them are highly expressed only in embryonic brains. On the basis of function, these molecules are classified mainly into two categories. Cytoskeletal proteins, such as tubulin, microtubule-associated proteins, and intermediate filament proteins, are responsible for changing both glial and neuronal morphology and location. Cell adhesion molecules, belonging to immunoglobulin superfamily proteins and extracellular matrix glycoproteins, also participate in neuronal-glial, neuronal-neuronal, and glial-glial recognition and guidance. Thus, the hypothalamo-neurohypophysial system is an interesting model for elucidating physiological significance and molecular mechanisms of activity-dependent structural plasticity in adult brains.
Since the time of the initial studies of the nervous system, neurons were recognized as the cellular elements responsible for the information processing of the nervous system, while glial cells were considered as playing simple supportive roles to neurons. The fundamental attribute of neurons is their cellular electrical excitability, which is based on the expression of a plethora of ligand- and voltage-gated membrane channels that give rise to prominent membrane currents and membrane potential variations, which represent the biophysical substrate underlying the integration and transfer of information at the cellular level in the Central Nervous System (CNS). By contrast, glial cells are not electrically excitable. Although they are able to express some of the ion channels that are expressed by neurons, the level of expression of some key channels is not sufficiently high to support the generation of active electrical behaviors in response to different stimuli. Nevertheless, glial cells display a form of excitability that is based on variations of the Ca2+ concentration in the cytosol rather than electrical changes in the membrane potential. © Springer Science+Business Media, LLC 2009. All rights reserved.
This chapter provides a historical perspective that highlights the early period of glial research. Topics covered include Virchow's invention of the term neuroglia in 1856, Heinrich Müller' picture of a glial cell, stellate cells in white and gray matter, and Camillo Golgi's description of cells with characteristic features of astrocytes and oligodendrocytes.
“The ultimate aim of these investigations on plasticity would be to correlate the observed structural and functional changes and to understand the way these changes are brought about by use and disuse. Inevitably, such a programme involves questions relating to the control of the manufacture of transmitter substance and to its availability for release by the activated synapses. It would seem more profitable that use gives increased function by enhancing the manufacture and availability of the transmitter substance, although enlargement of synaptic knobs and even the sprouting of new knobs are alternative devices for securing an increased synaptic action.” [Eccles (1961)Brain Mechanisms and Learning.]
A monoclonal antibody to vimentin (RBA1) and a polyclonal antiserum to glial fibrillary acidic protein (GFAP) were used in double labeling experiments to examine astrocyte intermediate filaments in development and wounding. RBA1 bound to radial glia in newborn rat parietal cortex that are predominantly anti-GFAP-negative. The RBA1-positive radial fibers disappeared by postnatal day 20 with the greatest rate of disappearance occurring between day 8 and day 15. Between birth and day 20, the anti-GFAP staining increased to the adult pattern in mature shaped astrocytes. Some overlay was observed between the binding patterns of the two antibodies.Stab wounds to cortical areas were made at a developmental time when there were normally no RBA1-positive astrocytes. RBA1-positivity was present in some astrocytes but only at the edges of the wounds. The distribution patterns of RBA1-positive cells led to hypotheses concerning the possible function of vimentin in astrocytes and its regulation during development and wounding.
Golgi and electronmicroscopic methods were used to define the shapes and intercellular relationships of cells migrating from their sites of origin near the ventricular surface across the intermediate zone to the superficial neocortical layers of the parietooccipital region in the brains of 75- to 97-day monkey fetuses. After mitotic division in either ventricular or subventricular zones, the cells enter the intermediate zone and assume an elongated bipolar form oriented toward the cortical plate. The leading processes, 50 to 70 μ long, are irregular cytoplasmic cylinders containing prominent Golgi apparatus, mitochondria, microtubules, ribosomal rosettes, immature endoplasmic reticulum and occasional centrioles. They usually terminate in several attenuated expansions, the longest one oriented toward the cortical plate. The trailing processes are more slender, relatively uniform in caliber and display few organelles.
Layoutgetreues Digitalisat der Ausg.: Berlin : Hirschwald, 1858 Zentralbibliothek Sign.: XId C 535 rz
Golgi techniques, immunofluorescence for glial fibrillary acidic (GFA) protein, and electron microscopy (EM) were used to determine the nature of radial glia in the cerebrum of human fetuses ranging from 7 to 20 weeks of ovulation age. Successful Golgi impregnation of radial fibers was achieved in fetuses 12 weeks of age and older. These fibers spanned the entire thickness of the hemisphere. At the pial surface many of them branched and terminated in pyramidal end feet expansions. Indirect immunofluorescent preparations utilizing antiserum to GFA protein, a protein specific for astrocytes, demonstrated numerous radially oriented nearly parallel fluorescent fibres between the ventricular zone and pia mater. GFA protein-positive fibers were demonstrated in all fetal specimens examined with this technique (10 weeks of age and older). Along the outer border of the marginal zone they formed a horizontal GFA protein-containing subpial membrane. By EM there were numerous linear electron lucent astrocytic processes containing 8-9 nm filaments and occasional glycogen granules at all levels of the cerebrum. They were interspersed among smaller and darker neuronal processes containing 20-25 nm neurotubules, and were demonstrable at all fetal ages between 7 and 18 weeks. They formed pericapillary investments and subpial terminal expansions closely abutting basal lamina of pia mater in every specimen examined. On the basis of these combined analyses, we conclude that radial glial fibers in early human fetal cerebrum represent processes of immature astrocytes. Although subsequently undergoing further maturation, radial glia already possess fundamental immunocytochemical and morphological characteristics indicative of astrocytic differentiation. A significant implication of our findings is that the development of astrocytes in the human fetal brain occurs much earlier than formerly believed.
The finding that astrocytes possess glutamate-sensitive ion channels hinted at a previously unrecognized signaling role for
these cells. Now it is reported that cultured hippocampal astrocytes can respond to glutamate with a prompt and oscillatory
elevation of cytoplasmic free calcium, visible through use of the fluorescent calcium indicator fluo-3. Two types of glutamate
receptor--one preferring quisqualate and releasing calcium from intracellular stores and the other preferring kainate and
promoting surface-membrane calcium influx--appear to be involved. Moreover, glutamate-induced increases in cytoplasmic free
calcium frequently propagate as waves within the cytoplasm of individual astrocytes and between adjacent astrocytes in confluent
cultures. These propagating waves of calcium suggest that networks of astrocytes may constitute a long-range signaling system
within the brain.
Adult neurogenesis in birds offers unique opportunities to study basic questions addressing the birth, migration and differentiation of neurons. Neurons in adult canaries originate from discrete proliferative regions on the walls of the lateral ventricles. They migrate away from their site of birth, initially at high rates, along the processes of radical cells. The rates of dispersal diminish as the young neurons invade regions devoid of radial fibers, probably under the guidance of other cues. The discrete sites of birth in the ventricular zone generate neurons that end up differentiating throughout the telencephalon. New neurons may become interneurons or projection neurons; the latter connect two song control nuclei between neostriatum and archistriatum. Radial cells, that in mammals disappear as neurogenesis comes to an end, persist in the adult avian brain. The presence of radial cells may be key to adult neurogenesis. Not only do they serve as guides for initial dispersal, they also divide and may be the progenitors of new neurons.
The evolution of concepts concerning the identity and the functions of neuroglia is traced. Some of the main ideas in the works of Virchow, Deiters, Golgi, Lenhossék, Lugaro, Ramón y Cajal, del Río-Hortega, Achúcarro, Penfield, and others are highlighted.
A general rule in the developing central nervous system is that cells are generated in sites different from those in which they will later reside. The intervening migrations, particularly in the human nervous system, form the subject of this review. The basic columnar organization in the early stages of development favors radial migration of cells. During later stages in primates, when young neurons migrate to the distant cerebral cortex, they follow radial glial guides across the widening intermediate zone as they pass from the juxtaventricular site of genesis to the cortical plate. Somas of later-generated cells take positions external to somas of their predecessors. The final position along the radial vector may be influenced by afferent axons. Cell relationships in the developing cerebellar cortex are essentially similar, though the key migration of granule cell neurons is in the reverse direction, from the external surface inward past Purkinje dendrites and somas. Bergmann glial fibers provide the radial guidance in this instance. The degree of dependence of developing neurons upon other cells and cell processes in their immediate environment has been clarified by study of mutant mice in which cerebral or cerebellar cortices are malformed. Other special migrations in the fetal human brain are reviewed, particularly the passage of neurons from the rhombic lip through the transient corpus pontobulbare to mainly the inferior olives and pontine gray nuclei, and from the ganglionic eminence of the cerebrum through the corpus gangliothalamicum into the pulvinar region of the thalamus. It was suggested that the special relationships involved in these various migrations are probably mediated by cell surface properties, and that such surface properties will come to be defined through analysis of reaggregation tissue cultures, experimental and natural chimeras, and by immunological definition of antigens on CNS cells at different stages of development.
The gold chloride sublimate method of Ramón y Cajal was used to stain astrocytes in the corpus callosum of 60–80 g male rats. The opacity of the metal-stained astrocytes to the electron beam allowed their study in the electron microscope. It was thus found that metal deposits are absent from the nucleus but are present over two cytoplasmic structures: bundles of filaments and dense bodies.
Once the features of astrocytes had been observed in metal stained preparations, it became possible to identify them in sections stained for electron microscopy by the routine uranyl-lead sequence. The presence of filament bundles in the cytoplasm was the most useful diagnostic feature. Dense bodies also helped. The nucleus was relatively large and light with a very distinct, often irregular nuclear envelope. The rather light cytoplasm contained glycogen granules, with few ribosomes and cisterns of endoplasmic reticulusm. However, a few otherwise typical astrocytes had fairly dark nucleus and cytoplasm.
After gaining experience with the electron microscope, it was possible to identify most astrocytes in the light microscope by their nuclear features. Counts of the frequency of astrocytes using either the light or electron microscope revealed that they make up about one quarter of the glial population of the corpus callosum.
The main properties of astrocytes seemed to be: (1) plasticity of the cell surface, which infiltrates all available spaces around it; (2) apparent rigidity of the bundles of filaments present in the processes, where they may play a supporting role; and (3) ability of astrocytes to undergo division, as demonstrated both by the uptake of 3H-thymidine into 1.7% of these cells and by the presence of mitotic figures.
The prenatal developmental histories of layer I, fibrous (white matter), and protoplasmic (gray matter) astrocytes have been studied in the human neocortex by the rapid Golgi method. The developmental route followed by each of these astrocytes is a distinct process which evolves from a specific precursor, occurs at a different time, and is linked to a specific event. The differentiation of layer I astrocytes is linked to the neocortex external glial limiting membrane (EGLM), that of fibrous astrocytes to the early white matter vascularization and maturation, and that of protoplasmic astrocytes to the late gray matter ascending vascularization and maturation. At the start of development, three glial precursors are established in the neocortex: 1) original radial neuroectodermal cells with nuclei above the primordial plexiform layer (PPL) by losing their ependymal and retaining their pial attachments become early astrocytes of layer I and EGLM components; 2) neuroectodermal cells with nuclei below the PPL that retain their pial and ependymal attachments become type I radial glial cells which are committed to the guidance of neurons and the early EGLM maintenance; and, 3) neuroectodermal cells that lose their pial but retain their ependymal attachment are transformed into type II radial glial precursors. By progressively losing their ependymal attachment, type II radial glia precursors become freely migrating cells, establish vascular contacts, and differentiate into fibrous astrocytes (and into oligodendrocytes?) throughout the subplate, developing white matter, and paraventricular regions. After the formation of the gray matter, additional layer I astrocytes are needed for the EGLM late prenatal and postnatal maintenance because type I radial glia cells start to regress and to reabsorb their EGLM endfeet. A late ependyma-to-pia migration of glial precursors progressively repopulates layer I with additional astrocytes and establishes the ephemeral subpial granular layer (SGL) of Ranke. From the 15th week of gestation to the time of birth, late astrocytes of layer I lose their EGLM attachments, migrate freely into the maturing gray matter, establish vascular contacts, and differentiate into protoplasmic astrocytes. The protoplasmic astrocytes of the gray matter evolve from transformation of layer I astrocytes rather than from radial glia cells as is generally believed.
It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels.
Estradiol induces coordinated modifications in the extension of glial and neuronal processes in the arcuate nucleus of the hypothalamus of adult female rats. This hormonal effect results in natural fluctuations in the ensheathing of arcuate neurons by glial processes and these glial changes are linked to a remodelling of inhibitory GABAergic synapses during the estrous cycle. Hormonally induced glial and synaptic changes appear to be dependent on specific recognition or adhesion molecules on the neuronal and/or glial membranes.
Although previous studies have revealed that the prenatal rat ventricular zone contains separate progenitor cells for neurons, astrocytes, and oligodendrocytes during the development of the cerebral cortex as early as the beginning of neurogenesis (Luskin et at., 1993; Grove et al., 1993), it is still unclear whether there are bipotential progenitor cells in the neonatal telencephalic subventricular zone which give rise to both astrocytes and oligodendrocytes during the peak of gliogenesis. To investigate this possibility, discrete groups of clonally related cells, generated by infecting progenitor cells of the neonatal subventricular zone with a retroviral lineage tracer, were analyzed ultrastructurally.
An intracerebral injection of retrovirus encoding the reporter gene E. coli ß-galactosidase (lacZ) was made into the subventricular zone of newborn rats. Two weeks later their brains were perfused, sectioned, and histochemically reacted with X-Gal to identify at the light microscopic level clones of lacZ-positive cells. The sections were processed for electron microscopy to enable the identity of clonally related cells to be assessed at the ultrastructural level.
All of the clones analyzed contained cells of the same phenotype and could be divided into four distinct types: immature cell clones situated in the subependymal zone surrounding the lateral ventricle, oligodendrocytes clones, and white or gray matter astrocyte clones. Not all of the cells in every clone displayed ultrastructural features of a mature cell. Rather, in some glial clones the lacZ-positive cells appeared to be at different stages of differentiation. However, we never encountered clones which contained both macroglial subtypes or clones containing neurons. Although the existence of bipotential progenitor cells cannot be completely dismissed, our results indicate the absence of progenitor cells in vivo in the neonatal subventricular zone which divide and generate astrocytes and oligodendrocytes.
The influence of gonadal steroids on the ultrastructure of glial cells and on the immunoreactivity for the specific astrocytic marker glial fibrillary acidic protein (GFAP) has been assessed in the neuroendocrine hypothalamus. The following parameters were analyzed in the arcuate nucleus of adult female rats: the number and the surface density of cells immunoreactive for GFAP, the number of glial profiles showing bundles of glial filaments, the size of the bundles of glial filaments, and the proportion of neuronal perikaryal membrane apposed by glial processes. These parameters were studied during the different phases of the estrous cycle, after ovariectomy, and after the administration of estradiol or progesterone to ovariectomized rats. No significant differences were detected in the number of GFAP-immunoreactive cells among the different experimental groups. The surface density of GFAP-immunoreactive material, the number of glial profiles in the neuropil, and the proportion of neuronal perikaryal membrane covered by glia were increased in the afternoon of proestrus and in the morning of estrus compared with other phases of the estrous cycle or to ovariectomized rats and showed a rapid (5 h) and reversible increase in ovariectomized rats injected with 17 beta estradiol, with a maximal effect by 24 h after the administration of the hormone. In contrast, the size of the bundles of glial filaments was decreased in the afternoon of proestrus, in the morning of estrus, and by the administration of estradiol to ovariectomized rats. The parameters studied were not affected by the administration of progesterone. However, progesterone (300 micrograms/rat) blocked the effects of 17 beta estradiol (1, 10, and 300 micrograms). The results suggest that glial cells may be actively involved in the modulation of neuroendocrine events by the hypothalamus.
The developmental fates of subventricular zone (SVZ) cells of the postnatal rat forebrain were determined by retroviral-mediated gene transfer and immunolabeling for glial antigens. A beta-galactosidase-containing retrovirus injected stereotactically into the SVZ infected small, immature cells. By 28 days post-injection labeled cells had appeared in both gray and white matter of the ipsilateral hemisphere. White matter contained labeled oligodendrocytes, but few astrocytes, while neocortex and striatum contained both glial types, often appearing in tightly knit clusters. An analysis after simultaneously injecting alkaline phosphatase- and beta-galactosidase-containing retroviruses showed that cells in each cortical cluster were related. Most clusters contained a single cell type, but approximately 15% contained both astrocytes and oligodendrocytes. These observations strongly suggest that a single SVZ cell can differentiate into both glial types.
Physiological activation of the magnocellular hypothalamo-neurohypophysial system induces a coordinated astrocytic withdrawal from between the magnocellular somata and the parallel-projecting dendrites of the supraoptic nucleus. Neural lobe astrocytes release engulfed axons and retract from their usual positions along the basal lamina. Occurring on a minutes-to-hours time scale, these changes are accompanied by increased direct apposition of both somatic and dendritic membrane, the formation of dendritic bundles, the appearance of novel multiple synapses in both the somatic and dendritic zones, and increased neural occupation of the perivascular basal lamina. Reversal, albeit with varying time courses, is achieved by removing the activating stimuli. Additionally, activation results in interneuronal coupling increases that are capable of being modulated synaptically via second messenger-dependent mechanisms. These changes appear to play important roles in control and coordination of oxytocin and vasopressin release during such conditions as lactation and dehydration.
Despite the increasing evidence for a prominent role played by the perivascular endfeet of astrocytes in the functional metabolic coupling between astrocytes and neurons, a clear picture of their spatial organization is still lacking. To examine the three-dimensional structure of the astrocyte endfeet and their relationships with the endothelial cells, coronal rat brain sections immunolabeled for the two astroglial markers [glial fibrillary acidic protein (GFAP)/S-100β] and the endothelial glucose transporter (GLUT1) were analyzed under the confocal microscope. Double immunolabeling of GFAP and S-100β showed numerous well-defined astrocytes sending one or more endfeet to the vasculature. Examination of GFAP immunolabeling at higher magnification showed that these endfeet consist of well-defined rosette-like structures lying on the vessel wall.
Double immunostaining of GFAP and GLUT1 showed that the endothelial cells were the main targets of these repeated geometrical units formed by the astrocyte endfeet. When three-dimensional images were reconstructed, obvious privileged anatomical relationships were observed between endfeet and individual endothelial cells. These anatomical data provide strong support for the involvement of astrocytes in cerebral metabolic coupling. The finger-like appearance of astrocyte endfeet could allow direct metabolic exchanges between intracerebral vessels and non-glial elements such as nerve terminals. GLIA 23:1–10, 1998.
According to the classical view of the nervous system, the numerically superior glial cells have inferior roles in that they provide an ideal environment for neuronal-cell function. However, there is a wave of new information suggesting that glia are intimately involved in the active control of neuronal activity and synaptic neurotransmission. Recent evidence shows that glia respond to neuronal activity with an elevation of their internal Ca2+ concentration, which triggers the release of chemical transmitters from glia themselves and, in turn, causes feedback regulation of neuronal activity and synaptic strength. In view of these new insights, this article suggests that perisynaptic Schwann cells and synaptically associated astrocytes should be viewed as integral modulatory elements of tripartite synapses.
Neural stem cells reside in the subventricular zone (SVZ) of the adult mammalian brain. This germinal region, which continually generates new neurons destined for the olfactory bulb, is composed of four cell types: migrating neuroblasts, immature precursors, astrocytes, and ependymal cells. Here we show that SVZ astrocytes, and not ependymal cells, remain labeled with proliferation markers after long survivals in adult mice. After elimination of immature precursors and neuroblasts by an antimitotic treatment, SVZ astrocytes divide to generate immature precursors and neuroblasts. Furthermore, in untreated mice, SVZ astrocytes specifically infected with a retrovirus give rise to new neurons in the olfactory bulb. Finally, we show that SVZ astrocytes give rise to cells that grow into multipotent neurospheres in vitro. We conclude that SVZ astrocytes act as neural stem cells in both the normal and regenerating brain.
The role of radial glial cells as guides for migrating neurons is well established, whereas their role as precursor cells is less understood. Here we examined the composition of radial glial cells and their proliferation in the mouse telencephalon during development. We found that almost all radial glial cells proliferate throughout neurogenesis. They consist of three distinct subsets identified by immunostaining for the antigens RC2, the astrocyte-specific glutamate transporter (GLAST), and the brain-lipid-binding protein (BLBP). In addition, RC2, GLAST, and BLBP antisera label precursor cells with different morphologies and thereby cover almost the entire progenitor pool in the developing cerebral cortex. The subsets identified by differential expression of these antigens differ also in their transcription factor expression and cell cycle characteristics. Moreover, the content of BLBP seems correlated to the fate of the progeny. BLBP-negative precursors are detected only during neurogenesis and persist into postnatal stages solely in the rostral migratory stream, a region of ongoing neurogenesis. In contrast, an enriched population of multipotential cells, neurosphere cultures derived from the adult or embryonic telencephalon, is immunoreactive for RC2, GLAST, and BLBP. Taken together, we have identified novel, functionally distinct subsets of CNS precursor cells.
Recent studies demonstrated the neuronogenic role of radial glial cells (RGCs) in the rodent. To reveal the fate of radial glial processes, we intensively monitored divisions of RGCs in DiI-labeled slices from the embryonic day 14 mouse cortex. During RGC division, each pia-connected fiber becomes thin but is neither lost nor divided; it is inherited asymmetrically by one daughter cell. In divisions that produce a neuron and a progenitor, the neuron inherits the pial fiber, also grows a thick ventricular process for several hours, and is therefore indistinguishable from the progenitor RGC. The ventricular process in the radial glial-like neuron ("radial neuron") then collapses, leading to ascent of the neuron by using the "recycled" radial fiber.
During the development of the cerebral cortex, radial glia serve as a scaffold to support and direct neurons during their migration. This view is now changing in the light of emerging evidence showing that these cells have a much more dynamic and diverse role. A recent series of studies has provided strong support for their role as precursor cells in the ventricular zone that generate cortical neurons and glia, in addition to providing migration guidance.