Precursors of neurons, neuroglia, and ependymal cells in the CNS: what are they? Where are they from? How do they get where they are going?

Glial Populations in the Human Brain Following Ischemic Injury

There is a growing interest in glial cells in the central nervous system due to their important role in maintaining brain homeostasis under physiological conditions and after injury. A significant amount of evidence has been accumulated regarding their capacity to exert either pro-inflammatory or anti-inflammatory effects under different pathological conditions. In combination with their proliferative potential, they contribute not only to the limitation of brain damage and tissue remodeling but also to neuronal repair and synaptic recovery. Moreover, reactive glial cells can modulate the processes of neurogenesis, neuronal differentiation, and migration of neurons in the existing neural circuits in the adult brain. By discovering precise signals within specific niches, the regulation of sequential processes in adult neurogenesis holds the potential to unlock strategies that can stimulate the generation of functional neurons, whether in response to injury or as a means of addressing degenerative neurological conditions. Cerebral ischemic stroke, a condition falling within the realm of acute vascular disorders affecting the circulation in the brain, stands as a prominent global cause of disability and mortality. Extensive investigations into glial plasticity and their intricate interactions with other cells in the central nervous system have predominantly relied on studies conducted on experimental animals, including rodents and primates. However, valuable insights have also been gleaned from in vivo studies involving poststroke patients, utilizing highly specialized imaging techniques. Following the attempts to map brain cells, the role of various transcription factors in modulating gene expression in response to cerebral ischemia is gaining increasing popularity. Although the results obtained thus far remain incomplete and occasionally ambiguous, they serve as a solid foundation for the development of strategies aimed at influencing the recovery process after ischemic brain injury.

Neural Progenitor Cells and the Hypothalamus

Neural progenitor cells (NPCs) are multipotent neural stem cells (NSCs) capable of self-renewing and differentiating into neurons, astrocytes and oligodendrocytes. In the postnatal/adult brain, NPCs are primarily located in the subventricular zone (SVZ) of the lateral ventricles (LVs) and subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). There is evidence that NPCs are also present in the postnatal/adult hypothalamus, a highly conserved brain region involved in the regulation of core homeostatic processes, such as feeding, metabolism, reproduction, neuroendocrine integration and autonomic output. In the rodent postnatal/adult hypothalamus, NPCs mainly comprise different subtypes of tanycytes lining the wall of the 3^rd ventricle. In the postnatal/adult human hypothalamus, the neurogenic niche is constituted by tanycytes at the floor of the 3^rd ventricle, ependymal cells and ribbon cells (showing a gap-and-ribbon organization similar to that in the SVZ), as well as suprachiasmatic cells. We speculate that in the postnatal/adult human hypothalamus, neurogenesis occurs in a highly complex, exquisitely sophisticated neurogenic niche consisting of at least four subniches; this structure has a key role in the regulation of extrahypothalamic neurogenesis, and hypothalamic and extrahypothalamic neural circuits, partly through the release of neurotransmitters, neuropeptides, extracellular vesicles (EVs) and non-coding RNAs (ncRNAs).

Restoration of spinal cord injury: From endogenous repairing process to cellular therapy

Spinal cord injury (SCI) disrupts neurological pathways and impacts sensory, motor, and autonomic nerve function. There is no effective treatment for SCI currently. Numerous endogenous cells, including astrocytes, macrophages/microglia, and oligodendrocyte, are involved in the histological healing process following SCI. By interfering with cells during the SCI repair process, some advancements in the therapy of SCI have been realized. Nevertheless, the endogenous cell types engaged in SCI repair and the current difficulties these cells confront in the therapy of SCI are poorly defined, and the mechanisms underlying them are little understood. In order to better understand SCI and create new therapeutic strategies and enhance the clinical translation of SCI repair, we have comprehensively listed the endogenous cells involved in SCI repair and summarized the six most common mechanisms involved in SCI repair, including limiting the inflammatory response, protecting the spared spinal cord, enhancing myelination, facilitating neovascularization, producing neurotrophic factors, and differentiating into neural/colloidal cell lines.

Heparin Protects Human Neural Progenitor Cells from Zika Virus-Induced Cell Death While Preserving Their Differentiation into Mature Neuroglial Cells

Zika virus (ZIKV) is an arbovirus member of the Flaviviridae family that causes severe congenital brain anomalies in infected fetuses. The key target cells of ZIKV infection, human neural progenitor cells (hNPCs), are highly permissive to infection that causes the inhibition of cell proliferation and induces cell death. We have previously shown that pharmaceutical-grade heparin inhibits virus-induced cell death with negligible effects on in vitro virus replication in ZIKV-infected hNPCs at the “high” multiplicity of infection (MOI) of 1. Here, we show that heparin inhibits formation of ZIKV-induced intracellular vacuoles, a signature of paraptosis, and inhibits necrosis and apoptosis of hNPCs grown as neurospheres (NS). To test whether heparin preserved the differentiation of ZIKV-infected hNPCs into neuroglial cells, hNPCs were infected at the MOI of 0.001. In this experimental condition, heparin inhibited ZIKV replication by ca. 2 log₁₀, mostly interfering with virion attachment, while maintaining its protective effect against ZIKV-induced cytopathicity. Heparin preserved differentiation into neuroglial cells of hNPCs that were obtained from either human-induced pluripotent stem cells (hiPSC) or by fetal tissue. Quite surprisingly, multiple additions of heparin to hNPCs enabled prolonged virus replication while preventing virus-induced cytopathicity. Collectively, these results highlight the potential neuroprotective effect of heparin that could serve as a lead compound to develop novel agents for preventing the damage of ZIKV infection on the developing brain.

IMPORTANCE ZIKV is a neurotropic virus that invades neural progenitor cells (NPCs), causing inhibition of their proliferation and maturation into neurons and glial cells. We have shown previously that heparin, an anticoagulant also used widely during pregnancy, prevents ZIKV-induced cell death with negligible inhibition of virus replication. Here, we demonstrate that heparin also exerts antiviral activity against ZIKV replication using a much lower infectious inoculum. Moreover, heparin interferes with different modalities of virus-induced cell death. Finally, heparin-induced prevention of virus-induced NPC death allows their differentiation into neuroglial cells despite the intracellular accumulation of virions. These results highlight the potential use of heparin, or pharmacological agents derived from it, in pregnant women to prevent the devastating effects of ZIKV infection on the developing brain of their fetuses.

Immediate Early Gene c-fos in the Brain: Focus on Glial Cells

The c-fos gene was first described as a proto-oncogene responsible for the induction of bone tumors. A few decades ago, activation of the protein product c-fos was reported in the brain after seizures and other noxious stimuli. Since then, multiple studies have used c-fos as a brain activity marker. Although it has been attributed to neurons, growing evidence demonstrates that c-fos expression in the brain may also include glial cells. In this review, we collect data showing that glial cells also express this proto-oncogene. We present evidence demonstrating that at least astrocytes, oligodendrocytes, and microglia express this immediate early gene (IEG). Unlike neurons, whose expression changes used to be associated with depolarization, glial cells seem to express the c-fos proto-oncogene under the influence of proliferation, differentiation, growth, inflammation, repair, damage, plasticity, and other conditions. The collected evidence provides a complementary view of c-fos as an activity marker and urges the introduction of the glial cell perspective into brain activity studies. This glial cell view may provide additional information related to the brain microenvironment that is difficult to obtain from the isolated neuron paradigm. Thus, it is highly recommended that detection techniques are improved in order to better differentiate the phenotypes expressing c-fos in the brain and to elucidate the specific roles of c-fos expression in glial cells.

Bioelectric Potential in Next-Generation Organoids: Electrical Stimulation to Enhance 3D Structures of the Central Nervous System

Pluripotent stem cell-derived organoid models of the central nervous system represent one of the most exciting areas in in vitro tissue engineering. Classically, organoids of the brain, retina and spinal cord have been generated via recapitulation of in vivo developmental cues, including biochemical and biomechanical. However, a lesser studied cue, bioelectricity, has been shown to regulate central nervous system development and function. In particular, electrical stimulation of neural cells has generated some important phenotypes relating to development and differentiation. Emerging techniques in bioengineering and biomaterials utilise electrical stimulation using conductive polymers. However, state-of-the-art pluripotent stem cell technology has not yet merged with this exciting area of bioelectricity. Here, we discuss recent findings in the field of bioelectricity relating to the central nervous system, possible mechanisms, and how electrical stimulation may be utilised as a novel technique to engineer “next-generation” organoids.

Coordinating cerebral cortical construction and connectivity: Unifying influence of radial progenitors

Radial progenitor development and function lay the foundation for the construction of the cerebral cortex. Radial glial scaffold, through its functions as a source of neurogenic progenitors and neuronal migration guide, is thought to provide a template for the formation of the cerebral cortex. Emerging evidence is challenging this limited view. Intriguingly, radial glial scaffold may also play a role in axonal growth, guidance, and neuronal connectivity. Radial glial cells not only facilitate the generation, placement, and allocation of neurons in the cortex but also regulate how they wire up. The organization and function of radial glial cells may thus be a unifying feature of the developing cortex that helps to precisely coordinate the right patterns of neurogenesis, neuronal placement, and connectivity necessary for the emergence of a functional cerebral cortex. This perspective critically explores this emerging view and its impact in the context of human brain development and disorders.

Reelin Is Required for Maintenance of Granule Cell Lamination in the Healthy and Epileptic Hippocampus

One characteristic feature of mesial temporal lobe epilepsy is granule cell dispersion (GCD), a pathological widening of the granule cell layer in the dentate gyrus. The loss of the extracellular matrix protein Reelin, an important positional cue for neurons, correlates with GCD formation in MTLE patients and in rodent epilepsy models. Here, we used organotypic hippocampal slice cultures (OHSC) from transgenic mice expressing enhanced green fluorescent protein (eGFP) in differentiated granule cells (GCs) to monitor GCD formation dynamically by live cell video microscopy and to investigate the role of Reelin in this process. We present evidence that following treatment with the glutamate receptor agonist kainate (KA), eGFP-positive GCs migrated mainly toward the hilar region. In the hilus, Reelin-producing neurons were rapidly lost following KA treatment as shown in a detailed time series. Addition of recombinant Reelin fragments to the medium effectively prevented the KA-triggered movement of eGFP-positive GCs. Placement of Reelin-coated beads into the hilus of KA-treated cultures stopped the migration of GCs in a distance-dependent manner. In addition, quantitative Western blot analysis revealed that KA treatment affects the Reelin signal transduction pathway by increasing intracellular adaptor protein Disabled-1 synthesis and reducing the phosphorylation of cofilin, a downstream target of the Reelin pathway. Both events were normalized by addition of recombinant Reelin fragments. Finally, following neutralization of Reelin in healthy OHSC by incubation with the function-blocking CR-50 Reelin antibody, GCs started to migrate without any direction preference. Together, our findings demonstrate that normotopic position of Reelin is essential for the maintenance of GC lamination in the dentate gyrus and that GCD is the result of a local Reelin deficiency.

Local gene regulation in radial glia: Lessons from across the nervous system

Radial glial cells (RGCs) are progenitors of the cerebral cortex which produce both neurons and glia during development. Given their central role in development, RGC dysfunction can result in diverse neurodevelopmental disorders. RGCs have an elongated bipolar morphology that spans the entire radial width of the cortex and ends in basal endfeet connected to the pia. The basal process and endfeet are important for proper guidance of migrating neurons and are implicated in signaling. However, endfeet must function at a great distance from the cell body. This spatial separation suggests a role for local gene regulation in endfeet. Endfeet contain a local transcriptome enriched for cytoskeletal and signaling factors. These localized mRNAs are actively transported from the cell body and can be locally translated in endfeet. Yet, studies of local gene regulation in RGC endfeet are still in their infancy. Here, we draw comparisons of RGCs with foundational work in anatomically and phylogenetically related cell types, neurons and astrocytes. Our review highlights a striking overlap in the types of RNAs localized, as well as principles of local translation between these three cell types. Thus, studies in neurons, astrocytes and RGCs can mutually inform an understanding of RNA localization across the nervous system.

On being the right shape: Roles for motile cilia and cerebrospinal fluid flow in body and spine morphology

How developing and growing organisms attain their proper shape is a central problem of developmental biology. In this review, we investigate this question with respect to how the body axis and spine form in their characteristic linear head-to-tail fashion in vertebrates. Recent work in the zebrafish has implicated motile cilia and cerebrospinal fluid flow in axial morphogenesis and spinal straightness. We begin by introducing motile cilia, the fluid flows they generate and their roles in zebrafish development and growth. We then describe how cilia control body and spine shape through sensory cells in the spinal canal, a thread-like extracellular structure called the Reissner fiber, and expression of neuropeptide signals. Last, we discuss zebrafish mutants in which spinal straightness breaks down and three-dimensional curves form. These curves resemble the common but little-understood human disease Idiopathic Scoliosis. Zebrafish research is therefore poised to make progress in our understanding of this condition and, more generally, how body and spine shape is acquired and maintained through development and growth.

Biomaterial strategies for creating in vitro astrocyte cultures resembling in vivo astrocyte morphologies and phenotypes

Astrocytes are dynamic cells residing in the central nervous system exhibiting many diverse functions. Astrocytes quickly change and present unique phenotypes in response to injury or disease. Here, we briefly summarize recent information regarding astrocyte morphology and function and provide brief insight into their phenotypic changes following injury or disease. We also present the utility of in vitro astrocyte cultures and present recent advances in biomaterial development that enable better recapitulation of their in vivo behavior and morphology.

Toxoplasma gondii infection impairs radial glia differentiation and its potential to modulate brain microvascular endothelial cell function in the cerebral cortex

Congenital toxoplasmosis is a parasitic disease that occurs due vertical transmission of the protozoan Toxoplasma gondii (T. gondii) during pregnancy. The parasite crosses the placental barrier and reaches the developing brain, infecting progenitor, glial, neuronal and vascular cell types. Although the role of Radial glia (RG) neural stem cells in the development of the brain vasculature has been recently investigated, the impact of T. gondii infection in these events is not yet understood. Herein, we studied the role of T. gondii infection on RG cell function and its interaction with endothelial cells. By infecting isolated RG cultures with T. gondii tachyzoites, we observed a cytotoxic effect with reduced numbers of RG populations together with decrease neuronal and oligodendrocyte progenitor populations. Conditioned medium (CM) from RG control cultures increased ZO-1 protein levels and organization on endothelial bEnd.3 cells membranes, which was impaired by CM from infected RG, accompanied by decreased trans-endothelial electrical resistance (TEER). ELISA assays revealed reduced levels of anti-inflammatory cytokine TGF-β1 in CM from T. gondii-infected RG cells. Treatment with recombinant TGF-β1 concomitantly with CM from infected RG cultures led to restoration of ZO-1 staining in bEnd.3 cells. Congenital infection in Swiss Webster mice led to abnormalities in the cortical microvasculature in comparison to uninfected embryos. Our results suggest that infection of RG cells by T. gondii negatively modulates cytokine secretion, which might contribute to endothelial loss of barrier properties, thus leading to impairment of neurovascular interaction establishment.

Neural Progenitor Cell Terminology

Since descriptions of neural precursor cells (NPCs) were published in the late 19th century, neuroanatomists have used a variety of terms to describe these cells, each term reflecting contemporary understanding of cellular characteristics and function. As the field gained knowledge through a combination of technical advance and individual insight, the terminology describing NPCs changed to incorporate new information. While there is a trend toward consensus and streamlining of terminology over time, to this day scientists use different terms for NPCs that reflect their field and perspective, i.e., terms arising from molecular, cellular, or anatomical sciences. Here we review past and current terminology used to refer to NPCs, including embryonic and adult precursor cells of the cerebral cortex and hippocampus.

The nervous system and genomics in endometriosis

Endometriosis is a gynaecological disease that occurs in approximately 10% to 15% of women of reproductive age and up to 47% of infertile women. The presence of implants of endometrial-like glands and stroma outside the uterus, characteristic of this disease, induce a wide variety of symptoms, mainly pelvic pain and infertility. Women suffering from this condition experience great distress, which significantly affects their quality of life. Numerous studies attempting to decipher the pathogenic mechanisms of endometriosis have been conducted around the world, yet its aetiology still remains unknown. It is widely believed that in women with endometriosis, the endometrium has characteristic features that allow the formation of implants once fragments have entered the peritoneal cavity through retrograde menstruation. Furthermore, a strong genetic tendency to develop the disease has been reported among patients and first-degree relatives. Thanks to the recent technological advances achieved in genomics and bioinformatics, a number of studies have had the potential to analyse several aspects of the pathogenesis of endometriosis from a genetic perspective. Due to the recent identification of nerve fibres in the endometrium of women with endometriosis, research on the neurogenesis of the disease has increased in the past few years. However, the genetic aspects of nerve growth in endometriosis have not been analysed in depth and further research providing important insights into the mechanisms that mediate pain in affected patients has the potential to contribute substantially to the future management of the condition.

Aquaporins and Their Regulation after Spinal Cord Injury

After injury to the spinal cord, edema contributes to the underlying detrimental pathophysiological outcomes that lead to worsening of function. Several related membrane proteins called aquaporins (AQPs) regulate water movement in fluid transporting tissues including the spinal cord. Within the cord, AQP1, 4 and 9 contribute to spinal cord injury (SCI)-induced edema. AQP1, 4 and 9 are expressed in a variety of cells including astrocytes, neurons, ependymal cells, and endothelial cells. This review discusses some of the recent findings of the involvement of AQP in SCI and highlights the need for further study of these proteins to develop effective therapies to counteract the negative effects of SCI-induced edema.

Blood-brain barrier development: Systems modeling and predictive toxicology

The blood-brain barrier (BBB) serves as a gateway for passage of drugs, chemicals, nutrients, metabolites, and hormones between vascular and neural compartments in the brain. Here, we review BBB development with regard to the microphysiology of the neurovascular unit (NVU) and the impact of BBB disruption on brain development. Our focus is on modeling these complex systems. Extant in silico models are available as tools to predict the probability of drug/chemical passage across the BBB; in vitro platforms for high-throughput screening and high-content imaging provide novel data streams for profiling chemical-biological interactions; and engineered human cell-based microphysiological systems provide empirical models with which to investigate the dynamics of NVU function. Computational models are needed that bring together kinetic and dynamic aspects of NVU function across gestation and under various physiological and toxicological scenarios. This integration will inform adverse outcome pathways to reduce uncertainty in translating in vitro data and in silico models for use in risk assessments that aim to protect neurodevelopmental health.

The role of neuro-epithelial-like and radial-glial stem and progenitor cells in development, plasticity, and repair

Neural stem and progenitor cells (NSPCs) are the primary source of new neurons in the brain and serve critical roles in tissue homeostasis and plasticity throughout life. Within the vertebrate brain, NSPCs are located within distinct neurogenic niches differing in their location, cellular composition, and proliferative behaviour. Heterogeneity in the NSPC population is hypothesized to reflect varying capacities for neurogenesis, plasticity and repair between different neurogenic zones. Since the discovery of adult neurogenesis, studies have predominantly focused on the behaviour and biological significance of adult NSPCs (aNSPCs) in rodents. However, compared to rodents, who show lifelong neurogenesis in only two restricted neurogenic niches, zebrafish exhibit constitutive neurogenesis across multiple stem cell niches that provide new neurons to every major brain division. Accordingly, zebrafish are a powerful model to probe the unique cellular and molecular profiles of NSPCs and investigate how these profiles govern tissue homeostasis and regenerative plasticity within distinct stem cell populations over time. Amongst the NSPC populations residing in the zebrafish central nervous system (CNS), proliferating radial-glia, quiescent radial-glia and neuro-epithelial-like cells comprise the majority. Here, we provide insight into the extent to which these distinct NSPC populations function and mature during development, respond to experience, and contribute to successful CNS regeneration in teleost fish. Together, our review brings to light the dynamic biological roles of these individual NSPC populations and showcases their diverse regenerative modes to achieve vertebrate brain repair later in life.

Astrocyte Neuroprotection and Dehydroepiandrosterone

Dehydroepiandrosterone (DHEA) and its sulfate ester (DHEAS) are the most abundant steroid hormones in the systemic circulation of humans. Due to their abundance and reduced production during aging, these hormones have been suggested to play a role in many aspects of health and have been used as drugs for a multiple range of therapeutic actions, including hormonal replacement and the improvement of aging-related diseases. In addition, several studies have shown that DHEA and DHEAS are neuroprotective under different experimental conditions, including models of ischemia, traumatic brain injury, spinal cord injury, glutamate excitotoxicity, and neurodegenerative diseases. Since astrocytes are responsible for the maintenance of neural tissue homeostasis and the control of neuronal energy supply, changes in astrocytic function have been associated with neuronal damage and the progression of different pathologies. Therefore, the aim of this chapter is to discuss the neuroprotective effects of DHEA against different types of brain and spinal cord injuries and how the modulation of astrocytic function by DHEA could represent an interesting therapeutic approach for the treatment of these conditions.

Distinct roles for hyaluronan in neural stem cell niches and perineuronal nets

Adult neurogenesis in mammals is a tightly regulated process where neural stem cells (NSCs), especially in the subgranular zone (SGZ) of the hippocampal dentate gyrus, proliferate and differentiate into new neurons that form new circuits or integrate into old circuits involved in episodic memory, pattern discrimination, and emotional responses. Recent evidence suggests that changes in the hyaluronan (HA)-based extracellular matrix of the SGZ may regulate neurogenesis by controlling NSC proliferation and early steps in neuronal differentiation. These studies raise the intriguing possibility that perturbations in this matrix, including HA accumulation with aging, could impact adult neurogenesis and cognitive functions, and that alterations to this matrix could be beneficial following insults to the central nervous system that impact hippocampal functions.

Thyroid hormone- and estrogen receptor interactions with natural ligands and endocrine disruptors in the cerebellum

Although the effects of phytoestrogens on brain function is widely unknown, they are often regarded as "natural" and thus as harmless. However, the effects of phytoestrogens or environmental pollutants on brain function is underestimated. Estrogen (17beta-estradiol, E2) and thyroid hormones (THs) play pivotal roles in brain development. In the mature brain, these hormones regulate metabolism on cellular and organismal levels. Thus, E2 and THs do not only regulate the energy metabolism of the entire organism, but simultaneously also regulate important homeostatic parameters of neurons and glia in the CNS. It is, therefore, obvious that the mechanisms through which these hormones exert their effects are pleiotropic and include both intra- and intercellular actions. These hormonal mechanisms are versatile, and the experimental investigation of simultaneous hormone-induced mechanisms is technically challenging. In addition, the normal physiological settings of metabolic parameters depend on a plethora of interactions of the steroid hormones. In this review, we discuss conceptual and experimental aspects of the gonadal and thyroid hormones as they relate to in vitro models of the cerebellum.

Radial Glia Cells Control Angiogenesis in the Developing Cerebral Cortex Through TGF-β1 Signaling

Neuroangiogenesis in the developing central nervous system is controlled by interactions between endothelial cells (ECs) and radial glia (RG) neural stem cells, although RG-derived molecules implicated in these events are not fully known. Here, we investigated the role of RG-secreted TGF-β1, in angiogenesis in the developing cerebral cortex. By isolation of murine microcapillary brain endothelial cells (MBECs), we demonstrate that conditioned medium from RG cultures (RG-CM) promoted MBEC migration and formation of vessel-like structures in vitro, in a TGF-β1-dependent manner. These events were followed by endothelial regulation of GPR124 and BAI-1 gene expression by RG-CM. Proteome profile of RG-CM identified angiogenesis-related molecules IGFBP2/3, osteopontin, endostatin, SDF1, fractalkine, TIMP1/4, Ang-1, pentraxin3, and Cyr61, some of them modulated by TGF-β1 induction. In vivo gain and loss of function assays targeting RG cells demonstrates a specific TGF-β1-dependent control of blood vessels branching in the cerebral cortex. Together, our results point to TGF-β1 signaling pathway as a potential mediator of the RG-EC interactions and shed light to the key role of RG in paving the brain vascular network.

"Targeting astrocytes in CNS injury and disease: A translational research approach"

Astrocytes are a major constituent of the central nervous system. These glia play a major role in regulating blood-brain barrier function, the formation and maintenance of synapses, glutamate uptake, and trophic support for surrounding neurons and glia. Therefore, maintaining the proper functioning of these cells is crucial to survival. Astrocyte defects are associated with a wide variety of neuropathological insults, ranging from neurodegenerative diseases to gliomas. Additionally, injury to the CNS causes drastic changes to astrocytes, often leading to a phenomenon known as reactive astrogliosis. This process is important for protecting the surrounding healthy tissue from the spread of injury, while it also inhibits axonal regeneration and plasticity. Here, we discuss the important roles of astrocytes after injury and in disease, as well as potential therapeutic approaches to restore proper astrocyte functioning.

The Spinal Ependymal Layer in Health and Disease

Ependymal cells are epithelial support cells that line the central canal and ventricular cavities of the central nervous system, providing the interface between the cerebrospinal fluid and the parenchyma of the brain and spinal cord. The spinal ependymal layer (SEL) is composed of 3 main cell types: tanycytes, ependymocytes, and cerebrospinal fluid-contacting neurons. A fourth cell type, termed the supraependymal cell, is also occasionally described. Cells of the SEL show restricted proliferative capacity in health but display neural stem cell properties both in vitro and in vivo in various disease states. A growing body of literature is devoted to the regenerative roles of the SEL, particularly in the context of spinal cord injury, where mechanical damage to the spinal cord leads to a significant increase in SEL proliferation. SEL-derived cell progeny migrate to sites of injury within the injured spinal cord parenchyma and contribute primarily to glial scar formation. In additional to their role as endogenous neural stem cells, cells of the SEL may be an important source of cytokines and other cell signaling molecules, such as tumor necrosis factor, heat shock proteins, and various growth factors. The SEL has become of recent interest to neuroscience researchers because of its potential to participate in and respond to diseases affecting the spinal cord (eg, traumatic spinal cord injury) and neurodegenerative disease. The intimate association of the SEL with the cerebrospinal fluid makes intrathecal therapies a viable option, and recent studies highlight the potential promise of treatments that augment SEL responses to disease.

An in vivo screen to identify candidate neurogenic genes in the developing Xenopus visual system

Neurogenesis in the brain of Xenopus laevis continues throughout larval stages of development. We developed a 2-tier screen to identify candidate genes controlling neurogenesis in Xenopus optic tectum in vivo. First, microarray and NanoString analyses were used to identify candidate genes that were differentially expressed in Sox2-expressing neural progenitor cells or their neuronal progeny. Then an in vivo, time-lapse imaging-based screen was used to test whether morpholinos against 34 candidate genes altered neural progenitor cell proliferation or neuronal differentiation over 3 days in the optic tectum of intact Xenopus tadpoles. We co-electroporated antisense morpholino oligonucleotides against each of the candidate genes with a plasmid that drives GFP expression in Sox2-expressing neural progenitor cells and quantified the effects of morpholinos on neurogenesis. Of the 34 morpholinos tested, 24 altered neural progenitor cell proliferation or neuronal differentiation. The candidates which were tagged as differentially expressed and validated by the in vivo imaging screen include: actn1, arl9, eif3a, elk4, ephb1, fmr1-a, fxr1-1, fbxw7, fgf2, gstp1, hat1, hspa5, lsm6, mecp2, mmp9, and prkaca. Several of these candidates, including fgf2 and elk4, have known or proposed neurogenic functions, thereby validating our strategy to identify candidates. Genes with no previously demonstrated neurogenic functions, gstp1, hspa5 and lsm6, were identified from the morpholino experiments, suggesting that our screen successfully revealed unknown candidates. Genes that are associated with human disease, such as such as mecp2 and fmr1-a, were identified by our screen, providing the groundwork for using Xenopus as an experimental system to probe conserved disease mechanisms. Together the data identify candidate neurogenic regulatory genes and demonstrate that Xenopus is an effective experimental animal to identify and characterize genes that regulate neural progenitor cell proliferation and differentiation in vivo.

Angiogenesis in the developing spinal cord: blood vessel exclusion from neural progenitor region is mediated by VEGF and its antagonists

Blood vessels in the central nervous system supply a considerable amount of oxygen via intricate vascular networks. We studied how the initial vasculature of the spinal cord is formed in avian (chicken and quail) embryos. Vascular formation in the spinal cord starts by the ingression of intra-neural vascular plexus (INVP) from the peri-neural vascular plexus (PNVP) that envelops the neural tube. At the ventral region of the PNVP, the INVP grows dorsally in the neural tube, and we observed that these vessels followed the defined path at the interface between the medially positioned and undifferentiated neural progenitor zone and the laterally positioned differentiated zone. When the interface between these two zones was experimentally displaced, INVP faithfully followed a newly formed interface, suggesting that the growth path of the INVP is determined by surrounding neural cells. The progenitor zone expressed mRNA of vascular endothelial growth factor-A whereas its receptor VEGFR2 and FLT-1 (VEGFR1), a decoy for VEGF, were expressed in INVP. By manipulating the neural tube with either VEGF or the soluble form of FLT-1, we found that INVP grew in a VEGF-dependent manner, where VEGF signals appear to be fine-tuned by counteractions with anti-angiogenic activities including FLT-1 and possibly semaphorins. These results suggest that the stereotypic patterning of early INVP is achieved by interactions between these vessels and their surrounding neural cells, where VEGF and its antagonists play important roles.

The cellular composition of the marsupial neocortex

In the current investigation we examined the number and proportion of neuronal and non-neuronal cells in the primary sensory areas of the neocortex of a South American marsupial, the short-tailed opossum (Monodelphis domestica). The primary somatosensory (S1), auditory (A1), and visual (V1) areas were dissected from the cortical sheet and compared with each other and the remaining neocortex using the isotropic fractionator technique. We found that although the overall sizes of V1, S1, A1, and the remaining cortical regions differed from each other, these divisions of the neocortex contained the same number of neurons, but the remaining cortex contained significantly more non-neurons than the primary sensory regions. In addition, the percent of neurons was higher in A1 than in the remaining cortex and the cortex as a whole. These results are similar to those seen in non-human primates. Furthermore, these results indicate that in some respects, such as number of neurons, the neocortex is homogenous across its extent, whereas in other aspects of organization, such as non-neuronal number and percentage of neurons, there is non-uniformity. Whereas the overall pattern of neuronal distribution is similar between short-tailed opossums and eutherian mammals, short-tailed opossum have a much lower cellular and neuronal density than other eutherian mammals. This suggests that the high neuronal density cortices of mammals such as rodents and primates may be a more recently evolved characteristic that is restricted to eutherians, and likely contributes to the complex behaviors we see in modern mammals.

Natural and lesion-induced decrease in cell proliferation in the medial nucleus of the trapezoid body during hearing development

The functional interactions between neurons and glial cells that are important for nervous system function are presumably established during development from the activity of progenitor cells. In this study we examined proliferation of progenitor cells in the medial nucleus of the trapezoid body (MNTB) located in the rat auditory brainstem. We performed DNA synthesis labeling experiments to demonstrate changes in cell proliferation activity during postnatal stages of development. An increase in cell proliferation correlated with MNTB growth and the presence of S100β-positive astrocytes among MNTB neurons. In additional experiments we analyzed the fate of newly born cells. At perinatal ages, newly born cells colabeled with the astrocyte marker S100β in higher numbers than when cells were generated at postnatal day 6. Furthermore, we identified newly born cells that were colabeled with caspase-3 immunohistochemistry and performed comparative experiments to demonstrate that there is a natural decrease in cell proliferation activity during postnatal development in rats, mice, gerbils, and ferrets. Lastly, we found that there is a stronger decrease in MNTB cell proliferation after performing bilateral lesions of the auditory periphery in rats. Altogether, these results identify important stages in the development of astrocytes in the MNTB and provide evidence that the proliferative activity of the progenitor cells is developmentally regulated. We propose that the developmental reduction in cell proliferation may reflect coordinated signaling between the auditory brainstem and the auditory periphery. J. Comp. Neurol. 522:971–985, 2014.

Repurposing Reelin: the new role of radial glia, Reelin and Notch in motor neuron migration

The role of Reelin during cerebral cortical neuron migration has long been studied, but the Reelin signaling pathway and its possible interactions are just beginning to be unraveled. Reelin is not only important in cerebral cortical migration, but has recently been shown to interact with the Notch signaling pathway and to be critical for radial glial cell number and morphology. Lee and Song (2013) show a new Notch- and Reelin-dependent role for radial glia in the mouse spinal cord: to act as a fine filter that allows somatic motor neuron axons but not cell bodies to traverse out of the CNS. Here, the types of neuronal migration are discussed, focusing on motor neurons and cues for proper localization. The interaction of Reelin signaling with the Notch pathway is reviewed, which dictates the proper formation of radial glia in the spinal cord in order to prevent ectopic motor neuron migration (Lee and Song, 2013). Future studies may reveal novel interactions and further insights as to how Reelin functions throughout the developing nervous system.

Clinical characteristics and surgical outcomes of spinal intramedullary ependymal cysts

OBJECT

Intramedullary ependymal cysts are exceedingly rare lesions, and have been previously reported in the literature as case reports. The aim of this study was to discuss the clinical presentation and the outcomes of microsurgery for these benign lesions.

METHODS

The authors retrospectively reviewed the records of ten patients who underwent microsurgery for intramedullary ependymal cysts. All patients had preoperative and postoperative magnetic resonance imaging. The surgical treatment included gross total resection and biopsy plus a cyst-subarachnoid shunt. The diagnosis of intramedullary ependymal cysts was based on radiological and pathological criteria. All patients were followed up, with a mean duration of 47.6 months.

RESULTS

Intramedullary ependymal cysts were hypointense on T1-weighted images and hyperintense on T2-weighted images. Contrast-enhanced T1-weighted images showed no enhancement. Gross total resection was achieved in one case. Biopsy of the cyst wall plus cyst-subarachnoid shunt placement was achieved in nine cases. Long-term neurological function was improved in eight patients. No recurrence was observed on magnetic resonance imaging.

CONCLUSIONS

Ependymal cysts should be considered in the differential diagnosis of intramedullary cysts. For symptomatic patients, early surgery should be performed before neurological deficits deteriorate. Complete decompression and cyst-subarachnoid shunt placement is the optimal treatment and the outcome may be favorable.

Radial glial cells: key organisers in CNS development

Radial glia are elongated bipolar cells present in the CNS during development. Our understanding of the unique roles these cells play has significantly expanded in the last decade. Historically, radial glial cells were primarily thought to provide an architectural framework for neuronal migration. Recent research reveals that radial glia play a more dynamic and integrated role in the development of the brain and spinal cord. They represent a major progenitor pool during early development and can give rise to a small population of multipotent cells in neurogenic niches of the adult CNS. Radial glial cells are a heterogeneous population, with divergent and often poorly understood roles across different brain and spinal cord regions during development; this heterogeneity extends to specialised adult subtypes, such as tanycytes, Müller glial cells and Bergman glial cells which possess morphological similarities to radial glial but play distinct functional roles in the CNS.

Differential changes in the cellular composition of the developing marsupial brain

Throughout development both the body and the brain change at remarkable rates. Specifically, the number of cells in the brain undergoes dramatic nonlinear changes, first exponentially increasing in cell number and then decreasing in cell number. Different cell types, such as neurons and glia, undergo these changes at different stages of development. The current investigation used the isotropic fractionator method to examine the changes in cellular composition at multiple developmental milestones in the short-tailed opossum, Monodelphis domestica. Here we report several novel findings concerning marsupial brain development and organization. First, during the later stages of neurogenesis (P18), neurons make up most of the cells in the neocortex, although the total number of neurons remains the same throughout the life span. In contrast, in the subcortical regions, the number of neurons decreases dramatically after P18, and a converse relationship is observed for nonneuronal cells. In the cerebellum, the total number of cells gradually increases until P180 and then remains constant, and then the number of neurons is consistent across the developmental ages examined. For the three major structures examined, neuronal density and the percentage of neurons within a structure are highest during neurogenesis and then decrease after this point. Finally, the total number of neurons in the opossum brain is relatively low compared with other small-brained mammals such as mice. The relatively low number of neurons and correspondingly high number of nonneurons suggests that in the marsupial brain nonneurons may play a significant role in signal processing.

Spinal intramedullary ependymal cysts : a case report and review of the literature

We report a rare case of a spinal intramedullary ependymal cyst in a 46-year-old female and review the 17 pathologically proven cases in the literature. The patient presented with a two-week history of gradually increasing tingling in her left posterior thigh and calf. A preoperative magnetic resonance image revealed a well-defined intramedullary cystic lesion on the ventral side of the spinal cord at the T11 to T12 levels. The lesion was hyper intense in T2-weighted images and hypointense in T1-weighted. The patient underwent a right-side hemilaminectomy at the T11 to T12 levels and fenestration of the cyst wall. After having the cyst wall partially removed and communication established between the cyst and the subarachnoid space, the patient improved neurologically. A histological study of the surgical specimens revealed that the cyst wall consisted of glial cells lined by a simple cuboidal to columnar epithelium. An immunohistochemical examination of the cells lining the cyst wall was positive for S-100 protein, glial fibrillary acidic protein, epithelial membrane antigen, and cytokeratin. We suggest that the optimal treatment of intramedullary ependymal cysts creates adequate communication between the cyst and the subarachnoid space.

Ligand-induced changes in Oestrogen and thyroid hormone receptor expression in the developing rat cerebellum: A comparative quantitative PCR and Western blot study

Oestrogen (E2) and thyroid hormones (THs) are key regulators of cerebellar development. Recent reports implicate a complex mechanism through which E2 and THs influence the expression levels of each other's receptors (ERs and TRs) to precisely mediate developmental signals and modulate signal strength. We examined the modulating effects of E2 and THs on the expression levels of their receptor mRNAs and proteins in cultured cerebellar cells obtained from 7-day-old rat pups. Cerebellar granule cell cultures were treated with either E2, THs or a combination of these hormones, and resulting receptor expression levels were determined by quantitative PCR and Western blot techniques. The results were compared to non-treated controls and to samples obtained from 14-day-old in situ cerebella. Additionally, we determined the glial effects on the regulation of ER-TR expression levels. The results show that (i) ER and TR expression depends on the combined presence of E2 and THs; (ii) glial cells mediate the hormonal regulation of neuronal ER-TR expression and (iii) loss of tissue integrity results in characteristic changes in ER-TR expression levels. These observations suggest that both E2 and THs, in adequate amounts, are required for the precise orchestration of cerebellar development and that alterations in the ratio of E2/THs may influence signalling mechanisms involved in neurodevelopment. Comparison of data from in vitro and in situ samples revealed a shift in receptor expression levels after loss of tissue integrity, suggesting that such adjusting/regenerative mechanisms may function after cerebellar tissue injury as well.

In vivo time-lapse imaging of cell proliferation and differentiation in the optic tectum of Xenopus laevis tadpoles

We analyzed the function of neural progenitors in the developing central nervous system of Xenopus laevis tadpoles by using in vivo time-lapse confocal microscopy to collect images through the tectum at intervals of 2-24 hours over 3 days. Neural progenitor cells were labeled with fluorescent protein reporters based on expression of endogenous Sox2 transcription factor. With this construct, we identified Sox2-expressing cells as radial glia and as a component of the progenitor pool of cells in the developing tectum that gives rise to neurons and other radial glia. Lineage analysis of individual radial glia and their progeny demonstrated that less than 10% of radial glia undergo symmetric divisions resulting in two radial glia, whereas the majority of radial glia divide asymmetrically to generate neurons and radial glia. Time-lapse imaging revealed the direct differentiation of radial glia into neurons. Although radial glia may guide axons as they navigate to the superficial tectum, we find no evidence that radial glia function as a scaffold for neuronal migration at early stages of tectal development. Over 3 days, the number of labeled cells increased 20%, as the fraction of radial glia dropped and the proportion of neuronal progeny increased to approximately 60% of the labeled cells. Tadpoles provided with short-term visual enhancement generated significantly more neurons, with a corresponding decrease in cell proliferation. Together these results demonstrate that radial glial cells are neural progenitors in the developing optic tectum and reveal that visual experience increases the proportion of neurons generated in an intact animal.

The two-pore domain K+ channel TASK-1 is closely associated with brain barriers and meninges

Impairment of the blood-brain barrier (BBB), the blood-cerebrospinal fluid (CSF) barrier and brain-CSF barrier has been implicated in neuropathology of several brain disorders, such as amyotrophic lateral sclerosis, cerebral edema, multiple sclerosis, neural inflammation, ischemia and stroke. Two-pore domain weakly inward rectifying K+ channel (TWIK)-related acid-sensitive potassium (TASK)-1 channels (K2p3.1; KCNK3) are among the targets that contribute to the development of these pathologies. For example TASK-1 activity is inhibited by acidification, ischemia, hypoxia and several signaling molecules released under pathologic conditions. We have used immuno-histochemistry to examine the distribution of the TASK-1 protein in structures associated with the BBB, blood-CSF barrier, brain-CSF barrier, and in the meninges of adult rat. Dense TASK-1 immuno-reactivity (TASK-1-IR) was observed in ependymal cells lining the fourth ventricle at the brain-CSF interface, in glial cells that ensheath the walls of blood vessels at the glio-vascular interface, and in the meninges. In these structures, TASK-1-IR often co-localized with glial fibrillary associated protein (GFAP) or vimentin. This study provides anatomical evidence for localization of TASK-1 K+ channels in cells that segregate distinct fluid compartments within and surrounding the brain. We suggest that TASK-1 channels, in coordination with other ion channels (e.g., aquaporins and chloride channels) and transporters (e.g., Na+-K+-ATPase and Na+-K+-2Cl⁻ and by virtue of its heterogeneous distribution, may differentially contribute to the varying levels of K+ vital for cellular function in these compartments. Our findings are likely to be relevant to recently reported roles of TASK-1 in cerebral ischemia, stroke and inflammatory brain disorders.

Functions of mature mammalian astrocytes: a current view

Before the roles of normal, mature astrocytes in the mammalian CNS can be discussed, we first need to define these cells. A definition proposed here is that such a class is best defined as consisting of the protoplasmic and fibrous astrocytes of the gray and white matter, respectively, the Bergmann glia of the molecular layer of the cerebellum, and the Muller cells of the retina. It is concluded that the established properties and functions of these mature astrocytes are essential support for neuronal activity, in the sense of Claude Bernard's principle of maintaining "la fixité du milieu intérieur." This milieu would be the extracellular space common to astrocytes and neurons. More specialized roles, such as the recently described "light guides" for retinal Muller cells can also be viewed as support and facilitation. The ECS is also, of course, common to all other neural cells, but here, I limit the discussion to perturbations of the ECS caused only by neuronal activities and the resolution of these perturbations by astrocytes, such as control of increases in extracellular K(+), uptake of excitatory amino acids, and alterations in blood vessel diameter and therefore blood flow. It is also proposed how this fits into the current morphological picture for the protoplasmic astrocytes as having small cell bodies with up to 100,000 process endings that occupy separate territories on which the processes of neighboring astrocytes scarcely intrude.

Basal enrichment within neuroepithelia suggests novel function(s) for Celsr1 protein

A characteristic of the 7TM-cadherins, Flamingo and Celsr1, is their asymmetric protein distribution and polarized activity at neighboring epithelial cell interfaces along defined axes of planar cell polarity. Here, we describe a novel distribution of Celsr1 protein to the basal surface of neuroepithelial cells within both the early neural tube and a less well-defined group of ventricular zone cells at the midline of the developing spinal cord. Importantly, this basal enrichment is lost in embryos homozygous for a mutant Celsr1 allele. We also demonstrate an intimate association between basal enrichment of Celsr1 protein and dorsal sensory tract morphogenesis, an intriguing spatio-temporal organization of Celsr1 protein along the apico-basal neuroepithelial axis suggestive of multiple Celsr1 protein isoforms and the existence of distinct cell surface Celsr1 protein species with direct signaling potential. Together, these data raise compelling new questions concerning the role of Celsr1 during neural development.

Serotonergic neurons migrate radially through the neuroepithelium by dynamin-mediated somal translocation

Embryonic CNS neurons can migrate from the ventricular zone to their final destination by radial glial-guided locomotion. Another less appreciated mechanism is somal translocation, where the young neuron maintains its primitive ventricular and pial processes, through which the cell body moves. A major problem in studying translocation has been the identification of neuronal-specific markers that appear in primitive, radially shaped cells. We used enhanced yellow fluorescent protein under control of the Pet-1 enhancer/promoter region (ePet-EYFP), a specific marker of early differentiated serotonergic neurons, to study their migration via immunohistology and time-lapse imaging of living slice cultures. As early as E10.0, ePet-EYFP-expressing neurons were axonless, radially oriented, and spanned the entire neuroepithelium. The soma translocated within the pial process toward the pial surface and could also translocate through its neurites, which sprouted from the pial process. The dynamin inhibitor dynasore significantly reduced translocation velocity, while the nonmuscle myosin II inhibitor blebbistatin and the kinesin inhibitor AMP-PNP had no significant effect. Here we show for the first time that serotonergic neurons migrate by somal translocation mediated, in part, by dynamin.

Ependymal cells: biology and pathology

The literature was reviewed to summarize the current understanding of the role of ciliated ependymal cells in the mammalian brain. Previous reviews were summarized. Publications from the past 10 years highlight interactions between ependymal cells and the subventricular zone and the possible role of restricted ependymal populations in neurogenesis. Ependymal cells provide trophic support and possibly metabolic support for progenitor cells. Channel proteins such as aquaporins may be important for determining water fluxes at the ventricle wall. The junctional and anchoring proteins are now fairly well understood, as are proteins related to cilia function. Defects in ependymal adhesion and cilia function can cause hydrocephalus through several different mechanisms, one possibility being loss of patency of the cerebral aqueduct. Ependymal cells are susceptible to infection by a wide range of common viruses; while they may act as a line of first defense, they eventually succumb to repeated attacks in long-lived organisms. Ciliated ependymal cells are almost certainly important during brain development. However, the widespread absence of ependymal cells from the adult human lateral ventricles suggests that they may have only regionally restricted value in the mature brain of large size.

Astrocytes: biology and pathology

Astrocytes are specialized glial cells that outnumber neurons by over fivefold. They contiguously tile the entire central nervous system (CNS) and exert many essential complex functions in the healthy CNS. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis, which has become a pathological hallmark of CNS structural lesions. Substantial progress has been made recently in determining functions and mechanisms of reactive astrogliosis and in identifying roles of astrocytes in CNS disorders and pathologies. A vast molecular arsenal at the disposal of reactive astrocytes is being defined. Transgenic mouse models are dissecting specific aspects of reactive astrocytosis and glial scar formation in vivo. Astrocyte involvement in specific clinicopathological entities is being defined. It is now clear that reactive astrogliosis is not a simple all-or-none phenomenon but is a finely gradated continuum of changes that occur in context-dependent manners regulated by specific signaling events. These changes range from reversible alterations in gene expression and cell hypertrophy with preservation of cellular domains and tissue structure, to long-lasting scar formation with rearrangement of tissue structure. Increasing evidence points towards the potential of reactive astrogliosis to play either primary or contributing roles in CNS disorders via loss of normal astrocyte functions or gain of abnormal effects. This article reviews (1) astrocyte functions in healthy CNS, (2) mechanisms and functions of reactive astrogliosis and glial scar formation, and (3) ways in which reactive astrocytes may cause or contribute to specific CNS disorders and lesions.

Modes and regulation of glial migration in vertebrates and invertebrates

Neurons and glial cells show mutual interdependence in many developmental and functional aspects of their biology. To establish their intricate relationships with neurons, glial cells must migrate over what are often long distances. In the CNS glial cells generally migrate as single cells, whereas PNS glial cells tend to migrate as cohorts of cells. How are their journeys initiated and directed, and what stops the migratory phase once glial cells are aligned with their neuronal counterparts? A deeper understanding of glial migration and the underlying neuron-glia interactions may contribute to the development of therapeutics for demyelinating diseases or glial tumours.

Electrical excitability of early neurons in the human cerebral cortex during the second trimester of gestation

Information about development of the human cerebral cortex (proliferation, migration, and differentiation of neurons) is largely based on postmortem histology. Physiological properties of developing human cortical neurons are difficult to access experimentally and therefore remain largely unexplored. Animal studies have shown that information about the arousal of electrical activity in individual cells within fundamental cortical zones (subventricular zone [SVZ], intermediate zone, subplate [SP], and cortical plate [CP]) is necessary for understanding normal brain development. Here we ask where, in what cortical zone, and when, in what gestational week (gw), human neurons acquire the ability to generate nerve impulses (action potentials [APs]). We performed electrical recordings from individual cells in acute brain slices harvested postmortem from the human fetal cerebral cortex (16-22 gw). Tetrodotoxin-sensitive Na(+) current occurs more frequently among CP cells and with significantly greater peak amplitudes than in SVZ. As early as 16 gw, a relatively small population of CP neurons (27%) was able to generate sodium APs upon direct current injection. Neurons located in the SP exhibited the highest level of cellular differentiation, as judged by their ability to fire repetitive APs. At 19 gw, a fraction of human CP and SP neurons possess beta IV spectrin-positive axon initial segments populated with voltage-gated sodium channels (PanNav). These results yield the first physiological characterization of developing human fetal cortical neurons with preserved morphologies in intact surrounding brain tissue.

Epigenetic gene regulation in stem cells and correlation to cancer

Through the classic study of genetics, much has been learned about the regulation and progression of human disease. Specifically, cancer has been defined as a disease driven by genetic alterations, including mutations in tumor-suppressor genes and oncogenes, as well as chromosomal abnormalities. However, the study of normal human development has identified that in addition to classical genetics, regulation of gene expression is also modified by 'epigenetic' alterations including chromatin remodeling and histone variants, DNA methylation, the regulation of polycomb group proteins, and the epigenetic function of non-coding RNA. These changes are modifications inherited during both meiosis and mitosis, yet they do not result in alterations of the actual DNA sequence. A number of biological questions are directly influenced by epigenetics, such as how does a cell know when to divide, differentiate or remain quiescent, and more importantly, what happens when these pathways become altered? Do these alterations lead to the development and/or progression of cancer? This review will focus on summarizing the limited current literature involving epigenetic alterations in the context of human cancer stems cells (CSCs). The extent to which epigenetic changes define cell fate, identity, and phenotype are still under intense investigation, and many questions remain largely unanswered. Before discussing epigenetic gene silencing in CSCs, the different classifications of stem cells and their properties will be introduced. This will be followed by an introduction to the different epigenetic mechanisms. Finally, there will be a discussion of the current knowledge of epigenetic modifications in stem cells, specifically what is known from rodent systems and established cancer cell lines, and how they are leading us to understand human stem cells.