O2A progenitor cells transplanted into the neonatal rat brain develop into oligodendrocytes but not astrocytes

Ultrastructural characteristics of oligodendrocyte precursor cells in the early postnatal mouse optic nerve observed by serial block-face scanning electron microscopy

Oligodendrocyte precursor cells (OPC) arise from restricted regions of the central nervous system (CNS) and differentiate into myelin-forming cells after migration, but their ultrastructural characteristics have not been fully elucidated. This study examined the three-dimensional ultrastructure of OPCs in comparison with other glial cells in the early postnatal optic nerve by serial block-face scanning electron microscopy. We examined 70 putative OPCs (pOPC) that were distinct from other glial cells according to established morphological criteria. The pOPCs were unipolar in shape with relatively few processes, and their Golgi apparatus were localized in the perinuclear region with a single cisterna. Astrocytes abundant in the optic nerve were distinct from pOPCs and had a greater number of processes and more complicated Golgi apparatus morphology. All pOPCs and astrocytes contained a pair of centrioles (basal bodies). Among them, 45% of pOPCs extended a short cilium, and 20% of pOPCs had centrioles accompanied by vesicles, whereas all astrocytes with basal bodies had cilia with invaginated ciliary pockets. These results suggest that the fine structures of pOPCs during the developing and immature stages may account for their distinct behavior. Additionally, the vesicular transport of the centrioles, along with a short cilium length, suggests active ciliogenesis in pOPCs.

A2B5 Expression in Central Nervous System and Gliomas

A2B5 IgM recognizes c-series gangliosides with three sialic acids. The aim of this review was to focus on A2B5 expression in the central nervous system and gliomas. In brain development, A2B5+ cells are recorded in areas containing multipotent neural stem cells (NSC). In adults, A2B5+ cells persist in neurogenic areas and in white matter where it identifies oligodendrocyte precursor cells (OPCs) but also cells with NSC properties. Although the expression of A2B5 has been widely studied in culture, where it characterizes bipotential glial progenitor cells, its expression in vivo is less characterized mainly because of technical issues. A new interest was given to the NSCs and OPCs since the discovery of cancer stem cells (CSC) in gliomas. Among other cell surface molecules, A2B5 has been identified as an accurate marker to identify glioma CSCs. We and others have shown that all types of gliomas express A2B5, and that only A2B5+ cells, and not A2B5- cells, can generate a tumor after orthotopic implantation in immunocompromised animals. Moreover, A2B5 epitope expression is positively correlated with stemness and tumor growth. This review highlights that A2B5 is an attractive target to tackle glioma CSCs, and a better characterization of its expression in the developing and adult CNS will benefit to a better understanding of gliomagenesis.

Glial restricted precursor cells in central nervous system disorders: Current applications and future perspectives

The crosstalk between glial cells and neurons represents an exceptional feature for maintaining the normal function of the central nervous system (CNS). Increasing evidence has revealed the importance of glial progenitor cells in adult neurogenesis, reestablishment of cellular pools, neuroregeneration, and axonal (re)myelination. Several types of glial progenitors have been described, as well as their potentialities for recovering the CNS from certain traumas or pathologies. Among these precursors, glial-restricted precursor cells (GRPs) are considered the earliest glial progenitors and exhibit tripotency for both Type I/II astrocytes and oligodendrocytes. GRPs have been derived from embryos and embryonic stem cells in animal models and have maintained their capacity for self-renewal. Despite the relatively limited knowledge regarding the isolation, characterization, and function of these progenitors, GRPs are promising candidates for transplantation therapy and reestablishment/repair of CNS functions in neurodegenerative and neuropsychiatric disorders, as well as in traumatic injuries. Herein, we review the definition, isolation, characterization and potentialities of GRPs as cell-based therapies in different neurological conditions. We briefly discuss the implications of using GRPs in CNS regenerative medicine and their possible application in a clinical setting. MAIN POINTS: GRPs are progenitors present in the CNS with differentiation potential restricted to the glial lineage. These cells have been employed in the treatment of a myriad of neurodegenerative and traumatic pathologies, accompanied by promising results, herein reviewed.

Role of glia in optic nerve

Glial cells are critically important for maintenance of neuronal activity in the central nervous system (CNS), including the optic nerve (ON). However, the ON has several unique characteristics, such as an extremely high myelination level of retinal ganglion cell (RGC) axons throughout the length of the nerve (with virtually all fibers myelinated by 7 months of age in humans), lack of synapses and very narrow geometry. Moreover, the optic nerve head (ONH) - a region where the RGC axons exit the eye - represents an interesting area that is morphologically distinct in different species. In many cases of multiple sclerosis (demyelinating disease of the CNS) vision problems are the first manifestation of the disease, suggesting that RGCs and/or glia in the ON are more sensitive to pathological conditions than cells in other parts of the CNS. Here, we summarize current knowledge on glial organization and function in the ON, focusing on glial support of RGCs. We cover both well-established concepts on the important role of glial cells in ON health and new findings, including novel insights into mechanisms of remyelination, microglia/NG2 cell-cell interaction, astrocyte reactivity and the regulation of reactive astrogliosis by mitochondrial fragmentation in microglia.

Trophic factors are essential for the survival of grafted oligodendrocyte progenitors and for neuroprotection after perinatal excitotoxicity

The consequences of neonatal white matter injury are devastating and represent a major societal problem as currently there is no cure. Prematurity, low weight birth and maternal pre-natal infection are the most frequent causes of acquired myelin deficiency in the human neonate leading to cerebral palsy and cognitive impairment. In the developing brain, oligodendrocyte (OL) maturation occurs perinatally, and immature OLs are particularly vulnerable. Cell replacement therapy is often considered a viable option to replace progenitors that die due to glutamate excitotoxicity. We previously reported directed specification and mobilization of endogenous committed and uncommitted neural progenitors by the combination of transferrin and insulin growth factor 1 (TSC1). Here, considering cell replacement and integration as therapeutic goals, we examined if OL progenitors (OLPs) grafted into the brain parenchyma of mice that were subjected to an excitotoxic insult could rescue white matter injury. For that purpose, we used a well-established model of glutamate excitotoxic injury. Four-day-old mice received a single intraparenchymal injection of the glutamate receptor agonist N-methyl-D-aspartate alone or in conjunction with TSC1 in the presence or absence of OLPs grafted into the brain parenchyma. Energetics and expression of stress proteins and OL developmental specific markers were examined. A comparison of the proteomic profile per treatment was also ascertained. We found that OLPs did not survive in the excitotoxic environment when grafted alone. In contrast, when combined with TSC1, survival and integration of grafted OLPs was observed. Further, energy metabolism in OLPs was significantly increased by N-methyl-D-aspartate and modulated by TSC1. The proteomic profile after the various treatments showed elevated ubiquitination and stress/heat shock protein 90 in response to N-methyl-D-aspartate. These changes were reversed in the presence of TSC1 and ubiquitination was decreased. The results obtained in this pre-clinical study indicate that the use of a combinatorial intervention including both trophic support and healthy OLPs constitutes a promising approach for long-term survival and successful graft integration. We established optimal conditioning of the host brain environment to promote long-term survival and integration of grafted OLPs into an inflamed neonate host brain. Experimental procedures were performed under the United States Public Health Service Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care Committee at (UCLA) (ARC #1992-034-61) on July 1, 2010.

Origin of Oligodendrocytes in the Vertebrate Optic Nerve: A Review

One of the unsolved problems in the research field of oligodendrocyte (OL) development has been the site(s) of origin of optic nerve OLs and its precursor cells (OPCs). It is generally accepted that OLs in the optic nerve are derived from the brain, and thus optic nerve OLs are immigrant cells. We previously demonstrated the brain origin of optic nerve OPCs in chick embryos. However, the site of optic nerve OPC origin has not been examined experimentally in developing rodents for the past two decades. We have recently reported that optic nerve OPCs in mice arise in the preoptic area by E12.5 and gradually migrate caudally and enter the optic nerve. These OPCs give rise to myelinating OLs in the optic nerve in the postnatal or adult stages. Surprisingly, there are species differences with respect to the origin of optic nerve OPCs between chicks and mice. Here, we summarize the site of OPC origin in the optic nerve based on our own previous and recent results, and discuss possible mechanisms underlying these species differences.

Oligodendrocyte precursor cells in the mouse optic nerve originate in the preoptic area

The present study aims to examine the origin of oligodendrocyte progenitor cells (OPCs) in the mouse optic nerve (ON) by labeling OPCs in the fetal forebrain. The labeling of OPCs in the ON was performed by injection of a retrovirus vector carrying the lacZ gene into the lateral ventricle, or by inducible Cre/loxP of Olig2-positive cells. The retrovirus labeling revealed that ventricular zone-derived cells of the fetal forebrain relocated to the ON and differentiated into oligodendrocytes. In addition, lineage tracing of Olig2-positive cells and whole-mount staining of PDGFRα-positive cells demonstrated that OPCs appeared by E12.5 in the preoptic area, and spread caudally to enter the ON. Our results also suggest that OPCs generated during the early stage are depleted from the ON after maturation.

Efficient Generation of Viral and Integration-Free Human Induced Pluripotent Stem Cell-Derived Oligodendrocytes

Here we document three highly reproducible protocols: (1) a culture system for the derivation of human oligodendrocytes (OLs) from human induced pluripotent stem cells (hiPS) and their further maturation-our protocol generates viral- and integration-free OLs that efficiently commit and move forward in the OL lineage, recapitulating all the steps known to occur during in vivo development; (2) a method for the isolation, propagation and maintenance of neural stem cells (NSCs); and (3) a protocol for the production, isolation, and maintenance of OLs from perinatal rodent and human brain-derived NSCs. Our unique culture systems rely on a series of chemically defined media, specifically designed and carefully characterized for each developmental stage of OL as they advance from OL progenitors to mature, myelinating cells. We are confident that these protocols bring our field a step closer to efficient autologous cell replacement therapies and disease modeling. © 2016 by John Wiley & Sons, Inc.

Dysfunction in Motor Coordination in Neonatal White Matter Injury Model Without Apparent Neuron Loss

We made a white matter injury (WMI) model with mild hindlimb dysfunction by right common carotid artery occlusion followed by 6% oxygen for 60 min at postnatal day 3 (P3), in which actively proliferating oligodendrocyte (OL) progenitors are mainly damaged. To know whether this model is appropriate for cell therapy using OL progenitors, the pathological response to mild hypoxia-ischemia (H-I) in neurons and OL lineage cells and myelination failure were investigated along with gene expression analysis. In WMI model rats, coordinated motor function, as assessed by the accelerating rotarod test, was impaired. The dysfunction was accompanied by myelination failure in layers I-IV of the sensorimotor cortex. Although several oligo2-positive OLs stained positive for active caspase 3 in the cortex and white matter at 24 h after H-I, few NeuN-positive neurons were apoptotic. Argyrophil-III staining for damaged neurons revealed no increase in the number of degenerating cells in the model. Moreover, the total number of NeuN-positive neurons in the cortex was comparable to that of controls 7 days later. Retrograde labeling of the corticospinal tract with Fluoro-Gold revealed no significant loss of layer V neurons. In addition, no decrease in the numbers of cortical projecting neurons and layers V-VI neurons in both motor and sensory areas was observed. Interestingly, the numbers of inhibitory GABAergic cells immunoreactive for parvalbumin, calretinin, or somatostatin were preserved in the P26 cortex. Gene expression analysis at P5 revealed 98 upregulated and 65 downregulated genes that may relate to cell survival, myelin loss, and differentiation of OLs. These data suggest that impaired motor coordination was not induced by neuron loss but, rather, myelination failure in layers I-IV. As OL lineage cells are mainly damaged, this WMI model might be useful for cell-based therapy by replacing OL progenitors.

Myelin damage and repair in pathologic CNS: challenges and prospects

Injury to the central nervous system (CNS) results in oligodendrocyte cell death and progressive demyelination. Demyelinated axons undergo considerable physiological changes and molecular reorganizations that collectively result in axonal dysfunction, degeneration and loss of sensory and motor functions. Endogenous adult oligodendrocyte precursor cells and neural stem/progenitor cells contribute to the replacement of oligodendrocytes, however, the extent and quality of endogenous remyelination is suboptimal. Emerging evidence indicates that optimal remyelination is restricted by multiple factors including (i) low levels of factors that promote oligodendrogenesis; (ii) cell death among newly generated oligodendrocytes, (iii) inhibitory factors in the post-injury milieu that impede remyelination, and (iv) deficient expression of key growth factors essential for proper re-construction of a highly organized myelin sheath. Considering these challenges, over the past several years, a number of cell-based strategies have been developed to optimize remyelination therapeutically. Outcomes of these basic and preclinical discoveries are promising and signify the importance of remyelination as a mechanism for improving functions in CNS injuries. In this review, we provide an overview on: (1) the precise organization of myelinated axons and the reciprocal axo-myelin interactions that warrant properly balanced physiological activities within the CNS; (2) underlying cause of demyelination and the structural and functional consequences of demyelination in axons following injury and disease; (3) the endogenous mechanisms of oligodendrocyte replacement; (4) the modulatory role of reactive astrocytes and inflammatory cells in remyelination; and (5) the current status of cell-based therapies for promoting remyelination. Careful elucidation of the cellular and molecular mechanisms of demyelination in the pathologic CNS is a key to better understanding the impact of remyelination for CNS repair.

NG2-glia and their functions in the central nervous system

In the central nervous system, NG2-glia represent a neural cell population that is distinct from neurons, astrocytes, and oligodendrocytes. While in the past the main role ascribed to these cells was that of progenitors for oligodendrocytes, in the last years it has become more obvious that they have further functions in the brain. Here, we will discuss some of the most current and highly debated issues regarding NG2-glia: Do these cells represent a heterogeneous population? Can they give rise to different progenies, and does this change under pathological conditions? How do they respond to injury or pathology? What is the role of neurotransmitter signaling between neurons and NG2-glia? We will first give an overview on the developmental origin of NG2-glia, and then discuss whether their distinct properties in different brain regions are the result of environmental influences, or due to intrinsic differences. We will then review and discuss their in vitro differentiation potential and in vivo lineage under physiological and pathological conditions, together with their electrophysiological properties in distinct brain regions and at different developmental stages. Finally, we will focus on their potential to be used as therapeutic targets in demyelinating and neurodegenerative diseases. Therefore, this review article will highlight the importance of NG2-glia not only in the healthy, but also in the diseased brain.

Diverse subtypes of astrocytes and their development during corticogenesis

Astrocytes are one of the most abundant cell types in the mammalian central nervous system, and are known to have a wide variety of physiological functions, including maintenance of neurons, formation of the blood brain barrier, and regulation of synapse functions. Although the migration and positioning of neurons has been extensively studied over the last several decades and many aspects have been uncovered, the process underlying glial development was largely unknown until recently due to the existence of multiple subtypes of glia and the sustained proliferative ability of these cells through adulthood. To overcome these difficulties, new gene transfer techniques and genetically modified mice were developed, and have been gradually revealing when and how astrocytes develop during corticogenesis. In this paper, I review the diversity of astrocytes and summarize our knowledge about their production and migration.

Role of NG2 expressing cells in addiction: a new approach for an old problem

Neuron-glial antigen 2 (NG2) is a proteoglycan expressed predominantly in oligodendrocyte progenitor cells (OPCs). NG2-expressing OPCs (NG2-OPCs) are self-renewing cells that are widely distributed in the gray and white matter areas of the central nervous system. NG2-OPCs can mature into premyelinating oligodendrocytes and myelinating oligodendroglia which serve as the primary source of myelin in the brain. This review characterizes NG2-OPCs in brain structure and function, conceptualizes the role of NG2-OPCs in brain regions associated with negative reinforcement and relapse to drug seeking and discusses how NG2-OPCs are regulated by neuromodulators linked to motivational withdrawal. We hope to provide the readers with an overview of the role of NG2-OPCs in brain structure and function in the context of negative affect state in substance abuse disorders and to integrate our current understanding of the physiological significance of the NG2-OPCs in the adult brain.

Adult olfactory sphere cells are a source of oligodendrocyte and Schwann cell progenitors

The olfactory epithelial layer contains multipotent horizontal basal cells (HBCs) that differentiate into olfactory sensory neurons. Here, we show that rat HBCs express oligodendrocyte progenitor cell (OPC) and astrocyte markers. We generated olfactory sphere (OS) cells in cultures that were derived from adult rat olfactory mucosa. Fluorescence-activated cell sorting and immunofluorescence analyses showed that OS cells also express OPC and astrocyte markers. Interestingly, OS cells underwent oligodendrocyte differentiation in vitro. To study oligodendrocyte differentiation in vivo, OS cells were transplanted into injured rat spinal cords. The transplanted cells integrated into host tissue and differentiated into oligodendrocytes. When transected saphenous nerve ends were encased in collagen-containing silicone tubes with or without OS cells, the transplanted OS cells differentiated into Schwann cells. Our data provide new insights into of the stemness of OS cells.

The identification of transcriptional targets of Ascl1 in oligodendrocyte development

The basic helix-loop-helix (bHLH) transcription factor Ascl1 plays crucial roles in both oligodendrocyte development and neuronal development; however, the molecular target of Ascl1 in oligodendrocyte progenitor cells (OPCs) remains elusive. To identify the downstream targets of Ascl1 in OPCs, we performed gene expression microarray analysis and identified Hes5 as a putative downstream target of Ascl1. In vivo analysis revealed that Ascl1 and Hes5 were coexpressed in early developmental oligodendrocytes in both the telencephalon and the ventral spinal cord. We also found that Hes5 expression was reduced in the OPCs of Ascl1 mutant mice. Furthermore, we demonstrated that Ascl1 directly binds to an E-box region within the Hes5 promoter and regulates Hes5 expression at the transcriptional level. Taken together, these in vivo and in vitro data suggest that Ascl1 induces Hes5 expression in a cell-autonomous manner. Considering the previously known function of Hes5 as a repressor of Ascl1, our data indicate that Hes5 is involved in the negative feedback regulation of Ascl1.

Precursor cell biology and the development of astrocyte transplantation therapies: lessons from spinal cord injury

This review summarizes current progress on development of astrocyte transplantation therapies for repair of the damaged central nervous system. Replacement of neurons in the injured or diseased central nervous system is currently one of the most popular therapeutic goals, but if neuronal replacement is attempted in the absence of appropriate supporting cells (astrocytes and oligodendrocytes), then the chances of restoring neurological functional are greatly reduced. Although the past 20 years have offered great progress on oligodendrocyte replacement therapies, astrocyte transplantation therapies have been both less explored and comparatively less successful. We have now developed successful astrocyte transplantation therapies by pre-differentiating glial restricted precursor (GRP) cells into a specific population of GRP cell-derived astrocytes (GDAs) by exposing the GRP cells to bone morphogenetic protein-4 (BMP) prior to transplantation. When transplanted into transected rat spinal cord, rat and human GDAs(BMP) promote extensive axonal regeneration, rescue neuronal cell survival, realign tissue structure, and restore behavior to pre-injury levels on a grid-walk analysis of volitional foot placement. Such benefits are not provided by GRP cells themselves, demonstrating that the lesion environment does not direct differentiation in a manner optimally beneficial for the restoration of function. Such benefits also are not provided by transplantation of a different population of astrocytes generated from GRP cells exposed to ciliary neurotrophic factor (GDAs(CNTF)), thus providing the first transplantation-based evidence of functional heterogeneity in astrocyte populations. Moreover, lessons learned from the study of rat cells are strongly predictive of outcomes using human cells. Thus, these studies provide successful strategies for the use of astrocyte transplantation therapies for restoration of function following spinal cord injury.

A common progenitor for retinal astrocytes and oligodendrocytes

Developing neural tissue undergoes a period of neurogenesis followed by a period of gliogenesis. The lineage relationships among glial cell types have not been defined for most areas of the nervous system. Here we use retroviruses to label clones of glial cells in the chick retina. We found that almost every clone had both astrocytes and oligodendrocytes. In addition, we discovered a novel glial cell type, with features intermediate between those of astrocytes and oligodendrocytes, which we have named the diacyte. Diacytes also share a progenitor cell with both astrocytes and oligodendrocytes.

Culture system for rodent and human oligodendrocyte specification, lineage progression, and maturation

Here we document protocols for the production, isolation, and maintenance of the oligodendrocyte phenotype from rodent and human neural stem cells. Our unique method relies on a series of chemically defined media, specifically designed and carefully characterized for each developmental stage of oligodendrocytes as they advance from oligodendrocyte progenitors to mature, myelinating oligodendrocytes.

A role for the MAPK/ERK pathway in oligodendroglial differentiation in vitro: stage specific effects on cell branching

The mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway is important for both long-term survival and timing of the progression of oligodendrocyte differentiation. Oligodendroglial cells treated with MEK inhibitor were distinguished by using stage specific markers: NG2 proteoglycan, A2B5, 2'3'nucleotide-cyclic 3'phosphodiesterase (CNPase) and myelin basic protein (MBP), and classified according to their morphology into different developmental stages. Treatment significantly increased the number of cells with more immature morphologies and decreased the number of mature cells. Furthermore, it increased the number of rounded cells that could not be classified into any of the oligodendroglial developmental stages. The strongest effects were usually observed shortly after treatment. Rounded cells were CNPase/MBP positive and they were not stained by anti-NG2 or A2B5, indicating that they were mature cells unable either to extend and/or to maintain their processes. These data showed an effect of the MAPK/ERK pathway on oligodendroglial branching, with possible consequences for the formation of the myelin sheath.

An FGF-responsive astrocyte precursor isolated from the neonatal forebrain

Gliogenesis in the mammalian CNS continues after birth, with astrocytes being generated well into the first two postnatal weeks. In this study, we have isolated an A2B5(+) astrocyte precursor (APC) from the postnatal rat forebrain, which is capable of differentiating into mature astrocytes in serum-free medium without further trophic support. Exposure to basic fibroblast growth factor (bFGF) selectively induces the APCs to proliferate, forming clusters of vimentin(+) cells, which, within 2 weeks, differentiate into GFAP(+) astrocytes. While bFGF functions as a potent mitogen, neither is it necessary to induce or maintain astrocyte differentiation, nor is it capable of maintaining the precursors in an immature, proliferative state. APCs exit the cell cycle and differentiate, even in the continued presence of fibroblast growth factor alone or in combination with other mitogenic factors such as platelet-derived growth factor. Under the culture conditions used, it was not possible to cause the astrocytes to re-enter cell cycle. After transplantation into the neonatal forebrain, APCs differentiated exclusively into astrocytes, regardless of brain region. Initially distributed widely within the forebrain, the precursors are most greatly concentrated within the subventricular zone (SVZ) and subcortical white matter, where they are maintained throughout postnatal development. APCs can be isolated from the SVZ and white matter of animals as late as 4 weeks after birth.

Stem cells and the origin and propagation of brain tumors

In recent years there has been a flood of interest in the relationship between brain tumors and stem cells. Some investigators have focused on the sensitivity of normal stem cells to transformation, others have described phenotypic or functional similarities between tumor cells and stem cells, and still others have suggested that tumors contain a subpopulation of ;;cancer stem cells'' that is crucial for tumor maintenance or propagation. Although all these concepts are interesting and provide insight into the origins and properties of brain tumors, the use of similar terms to describe them has led to confusion. The goal of this review is to sort out some of that confusion and highlight what we know and what we have yet to learn.

NG2 cells generate both oligodendrocytes and gray matter astrocytes

NG2 glia constitute a fourth major glial cell type in the mammalian central nervous system (CNS) that is distinct from other cell types. Although circumstantial evidence suggests that some NG2 glia differentiate into oligodendrocytes, their in vivo fate has not been directly examined. We have used the bacterial artificial chromosome (BAC) modification technique to generate transgenic mice that express DsRed or Cre specifically in NG2-expressing (NG2+) cells. In NG2DsRedBAC transgenic mice, DsRed was expressed specifically in NG2+ cells throughout the postnatal CNS. When the differentiation potential of NG2+ cells in vitro was examined using DsRed+NG2+ cells purified from perinatal transgenic brains, the majority of the cells either remained as NG2+ cells or differentiated into oligodendrocytes. In addition, DsRed+NG2+ cells also differentiated into astrocytes. The in vivo fate of NG2 glia was examined in mice that were double transgenic for NG2creBAC and the Cre reporter Z/EG. In the double transgenic mice, the Cre reporter EGFP was detected in myelinating oligodendrocytes and in a subpopulation of protoplasmic astrocytes in the gray matter of ventrolateral forebrain but not in fibrous astrocytes of white matter. These observations suggest that NG2+ cells are precursors of oligodendrocytes and some protoplasmic astrocytes in gray matter.

Identification of two novel glial-restricted cell populations in the embryonic telencephalon arising from unique origins

Background

Considerably less attention has been given to understanding the cellular components of gliogenesis in the telencephalon when compared to neuronogenesis, despite the necessity of normal glial cell formation for neurological function. Early proposals of exclusive ventral oligodendrocyte precursor cell (OPC) generation have been challenged recently with studies revealing the potential of the dorsal telencephalon to also generate oligodendrocytes. The identification of OPCs generated from multiple regions of the developing telencephalon, together with the need of the embryonic telencephalon to provide precursor cells for oligodendrocytes as well as astrocytes in ventral and dorsal areas, raises questions concerning the identity of the precursor cell populations capable of generating macroglial subtypes during multiple developmental windows and in differing locations.

Results

We have identified progenitor populations in the ventral and dorsal telencephalon restricted to the generation of astrocytes and oligodendrocytes. We further demonstrate that the dorsal glial progenitor cells can be generated de novo from the dorsal telencephalon and we demonstrate their capacity for in vivo production of both myelin-forming oligodendrocytes and astrocytes upon transplantation.

Conclusion

Based on our results we offer a unifying model of telencephalic gliogenesis, with the generation of both oligodendrocytes and astrocytes from spatially separate, but functionally similar, glial restricted populations at different developmental times in the dorsal and ventral CNS.

Branching stochastic processes with immigration in analysis of renewing cell populations

This paper considers the utility of a new class of stochastic branching processes with non-homogeneous immigration in modeling complex renewing cell systems. Such systems typically include the population of stem cells that provides an inexhaustible supply of cells necessary for maintaining the cellular composition of a tissue. A stem cell may be induced to transform (differentiate) into a progenitor cell. Progenitor cells retain the ability to proliferate and their function is believed to provide a quick proliferative response to an increased demand for cells in the population. There may be several sub-types of progenitor cells. Terminally differentiated cells do not divide under normal conditions; they are responsible for maintaining tissue-specific functions. Recent advancements in experimental techniques offer considerable scope for quantitative studies of in vivo cell kinetics based on stochastic modeling of renewing cell populations. However, no ready-made theory is currently available to take full advantage of these advancements. This paper introduces such a theory with a special focus on its feasibility in biological applications.

Glioma stem cells: evidence and limitation

Gliomas, in particular the high-grade anaplastic glioma and glioblastoma multiforme (GBM), are manifested by morphological, genetic and phenotypic heterogeneity. Most of the studies hitherto have been performed on bulk glioma cells, with limited understanding on the origin and the relative contribution of particular glioma cell populations to glioma growth and progression. Recent studies have demonstrated the existence of a small fraction of glioma cells endowed with features of primitive neural progenitor cells and tumor-initiating function. Such cells have been defined as glioma stem cells. However, questions remain as to whether the currently identified glioma stem cells are the cell-of-origin for glioma initiation and progression, or the results of such processes. In this review, we discuss the current evidence and limitation in identifying glioma stem cells and the potential origin of glioma stem cells in the context of post-natal neural cell regeneration and their transformation mechanisms. The implication of these findings for glioma diagnosis and treatment will also be reviewed.

Postnatal cerebral cortical multipotent progenitors: regulatory mechanisms and potential role in the development of novel neural regenerative strategies

In the developing postnatal cerebral cortex, protracted generation of glia and neurons occurs and precise matching of local cell types is needed for the functional organization of regional microdomains characteristic of complex CNS tissues. Recent studies have suggested that multipotent progenitors play an important role in neural lineage elaboration during neurogenesis and gliogenesis after migration from paramedian generative zones. The presence of a separate reservoir of cerebral cortical multipotent cells under strict local environmental regulation would provide an appropriate mechanism for terminal developmental sculpting and for reconstitution of regional cellular pools after injury. We have isolated distinct pools of EGF- and bFGF-responsive multipotent progenitors from the postnatal mammalian cerebral cortex independent of the subventricular zone. These progenitor populations are under tight environmental regulation by specific hierarchies of cytokine subclasses that program the progressive elaboration of intermediate lineage-restricted progenitors and differentiated type I and II astrocytes, myelinating oligodendrocytes and neuronal subtypes that express specific neuromodulatory proteins. Neural lineage development from these cortical multipotent progenitors is a graded developmental process involving sequential induction of specific cytokine receptors, acquisition of factor responsiveness and complex lineage interdependence. The cortical multipotent progenitor pathways program the elaboration of neural lineage species with distinct cellular response properties when compared with analogous species derived from subventricular zone progenitors, indicating that the cortical multipotent cells contribute to the establishment of lineage diversity within the developing cortical cortex. In addition, the cortical multipotent cells generate dynamic intermediate progenitor pools that utilize temporally-coded environmental cues to alter neural fate decisions. These cumulative observations suggest that postnatal cerebral cortical multipotent cells represent a novel set of progenitor pathways necessary for normal mammalian cortical maturation, and may have important implications for our understanding of a wide variety of neuropathological conditions and for the development of more effective regenerative strategies to combat these pervasive neurological disorders.

Redox regulation of precursor cell function: insights and paradoxes

Studies on oligodendrocytes, the myelin-forming cells of the central nervous system, and on the progenitor cells from which they are derived, have provided several novel insights into the role of intracellular redox state in cell function. This review discusses our findings indicating that intracellular redox state is utilized by the organism as a means of regulating the balance between progenitor cell division and differentiation. This regulation is achieved in part through cell-intrinsic differences that modify the response of cells to extracellular signaling molecules, such that cells that are slightly more reduced are more responsive to inducers of cell survival and division and less responsive to inducers of differentiation or cell death. Cells that are slightly more oxidized, in contrast, show a greater response to inducers of differentiation or cell death, but less response to inducers of proliferation or survival. Regulation is also achieved by the ability of exogenous signaling molecules to modify intracellular redox state in a highly predictable manner, such that signaling molecules that promote self-renewal make progenitor cells more reduced and those that promote differentiation make cells more oxidized. In both cases, the redox changes induced by exposure to exogenous signaling molecules are a necessary component of their mode of action. Paradoxically, the results obtained through studies on the oligodendrocyte lineage are precisely the opposite of what might be predicted from a large number of studies demonstrating the ability of reactive oxidative species to enhance the effects of signaling through receptor tyrosine kinase receptors and to promote cell proliferation. Taken in sum, available data demonstrate clearly the existence of two distinct programs of cellular responses to changes in oxidative status. In one of these, becoming even slightly more oxidized is sufficient to inhibit proliferation and induce differentiation. In the second program, similar changes enhance proliferation. It is not yet clear how cells can interpret putatively identical signals in such opposite manners, but it does already seem clear that resolving this paradox will provide insights of considerable relevance to the understanding of normal development, tissue repair, and tumorigenesis.

Effects of glial transplantation on functional recovery following acute spinal cord injury

Numerous efforts have been made to maximize the efficacy of treatment for spinal cord injury (SCI). Recently, oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells have been reported to remyelinate focal areas of demyelinated spinal cord in adult rats. We conducted a study to investigate the therapeutic potential of transplantation of O-2A cells in a rat model of acute SCI. SCI was induced with an NYU Impactor at T9 of rats. O-2A cells labeled with bromodeoxyuridine (BrdU) were transplanted into sites of SCI at 1 week after the induction of SCI. At 6 weeks after cell transplantation, a behavioral test showed significant functional improvement in animals that had received O-2A-cell transplants as compared to animals given cell-culture medium alone. An electrophysiological study revealed that the transplants did not improve the amplitude or latency of somatosensory evoked potentials, but a recording of motor evoked potentials showed that the latency of these potentials in the O-2A-cell-transplant group was significantly shorter than that in the group treated with cell-culture medium. Following transplantation of BrdU-labeled O-2A cells, cells positive for BrdU were detected at and near sites of SCI. Cells labeled for both BrdU and 2',3' -cyclic nucleotide-3-phosphodiesterase were also detected, showing that the transplanted O-2A cells differentiated into oligodendrocytes. By contrast, cells labeled for BrdU and glial fibrillary acidic protein, or for neuronal nuclei antigen, were not detected. Furthermore, a tract-tracing study showed that numbers of retrogradely labeled neurons increased in areas of the brain stem after O-2A-cell transplantation. The study data showed that after being transplanted into an animal with SCI, O-2A cells migrated to the area adjacent to the site of injury and differentiated into oligodendrocytes. The behavioral test and the electrophysiological and morphological studies showed that transplantation of O-2A cells may play an important role in functional recovery and the regeneration of axons after SCI.

Getting a GR(i)P on oligodendrocyte development

One of the most extensively studied of mammalian cells is the oligodendrocyte, the myelin-forming cell of the central nervous system. The ancestry and development of this cell have been studied with every approach utilized by developmental biologists. Such detailed efforts have the potential of providing paradigms of relevance to those interested in analyzing the ancestry and development of any cell type. One of the striking features of studies on the development of oligodendrocytes is that different analytical approaches have led to strikingly different theoretical views regarding the ancestry of these cells. On one extreme is the hypothesis that the steps leading to the generation of oligodendrocytes begin with the generation of a glial-restricted precursor (GRP) cell from neuroepithelial stem cells. GRP cells are thought to be capable of giving rise to all glial cells (including oligodendrocytes and multiple astrocyte populations), but not to neurons, a process that appears to require progression through further stages of greater lineage restriction. On the other extreme is the hypothesis that oligodendrocytes are derived from a precursor cell that generates only motor neurons and oligodendrocytes, with astrocytes being generated through a separate lineage. In this review, we critically consider the various contributions to understanding the ancestry of oligodendrocytes, with particular attention to the respective merits of the GRP cell vs. the motor neuron-oligodendrocyte precursor (MNOP) cell hypothesis. We draw the conclusion that, at present, the strengths of the GRP cell hypothesis outweigh those of the MNOP hypothesis and other hypotheses suggesting oligodendrocytes are developmentally more related to motor neurons than to astrocytes. Moreover, it is clear from existing data that, following the period of motor neuron generation, the major glial precursor cell in the embryonic spinal cord is the GRP cell, and that multiple previous studies on the earliest stages of oligodendrocyte generation in the developing spinal cord have been focused on a differentiation stage of GRP cells.

Neural stem cells in development and regenerative medicine

In the last 10 years, enormous interest in neural stem cells has arisen from both basic and medical points of view. The discovery of neurogenesis in the adult brain has opened our imagination to consider novel strategies for the treatment of neurodegenerative diseases. Characterization of neurogenesis during development plays a fundamental role for the rational design of therapeutic procedures. In the present review, we describe recent progress in the characterization of embryo and adult neural stem cells (NSCs). We emphasize studies directed to determine the in vivo and in vitro differentiation potential of different NSC populations and the influence of the surrounding environment on NSC-specific differentiation. From a different perspective, the fact that NSCs and progenitors continuously proliferate and differentiate in some areas of the adult brain force us to ask how this process can be affected in neurodegenerative diseases. We propose that both abnormal cell death activation and decreased natural neuronal regeneration can contribute to the neuronal loss associated with aging, and perhaps even with that occurring in some neurodegenerative diseases. Furthermore, although NSC activation can be useful to treat neurodegenerative diseases, uncontrolled NSC proliferation, survival, and/or differentiation could cause tumorigenesis in the brain. NSC-mediated therapeutic procedures must take into account this latter possibility.

Inhibiting cell proliferation during formation of the glial scar: effects on axon regeneration in the CNS

Following a CNS lesion many glial cell types proliferate and/or migrate to the lesion site, forming the glial scar. The majority of these cells express chondroitin sulphate proteoglycans (CS-PGs), previously shown to inhibit axonal growth. In this study, in an attempt to diminish glial scar formation and improve axonal regeneration, proliferating cells were eliminated from the lesion site. Adult rats received a continuous infusion of 2% cytosine-D-arabinofuranoside (araC) or saline for 7 days over the lesion site, immediately following a unilateral transection of the right medial forebrain bundle. Additional groups of rats that received subdural infusions prior to the lesion, and lesioned rats which received no infusion, were also compared in the analyses. Animals were killed at 4, 7, 12 or 18 days post-lesion (dpl) and immunohistochemistry was used to determine the effects of these treatments on tyrosine hydroxylase (TH)-lesioned axons, and on the injury response of glial cells. Almost complete elimination of NG2 oligodendrocyte progenitor cells from the lesion site was seen up to 7 dpl in araC-infused animals; reduced numbers of reactive CD11b microglia were also seen but no effects were seen on the injury response of GFAP astrocytes. Significantly more TH axons were seen distal to the lesion in araC-treated brains, but these numbers dwindled by 18 dpl.

Genetic programs and responses of neural stem/progenitor cells during demyelination: potential insights into repair mechanisms in multiple sclerosis

In recent years, it has become evident that the adult mammalian CNS contains a population of neural stem cells (NSCs) described as immature, undifferentiated, multipotent cells, that may be called upon for repair in neurodegenerative and demyelinating diseases. NSCs may give rise to oligodendrocyte progenitor cells (OPCs) and other myelinating cells. This article reviews recent progress in elucidating the genetic programs and dynamics of NSC and OPC proliferation, differentiation, and apoptosis, including the response to demyelination. Emerging knowledge of the molecules that may be involved in such responses may help in the design of future stem cell-based treatment of demyelinating diseases such as multiple sclerosis.

Myelination of a fetus with Pelizaeus-Merzbacher disease: immunopathological study

We report an autopsied case of a 21-gestational-week fetus with duplication of the proteolipid protein (PLP) gene (PLP1). An immunohistochemical study, which can detect the specific expression of PLP, myelin basic protein, myelin-associated glycoprotein, and platelet-derived growth factor receptor alpha subunit in brain tissues, showed that the myelination was almost the same as that of age-matched controls. This result suggests that the development and migration of the oligodendrocyte is normal in Pelizaeus-Merzbacher disease until midgestation. To our knowledge, this is the first report of the myelination of a fetus with duplication of the PLP1 gene.

MR microscopy of magnetically labeled neurospheres transplanted into the Lewis EAE rat brain

Stem cell transplantation is being explored as a new paradigm for the treatment of demyelinating diseases. Magnetically labeled multipotential neural precursor cells were transplanted into the ventricles of rats with acute experimental allergic encephalomyelitis (EAE) and high-resolution (microscopic) MR images were obtained ex vivo. Migration patterns of live cells into periventricular white matter structures could be easily visualized, with a good correlation of the corresponding histopathology. The present results confirm that MR cell tracking can be used to guide the development of successful transplantation protocols.

Mechanisms regulating lineage diversity during mammalian cerebral cortical neurogenesis and gliogenesis

During mammalian cerebral cortical development, neural stem cells (NSCs) present within periventricular generative zones give rise to successive waves of neurons and radial glia, followed by oligodendrocytes and astrocytes. The molecular and cellular mechanisms that orchestrate these precisely timed and progressive maturational events are still largely undefined. These developmental processes are likely to involve the dynamic interplay of environmental signals, cell-cell interactions and transcriptional regulatory events. The bone morphogenetic proteins (BMPs), an expanding subclass of the transforming growth factor beta cytokine superfamily, may represent an important set of environmental cues for these progressive maturational events because of the broad profiles of developmental expression of the requisite BMP ligands, receptor subunits and intracellular transduction elements, and because of their versatile roles in promoting a spectrum of cellular processes intimately involved in progressive neural fate decisions. The BMPs also interact with complementary regional environmental signals such as the basic fibroblast growth factor (bFGF) and sonic hedgehog (Shh) that promote earlier stages of NSC expansion, self-renewal, lineage restriction and incipient lineage commitment. The ability of these cytokines and trophic signals to act within specific neurodevelopmental contexts may, in turn, depend on the composite actions of cell-cell contact-associated signals, such as Notch-Hes-mediated lateral inhibitory pathways, and additional transcriptional modulatory events, such as those mediated by members of the inhibitor of differentiation (ID) gene family that encode a novel set of negative basic helix-loop-helix (bHLH) transcription factors. In this chapter, we will examine the distinct roles of these different classes of developmental cues in defining the biological properties of an integrated cerebral cortical developmental signaling network. Ongoing studies in this exciting area of mammalian central nervous system (CNS) development will help to identify important molecular and cellular targets for evolving pharmacological, gene and stem cell therapeutic interventions to combat the pathological sequelae of a spectrum of acquired and genetic disorders of the central nervous system.

Control of cell survival and proliferation of postnatal PSA-NCAM(+) progenitors

In the present work, we studied the effects of several growth factors on survival and proliferation of freshly isolated neural progenitors expressing the polysialylated form of neural cell adhesion molecule (PSA-NCAM). Cells were obtained from postnatal day 2 rat forebrain, using isolation method. We found that (1) insulin-like growth factor 1 (IGF-1) exerts a powerful survival effect by inhibiting apoptotic cell death, (2) epidermal growth factor (EGF) strongly increases cell proliferation, (3) the combination of IGF-1 plus EGF promotes cellular expansion, (4) basic fibroblast growth factor displays only a weak mitogenic effect, and (5) platelet-derived growth factor-AA (PDGF-AA) has no effect on cell survival and proliferation. These results suggest that the postnatal PSA-NCAM(+) progenitors characterized in the present work may represent a transitional stage, between the embryonic EGF-responsive neural progenitors and the postnatal PSA-NCAM(+) progenitors already described that are PDGF-responsive. For these "early PSA-NCAM(+) progenitors," insulin-like growth factor 1 and EGF seem to play a pivotal role in the control of cell death and cell proliferation.

Expression of the green fluorescent protein in the oligodendrocyte lineage: a transgenic mouse for developmental and physiological studies

We generated a transgenic mouse expressing the enhanced green fluorescent protein (EGFP) under the control of the 2'-3'-cyclic nucleotide 3'-phosphodiesterase (CNP) promoter. EGFP(+) cells were visualized in live tissue throughout embryonic and postnatal development. Immunohistochemical analysis in brain tissue and in sciatic nerve demonstrated that EGFP expression was restricted to cells of the oligodendrocyte and Schwann cell lineages. EGFP was also strongly expressed in "adult" oligodendrocyte progenitors (OPs) and in gray matter oligodendrocytes. Fluorescence-activated cell sorting allowed high-yield purification of EGFP(+) oligodendrocyte-lineage cells from transgenic brains. Electrophysiological patch clamp recordings of EGFP(+) cells in situ demonstrated that OP cells displayed large outward tetraethylammonium (TEA)-sensitive K(+) currents and very small inward currents, whereas mature oligodendrocytes were characterized by expression of large inward currents and small outward K(+) currents. The proliferation rate of EGFP(+) cells in developing white matter decreased with the age of the animals and was strongly inhibited by TEA. Oligodendrocyte development and physiology can be studied in live tissue of CNP-EGFP transgenic mice, which represent a source of pure EGFP(+) oligodendrocyte-lineage cells throughout development.

Selective specification of CNS stem cells into oligodendroglial or neuronal cell lineage: cell culture and transplant studies

Neural stem cells (NSCs) were isolated from embryonic day 16 Sprague-Dawley rats and cultured in a novel serum-free stem cell medium that selected for the growth of NSCs and against the growth of GFAP(+) cells (astrocytes). NSCs maintained in culture for extended periods of time retained immunoreactivity for both nestin and PSA-NCAM, two markers characteristic of the stem cell phenotype. Moreover, using an oligodendrocyte (OL) specification medium, NSCs differentiated into OL as evidenced by their morphology and expression of multiple oligodendrocyte/myelin-specific markers. In addition, NSCs are capable of acquiring a neuronal phenotype as evidenced by expressing neuronal markers, such as neurofilament (NF) and NeuN when cultured in a defined medium for neurons indicating that these cells are also a good source of neuroblasts, which could be used to replace neuronal populations in the brain. We also showed successful propagation and differentiation of NSCs into OL after cryostorage, allowing for the later use of stored NSCs. The long-term goal of culturing NSCs and committed oligodendrocyte progenitors (OLP) is to obtain homogeneous populations for transplantation with the goal of remyelinating the myelin-deficient CNS. Our preliminary experiments carried out on normal and myelin deficient rats demonstrate that these cells survive and migrate extensively in both types of hosts. NSCs grafted as such, as well as cells derived from NSCs exposed to selective specification before grafting, are able to differentiate within the host brain. As expected, NSCs are capable of giving rise to astrocytes in a medium favoring this phenotype.

Regulation of oligodendrocyte development in the vertebrate CNS

The vertebrate central nervous system (CNS) contains two major classes of macroglial cells, oligodendrocytes and astrocytes. Oligodendrocytes are responsible for the formation of myelin in the central nervous system, while the functions of astrocytes are more diverse and less well established. Recent studies have provided new insights into when, where and how these different classes of cell arise during CNS development. The founder cells of the oligodendrocyte lineage initially arise in distinct regions of the ventricular zone during early development as the result of local signals including sonic hedgehog. In the spinal cord, oligodendrocyte precursors appear to share a developmental lineage with motor neurons, although they may also develop from restricted glial precursors. Immature oligodendrocyte precursors are highly migratory. They migrate from their site of origin to developing white matter tracts using a variety of guidance cues including diffusible chemorepellents. The majority of oligodendrocyte precursor proliferation occurs in developing white matter as a result of the local expression of mitogenic signals. Oligodendrocyte precursor cell proliferation is regulated by a number of distinct growth factors that act at distinct stages in the lineage and whose activity is modulated by synergy with other molecules including chemokines. The final matching of oligodendrocyte and axon number is accomplished through a combination of local regulation of cell proliferation, differentiation and cell death. Not all oligodendrocyte precursors differentiate during development, and the adult CNS contains a significant population of precursors. Understanding the regulation of oligodendrogenesis will facilitate the use of these endogenous precursors to enhance repair in a variety of pathological conditions.

In vivo magnetic resonance tracking of magnetically labeled cells after transplantation

During the last few years, the therapeutic use of stem and progenitor cells as a substitute for malfunctioning endogenous cell populations has received considerable attention. Unlike their current use in animal models, the introduction of therapeutic cells in patients will require techniques that can monitor their tissue biodistribution noninvasively. Among the different imaging modalities, magnetic resonance (MR) imaging offers both near-cellular (i.e., 25- to 50-mu) resolution and whole-body imaging capability. In order to be visualized, cells must be labeled with an intracellular tracer molecule that can be detected by MR imaging. Methods have now been developed that make it possible to incorporate sufficient amounts of superparamagnetic iron oxide into cells, enabling their detection in vivo using MR imaging. This is illustrated for (neural stem cell-derived) magnetically labeled oligodendroglial progenitors, transplanted in the central nervous system of dysmyelinated rats. Cells can be followed in vivo for at least 6 weeks after transplantation, with a good histopathologic correlation including the formation of myelin. Now that MR tracking of magnetically labeled cells appears feasible, it is anticipated that this technique may ultimately become an important tool for monitoring the efficacy of clinical (stem) cell transplantation protocols.

Remyelination of the demyelinated CNS: the case for and against transplantation of central, peripheral and olfactory glia

Although originally developed as a research tool for studying glial-glial and glial-axonal interactions, the technique of transplanting glial cell into the central nervous system has more recently been employed as a potential means for repairing persistent demyelination in clinical disease. It has now been clearly established using various experimental models that oligodendrocyte lineage cells, Schwann cells and olfactory ensheathing cells can all produce new myelin sheaths around demyelinated or amyelinated axons following transplantation. However, this property alone does not necessarily mean that transplantation of these cells into demyelinated lesions in clinical disease will be successful. This article considers some of the properties that would be required of a transplanted myelinogenic cell and assesses the advantages and disadvantages of the currently available cell types.

Isolation of a glial-restricted tripotential cell line from embryonic spinal cord cultures

Neuroepithelial stem cells (NEPs), glial-restricted precursors (GRPs), and neuron-restricted precursors (NRPs) are present during early differentiation of the spinal cord and can be identified by cell surface markers. In this article, we describe the properties of GRP cells that have been immortalized using a regulatable v-myc retrovirus construct. Immortalized GRP cells can be maintained in an undifferentiated dividing state for long periods and can be induced to differentiate into two types of astrocytes and into oligodendrocytes in culture. A clonal cell line prepared from immortalized GRP cells, termed GRIP-1, was also shown to retain the properties of a glial-restricted tripotential precursor. Transplantation of green fluorescent protein (GFP)-labeled subclones of the immortalized cells into the adult CNS demonstrates that this cell line can also participate in the in vivo development of astrocytes and oligodendrocytes. Late passages of the immortalized cells undergo limited transdifferentiation into neurons as assessed by expression of multiple neuronal markers. The availability of a conditionally immortalized cell line obviates the difficulties of obtaining a large and homogeneous population of GRPs that can be used for studying the mechanism and signals for glial cell differentiation as well as their application in transplantation protocols.

The tripotential glial-restricted precursor (GRP) cell and glial development in the spinal cord: generation of bipotential oligodendrocyte-type-2 astrocyte progenitor cells and dorsal-ventral differences in GRP cell function

We have found that the tripotential glial-restricted precursor (GRP) cell of the embryonic rat spinal cord can give rise in vitro to bipotential cells that express defining characteristics of oligodendrocyte-type-2 astrocyte progenitor cells (O2A/OPCs). Generation of O2A/OPCs is regulated by environmental signals and is promoted by platelet-derived growth factor (PDGF), thyroid hormone (TH) and astrocyte-conditioned medium. In contrast to multiple observations indicating that oligodendrocyte precursor cells in the embryonic day 14 (E14) spinal cord are ventrally restricted, GRP cells are already present in both the dorsal and ventral spinal cord at E13.5. Ventral-derived GRP cells, however, were more likely to generate O2A/OPCs and/or oligodendrocytes than were their dorsal counterparts when exposed to TH, PDGF, or even bone morphogenetic protein-4. The simplest explanation of our results is that oligodendrocyte generation occurs as a result of generation of GRP cells from totipotent neuroepithelial stem cells, of O2A/OPCs from GRP cells and, finally, of oligodendrocytes from O2A/OPCs. In this respect, the responsiveness of GRP cells to modulators of this process may represent a central control point in the initiation of this critical developmental sequence. Our findings provide an integration between the earliest known glial precursors and the well-studied O2A/OPCs while opening up new questions concerning the intricate spatial and temporal regulation of precursor cell differentiation in the CNS.