Gliogenesis in rat optic nerve: astrocytes are generated in a single wave before oligodendrocytes

Astrocytes of the eye and optic nerve: heterogeneous populations with unique functions mediate axonal resilience and vulnerability to glaucoma

The role of glia, particularly astrocytes, in mediating the central nervous system's response to injury and neurodegenerative disease is an increasingly well studied topic. These cells perform myriad support functions under physiological conditions but undergo behavioral changes - collectively referred to as 'reactivity' - in response to the disruption of neuronal homeostasis from insults, including glaucoma. However, much remains unknown about how reactivity alters disease progression - both beneficially and detrimentally - and whether these changes can be therapeutically modulated to improve outcomes. Historically, the heterogeneity of astrocyte behavior has been insufficiently addressed under both physiological and pathological conditions, resulting in a fragmented and often contradictory understanding of their contributions to health and disease. Thanks to increased focus in recent years, we now know this heterogeneity encompasses both intrinsic variation in physiological function and insult-specific changes that vary between pathologies. Although previous studies demonstrate astrocytic alterations in glaucoma, both in human disease and animal models, generally these findings do not conclusively link astrocytes to causative roles in neuroprotection or degeneration, rather than a subsequent response. Efforts to bolster our understanding by drawing on knowledge of brain astrocytes has been constrained by the primacy in the literature of findings from peri-synaptic 'gray matter' astrocytes, whereas much early degeneration in glaucoma occurs in axonal regions populated by fibrous 'white matter' astrocytes. However, by focusing on findings from astrocytes of the anterior visual pathway - those of the retina, unmyelinated optic nerve head, and myelinated optic nerve regions - we aim to highlight aspects of their behavior that may contribute to axonal vulnerability and glaucoma progression, including roles in mitochondrial turnover and energy provisioning. Furthermore, we posit that astrocytes of the retina, optic nerve head and myelinated optic nerve, although sharing developmental origins and linked by a network of gap junctions, may be best understood as distinct populations residing in markedly different niches with accompanying functional specializations. A closer investigation of their behavioral repertoires may elucidate not only their role in glaucoma, but also mechanisms to induce protective behaviors that can impede the progressive axonal damage and retinal ganglion cell death that drive vision loss in this devastating condition.

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.

Neurovascular Unit-Derived Extracellular Vesicles: From Their Physiopathological Roles to Their Clinical Applications in Acute Brain Injuries

Extracellular vesicles (EVs) form a heterogeneous group of membrane-enclosed structures secreted by all cell types. EVs export encapsulated materials composed of proteins, lipids, and nucleic acids, making them a key mediator in cell–cell communication. In the context of the neurovascular unit (NVU), a tightly interacting multicellular brain complex, EVs play a role in intercellular communication and in maintaining NVU functionality. In addition, NVU-derived EVs can also impact peripheral tissues by crossing the blood–brain barrier (BBB) to reach the blood stream. As such, EVs have been shown to be involved in the physiopathology of numerous neurological diseases. The presence of NVU-released EVs in the systemic circulation offers an opportunity to discover new diagnostic and prognostic markers for those diseases. This review outlines the most recent studies reporting the role of NVU-derived EVs in physiological and pathological mechanisms of the NVU, focusing on neuroinflammation and neurodegenerative diseases. Then, the clinical application of EVs-containing molecules as biomarkers in acute brain injuries, such as stroke and traumatic brain injuries (TBI), is discussed.

SHANK3 deficiency leads to myelin defects in the central and peripheral nervous system

Mutations or deletions of the SHANK3 gene are causative for Phelan–McDermid syndrome (PMDS), a syndromic form of autism spectrum disorders (ASDs). We analyzed Shank3Δ11(−/−) mice and organoids from PMDS individuals to study effects on myelin. SHANK3 was found to be expressed in oligodendrocytes and Schwann cells, and MRI analysis of Shank3Δ11(−/−) mice revealed a reduced volume of the corpus callosum as seen in PMDS patients. Myelin proteins including myelin basic protein showed significant temporal and regional differences with lower levels in the CNS but increased amounts in the PNS of Shank3Δ11(−/−) animals. Node, as well as paranode, lengths were increased and ultrastructural analysis revealed region-specific alterations of the myelin sheaths. In PMDS hiPSC-derived cerebral organoids we observed an altered number and delayed maturation of myelinating cells. These findings provide evidence that, in addition to a synaptic deregulation, impairment of myelin might profoundly contribute to the clinical manifestation of SHANK3 deficiency.

Semaphorins in Adult Nervous System Plasticity and Disease

Semaphorins, originally discovered as guidance cues for developing axons, are involved in many processes that shape the nervous system during development, from neuronal proliferation and migration to neuritogenesis and synapse formation. Interestingly, the expression of many Semaphorins persists after development. For instance, Semaphorin 3A is a component of perineuronal nets, the extracellular matrix structures enwrapping certain types of neurons in the adult CNS, which contribute to the closure of the critical period for plasticity. Semaphorin 3G and 4C play a crucial role in the control of adult hippocampal connectivity and memory processes, and Semaphorin 5A and 7A regulate adult neurogenesis. This evidence points to a role of Semaphorins in the regulation of adult neuronal plasticity. In this review, we address the distribution of Semaphorins in the adult nervous system and we discuss their function in physiological and pathological processes.

Interaction of Microglia and Astrocytes in the Neurovascular Unit

The interaction between microglia and astrocytes significantly influences neuroinflammation. Microglia/astrocytes, part of the neurovascular unit (NVU), are activated by various brain insults. The local extracellular and intracellular signals determine their characteristics and switch of phenotypes. Microglia and astrocytes are activated into two polarization states: the pro-inflammatory phenotype (M1 and A1) and the anti-inflammatory phenotype (M2 and A2). During neuroinflammation, induced by stroke or lipopolysaccharides, microglia are more sensitive to pathogens, or damage; they are thus initially activated into the M1 phenotype and produce common inflammatory signals such as IL-1 and TNF-α to trigger reactive astrocytes into the A1 phenotype. These inflammatory signals can be amplified not only by the self-feedback loop of microglial activation but also by the unique anatomy structure of astrocytes. As the pathology further progresses, resulting in local environmental changes, M1-like microglia switch to the M2 phenotype, and M2 crosstalk with A2. While astrocytes communicate simultaneously with neurons and blood vessels to maintain the function of neurons and the blood-brain barrier (BBB), their subtle changes may be identified and responded by astrocytes, and possibly transferred to microglia. Although both microglia and astrocytes have different functional characteristics, they can achieve immune "optimization" through their mutual communication and cooperation in the NVU and build a cascaded immune network of amplification.

Innate immunity and cellular senescence: The good and the bad in the developmental and aged brain

Ongoing studies evidence cellular senescence in undifferentiated and specialized cells from tissues of all ages. Although it is believed that senescence plays a wider role in several stress responses in the mature age, its participation in certain physiological and pathological processes throughout life is coming to light. The "senescence machinery" has been observed in all brain cell populations, including components of innate immunity (e.g., microglia and astrocytes). As the beneficial versus detrimental implications of senescence is an open question, we aimed to analyze the contribution of immune responses in regulatory mechanisms governing its distinct functions in healthy (development, organogenesis, danger patrolling events) and diseased brain (glioma, neuroinflammation, neurodeneration), and the putative connection between cellular and molecular events governing the 2 states. Particularly this review offers new insights into the complex roles of senescence both as a chronological event as age advances, and as a molecular mechanism of brain homeostasis through the important contribution of innate immune responses and their crosstalk with neighboring cells in brain parenchyma. We also highlight the impact of the recently described glymphatic system and brain lymphatic vasculature in the interplay between peripheral and central immune surveillance and its potential implication during aging. This will open new ways to understand brain development, its deterioration during aging, and the occurrence of several oncological and neurodegenerative diseases.

Opiate Drugs with Abuse Liability Hijack the Endogenous Opioid System to Disrupt Neuronal and Glial Maturation in the Central Nervous System

The endogenous opioid system, comprised of multiple opioid neuropeptide and receptor gene families, is highly expressed by developing neural cells and can significantly influence neuronal and glial maturation. In many central nervous system (CNS) regions, the expression of opioid peptides and receptors occurs only transiently during development, effectively disappearing with subsequent maturation only to reemerge under pathologic conditions, such as with inflammation or injury. Opiate drugs with abuse liability act to modify growth and development by mimicking the actions of endogenous opioids. Although typically mediated by μ-opioid receptors, opiate drugs can also act through δ- and κ-opioid receptors to modulate growth in a cell-type, region-specific, and developmentally regulated manner. Opioids act as biological response modifiers and their actions are highly contextual, plastic, modifiable, and influenced by other physiological processes or pathophysiological conditions, such as neuro-acquired immunodeficiency syndrome. To date, most studies have considered the acute effects of opiates on cellular maturation. For example, activating opioid receptors typically results in acute growth inhibition in both neurons and glia. However, with sustained opioid exposure, compensatory factors become operative, a concept that has been largely overlooked during CNS maturation. Accordingly, this article surveys prior studies on the effects of opiates on CNS maturation, and also suggests new directions for future research in this area. Identifying the cellular and molecular mechanisms underlying the adaptive responses to chronic opiate exposure (e.g., tolerance) during maturation is crucial toward understanding the consequences of perinatal opiate exposure on the CNS.

The Indispensable Roles of Microglia and Astrocytes during Brain Development

Glia are essential for brain functioning during development and in the adult brain. Here, we discuss the various roles of both microglia and astrocytes, and their interactions during brain development. Although both cells are fundamentally different in origin and function, they often affect the same developmental processes such as neuro-/gliogenesis, angiogenesis, axonal outgrowth, synaptogenesis and synaptic pruning. Due to their important instructive roles in these processes, dysfunction of microglia or astrocytes during brain development could contribute to neurodevelopmental disorders and potentially even late-onset neuropathology. A better understanding of the origin, differentiation process and developmental functions of microglia and astrocytes will help to fully appreciate their role both in the developing as well as in the adult brain, in health and disease.

Glia-neuron interactions in the mammalian retina

The mammalian retina provides an excellent opportunity to study glia-neuron interactions and the interactions of glia with blood vessels. Three main types of glial cells are found in the mammalian retina that serve to maintain retinal homeostasis: astrocytes, Müller cells and resident microglia. Müller cells, astrocytes and microglia not only provide structural support but they are also involved in metabolism, the phagocytosis of neuronal debris, the release of certain transmitters and trophic factors and K(+) uptake. Astrocytes are mostly located in the nerve fibre layer and they accompany the blood vessels in the inner nuclear layer. Indeed, like Müller cells, astrocytic processes cover the blood vessels forming the retinal blood barrier and they fulfil a significant role in ion homeostasis. Among other activities, microglia can be stimulated to fulfil a macrophage function, as well as to interact with other glial cells and neurons by secreting growth factors. This review summarizes the main functional relationships between retinal glial cells and neurons, presenting a general picture of the retina recently modified based on experimental observations. The preferential involvement of the distinct glia cells in terms of the activity in the retina is discussed, for example, while Müller cells may serve as progenitors of retinal neurons, astrocytes and microglia are responsible for synaptic pruning. Since different types of glia participate together in certain activities in the retina, it is imperative to explore the order of redundancy and to explore the heterogeneity among these cells. Recent studies revealed the association of glia cell heterogeneity with specific functions. Finally, the neuroprotective effects of glia on photoreceptors and ganglion cells under normal and adverse conditions will also be explored.

Phenotype overlap in glial cell populations: astroglia, oligodendroglia and NG-2(+) cells

The extent to which NG-2(+) cells form a distinct population separate from astrocytes is central to understanding whether this important cell class is wholly an oligodendrocyte precursor cell (OPC) or has additional functions akin to those classically ascribed to astrocytes. Early immuno-staining studies indicate that NG-2(+) cells do not express the astrocyte marker GFAP, but orthogonal reconstructions of double-labeled confocal image stacks here reveal a significant degree of co-expression in individual cells within post-natal day 10 (P10) and adult rat optic nerve (RON) and rat cortex. Extensive scanning of various antibody/fixation/embedding approaches identified a protocol for selective post-embedded immuno-gold labeling. This first ultrastructural characterization of identified NG-2(+) cells revealed populations of both OPCs and astrocytes in P10 RON. NG-2(+) astrocytes had classic features including the presence of glial filaments but low levels of glial filament expression were also found in OPCs and myelinating oligodendrocytes. P0 RONs contained few OPCs but positively identified astrocytes were observed to ensheath pre-myelinated axons in a fashion previously described as a definitive marker of the oligodendrocyte lineage. Astrocyte ensheathment was also apparent in P10 RONs, was absent from developing nodes of Ranvier and was never associated with compact myelin. Astrocyte processes were also shown to encapsulate some oligodendrocyte somata. The data indicate that common criteria for delineating astrocytes and oligodendroglia are insufficiently robust and that astrocyte features ascribed to OPCs may arise from misidentification.

Multicolor analysis of oligodendrocyte morphology, interactions, and development with Brainbow

Oligodendrocytes are the myelinating cells of the central nervous system. Multiple markers are available to analyze the populations of oligodendroglial cells and their precursors during development and in pathological conditions. However, the behavior of oligodendrocytes remains poorly characterized in vivo, especially at the level of individual cells. Studying this aspect has been impaired so far by the lack of suitable methods for visualizing single oligodendrocytes, their processes, and their interactions during myelination. Here, we have used multicolor labeling technology to single-out simultaneously many individual oligodendrocytes in the postnatal mouse optic nerve. This method is based on Brainbow, a transgenic system for stochastic expression of multiple fluorescent protein genes through Cre-lox recombination, previously used for visualizing axons and neurons. We used tamoxifen-inducible recombination in myelinating cells of Brainbow transgenic mice to obtain multicolor labeling of oligodendrocytes. We show that the palette of colors expressed by labeled oligodendrocytes is tamoxifen dependent, with the highest doses producing the densest and most colorful labeling. At low doses of tamoxifen, the morphology of single or small clusters of fluorescent oligodendrocytes can be studied during postnatal development and in adult. Internodes are labeled to their extremities, revealing nodes of Ranvier. The new mouse model presented here opens new possibilities to explore the organization and development of the oligodendrocyte network with single-cell resolution.

Development of astrocytes in the vertebrate eye

Astrocytes represent the earliest glial population in the embryonic optic nerve, contributing critically to retinal angiogenesis and formation of brain-retinal-barrier. Despite of many developmental and clinical implications of astrocytes, answers to some of the most fundamental questions of this unique type of glial cells remain elusive. This review provides an overview of the current knowledge about the origination, proliferation, and differentiation of astrocytes, their journey from the optic nerve toward the neuroretina, and their involvement in physiological and pathological development of the visual system.

Interactions of HIV and drugs of abuse: the importance of glia, neural progenitors, and host genetic factors

Considerable insight has been gained into the comorbid, interactive effects of HIV and drug abuse in the brain using experimental models. This review, which considers opiates, methamphetamine, and cocaine, emphasizes the importance of host genetics and glial plasticity in driving the pathogenic neuron remodeling underlying neuro-acquired immunodeficiency syndrome and drug abuse comorbidity. Clinical findings are less concordant than experimental work, and the response of individuals to HIV and to drug abuse can vary tremendously. Host-genetic variability is important in determining viral tropism, neuropathogenesis, drug responses, and addictive behavior. However, genetic differences alone cannot account for individual variability in the brain "connectome." Environment and experience are critical determinants in the evolution of synaptic circuitry throughout life. Neurons and glia both exercise control over determinants of synaptic plasticity that are disrupted by HIV and drug abuse. Perivascular macrophages, microglia, and to a lesser extent astroglia can harbor the infection. Uninfected bystanders, especially astroglia, propagate and amplify inflammatory signals. Drug abuse by itself derails neuronal and glial function, and the outcome of chronic exposure is maladaptive plasticity. The negative consequences of coexposure to HIV and drug abuse are determined by numerous factors including genetics, sex, age, and multidrug exposure. Glia and some neurons are generated throughout life, and their progenitors appear to be targets of HIV and opiates/psychostimulants. The chronic nature of HIV and drug abuse appears to result in sustained alterations in the maturation and fate of neural progenitors, which may affect the balance of glial populations within multiple brain regions.

HIV-1 alters neural and glial progenitor cell dynamics in the central nervous system: coordinated response to opiates during maturation

HIV-associated neurocognitive disorders (HANDs) are common sequelae of human immunodeficiency virus (HIV) infection, even when viral titers are well controlled by antiretroviral therapy. Evidence in patients and animal models suggests that neurologic deficits are increased during chronic opiate exposure. We have hypothesized that central nervous system (CNS) progenitor cells in both adult and developing CNS are affected by HIV infection and that opiates exacerbate these effects. To examine this question, neural progenitors were exposed to HIV-1 Tat(1-86) in the developing brain of inducible transgenic mice and in vitro. We examined whether Tat affected the proliferation or balance of progenitor populations expressing nestin, Sox2, and Olig2. Disease relevance was further tested by exposing human-derived progenitors to supernatant from HIV-1 infected monocytes. Studies concentrated on striatum, a region preferentially targeted by HIV and opiates. Results were similar among experimental paradigms. Tat or HIV exposure reduced the proliferation of undifferentiated (Sox2(+)) progenitors and oligodendroglial (Olig2(+)) progenitors. Coexposure to morphine exacerbated the effects of Tat or HIV-1(SF162) supernatant, but partially reversed HIV-1(IIIB) supernatant effects. Populations of Sox2(+) and Olig2(+) cells were also reduced by Tat exposure, although progenitor survival was unaffected. In rare instances, p24 immunolabeling was detected in viable human progenitors by confocal imaging. The vulnerability of progenitors is likely to distort the dynamic balance among neuron/glial populations as the brain matures, perhaps contributing to reports that neurologic disease is especially prevalent in pediatric HIV patients. Pediatric disease is atypical in developed regions but remains a serious concern in resource-limited areas where infection occurs commonly at birth and through breast feeding.

Looking back

In this Perspective, I review my scientific career, which began after I trained in medicine in Montreal and in neurology in Boston. I started in immunology in London with Avrion Mitchison, using antibodies against cell-surface antigens to study the development and functions of mouse T and B cells. The finding that antibody binding causes immunoglobulin on B cells to redistribute rapidly on the cell surface and be endocytosed transformed me from an immunologist into a cell biologist. I moved with Mitchison to University College London, where my colleagues and I used the antibody approach to study cells of the rodent nervous system, focusing on the intrinsic and extrinsic molecular mechanisms that control the development and behavior of myelinating glial cells-Schwann cells and oligodendrocytes. I retired from active research in 2002 and now spend much of my time on scientific advisory boards and thinking about autism.

Early postnatal proteolipid promoter-expressing progenitors produce multilineage cells in vivo

Proteolipid promoter (plp promoter) activity in the newborn mouse CNS is restricted to NG2-expressing oligodendroglial progenitor cells and oligodendrocytes. There are two populations of NG2 progenitors based on their plp promoter expression. Whereas the general population of NG2 progenitors has been shown to be multipotent in vitro and after transplantation, it is not known whether the subpopulation of plp promoter-expressing NG2 progenitors [i.e., plp promoter-expressing NG2 progenitors (PPEPs)] has the potential to generate multilineage cells during normal development in vivo. We addressed this issue by fate mapping Plp-Cre-ER(T2)/Rosa26-EYFP (PCE/R) double-transgenic mice, which carried an inducible Cre gene under the control of the plp promoter. Expression of the enhanced yellow fluorescent protein (EYFP) reporter gene in PPEPs was elicited by administering tamoxifen to postnatal day 7 PCE/R mice. We have demonstrated that early postnatal PPEPs, which had been thought to be restricted to the oligodendroglial lineage, also unexpectedly gave rise to a subset of immature, postmitotic, protoplasmic astrocytes in the gray matter of the spinal cord and ventral forebrain, but not in white matter. Furthermore, these PPEPs also gave rise to small numbers of immature, DCX (doublecortin)-negative neurons in the ventral forebrain, dorsal cerebral cortex, and hippocampus. EYFP-labeled representatives of each of these lineages survived to adulthood. These findings indicate that there are regional differences in the fates of neonatal PPEPs, which are multipotent in vivo, giving rise to oligodendrocytes, astrocytes, and neurons.

Functional glutamate transport in rodent optic nerve axons and glia

Recent findings suggest that synaptic-type glutamate signaling operates between axons and their supporting glial cells. Glutamate reuptake will be a necessary component of such a system. Evidence for glutamate-mediated damage of oligodendroglia somata and processes in white matter suggests that glutamate regulation in white matter structures is also of clinical importance. The expression of glutamate transporters was examined in postnatal Day 14-17 (P14-17) mouse and in mature mouse and rat optic nerve using immuno-histochemistry and immuno-electron microscopy. EAAC1 was the major glutamate transporter detected in oligodendroglia cell membranes in both developing and mature optic nerve, while GLT-1 was the most heavily expressed transporter in the membranes of astrocytes. Both EAAC1 and GLAST were also seen in adult astrocytes, but there was little membrane expression of either at P14-17. GLAST, EAAC1, and GLT-1 were expressed in P14-17 axons with marked GLT-1 expression in the axolemma, while in mature axons EAAC1 was abundant at the node of Ranvier. Functional glutamate transport was probed in P14-17 mouse optic nerve revealing Na+-dependent, TBOA-blockable uptake of D-aspartate in astrocytes, axons, and oligodendrocytes. The data show that in addition to oligodendroglia and astrocytes, axons represent a potential source for extracellular glutamate in white matter during ischaemic conditions, and have the capacity for Na(+)-dependent glutamate uptake. The findings support the possibility of functional synaptic-type glutamate release from central axons, an event that will require axonal glutamate reuptake.

The mechanisms of acute ischemic injury in the cell processes of developing white matter astrocytes

Astrocytes are fundamentally important to the maintenance and proper functioning of the central nervous system. During the period of development when myelination is occurring, white matter astrocytes are particularly sensitive to ischemic injury and their failure to regulate glutamate during ischemic conditions may be an important factor in excitotoxic injury. Here, we have identified key mechanisms of injury that operate on the processes of immature white matter astrocytes during oxygen-glucose deprivation (OGD) using GFAP-GFP mice. Oxygen-glucose deprivation produced a parallel loss of astrocyte processes and somata, assessed by both the retention of GFP fluorescence within these structures and by quantitative electron microscopy. Oxygen-glucose deprivation-induced process loss was Ca(2+) independent and had two distinct mechanisms. Substituting either extracellular Na(+) or Cl(-), or perfusion with the Na-K-Cl co-transport blocker bumetanide, provided protection up to 40 mins of OGD but not beyond that point. HCO(3)(-) substitution or perfusion with 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid provided complete protection of the processes up to 60 mins of OGD. Zero-Na(+)/zero-K(+) conditions provided complete protection from OGD-induced injury of processes and somata at all time points. We conclude that acute ischemic-type injury of immature astrocytes follows a cytotoxic ion influx mediated in part by Na-K-Cl co-transport and in part by Na(+)- and K(+)-dependent HCO(3)(-) transport, a mechanism that is common to both cell processes and somata. This work provides a basis on which preventative strategies may be developed to protect white matter astrocytes from ischemic injury in susceptible individuals.

Oligodendrocyte precursor cells generate pituicytes in vivo during neurohypophysis development

In the vertebrate brain, much remains to be understood concerning the origin of glial cell diversity and the potential lineage relationships between the various types of glia. Besides astrocytes and myelin-forming oligodendrocytes, other macroglial cell populations are found in discrete areas of the central nervous system (CNS). They share functional features with astrocytes and oligodendrocytes but also display specific characteristics. Such specialized cells, called pituicytes, are located in the neurohypophysis (NH). Our work focuses on the lineage of the pituicytes during rodent development. First, we show that cells identified with a combination of oligodendrocyte precursor cell (OPC) markers are present in the developing rat NH. In culture, neonatal NH progenitors also share major functional characteristics with OPCs, being both migratory and bipotential, i.e. able to give rise to type 2 astrocytes and oligodendrocytes. We then observe that, either in vitro or after transplantation into myelin-deficient Shiverer brain, pieces of NH generate myelinating oligodendrocytes, confirming the oligodendrogenic potentiality of NH cells. However, no mature oligodendrocyte can be found in the NH. This led us to hypothesize that the OPCs present in the developing NH might be generating other glial cells, especially the pituicytes. Consistent with this hypothesis, the OPCs appear during NH development before pituicytes differentiate. Finally, we establish a lineage relationship between olig1+ cells, most likely OPCs, and the pituicytes by fate-mapping experiments using genetically engineered mice. This constitutes the first demonstration that OPCs generate glial cells other than oligodendrocytes in vivo.

A role for Noggin in the development of oligodendrocyte precursor cells

Oligodendrocyte precursor cells (OPCs) can be differentiated in culture into either oligodendrocytes or type-2 astrocytes (2As), depending on the culture conditions. Whereas oligodendrocyte development can occur in the absence of inducing signals, 2A development apparently cannot. Fetal calf serum (FCS) and bone morphogenetic proteins (BMPs) are powerful inducers of 2A development in culture, but there is no compelling evidence that OPCs develop into astrocytes in vivo. We show here that BMPs are made by glial cells in the developing rat optic nerve, raising the question of why 2As do not normally develop in the optic nerve. We demonstrate that the BMP antagonist Noggin is strongly expressed by both OPCs and type-1 astrocytes in the developing optic nerve. We also show that depletion of Noggin by a small interference RNA inhibits OPC proliferation and induces 2A differentiation in the presence of a low, non-2A-inducing concentration of FCS. By contrast, enforced expression of Noggin in OPCs blocks FCS-induced 2A differentiation. These findings suggest that BMPs in FCS are largely responsible for the 2A-inducing activity of FCS and that Noggin may normally inhibit the formation of 2As in the developing CNS.

Regulation of CNS glial phenotypes in N20.1 cells

The N20.1 oligodendroglial cell line, immortalized with SV40 T antigen, simultaneously expresses oligodendroglial markers and glial fibrillary acidic protein (GFAP), an astroglial marker. This study examines the plasticity of N20.1 cells with regard to GFAP expression, and its relationship to expression of SV40 T antigen, p53, and a novel nuclear antigen detected by the A007 monoclonal antibody. Marked changes occur in GFAP levels and cell morphology when N20.1 cells are switched from the permissive temperature (34 degrees C) to the non-permissive temperature (39 degrees C), and with cyclic AMP elevation at 39 degrees C. At 34 degrees C, levels of GFAP are high; when cells are switched to 39 degrees C, GFAP levels decrease significantly, then increase slightly when forskolin is added. At both temperatures, the cells display feathery GFAP immunostaining. When forskolin is added at 39 degrees C, however, cells display bright fibrous GFAP staining in elongated processes. The changes in GFAP were compared to changes in T antigen and p53. As expected, the decrease in T antigen at 39 degrees C was accompanied by movement of p53 from the nucleus to cytoplasm. Total p53 levels did not change, however, and forskolin did not alter the respective distribution or levels of p53 at either temperature. At both temperatures, the cell bodies and processes show internal expression of sulfatide, as demonstrated with the O4, Sulph I, and A007 antibodies. We show, for the first time, abundant nuclear immunoreactivity with the A007 monoclonal antibody in the N20.1 cells. This nuclear reactivity is seen at 34 degrees C, but not at 39 degrees C, similar to p53, and is not detected with the other sulfatide antibodies. Double-label immunostaining shows that the nuclear A007 immunoreactivity is co-localized in nuclear structures with T antigen and p53 at 34 degrees C, but is not found in every nucleus containing these antigens. We conclude that regulation of GFAP expression and morphology in N20.1 cells is dependent on a combination of T antigen expression and level of cAMP and may be related to regulation of p53 and the A007 nuclear antigen.

Oligodendroglia in developmental neurotoxicity

The developing nervous system has been long recognized as a primary target for a variety of toxicants. To date, most efforts to understand the impact of neurotoxic agents on the brain have focused primarily on neurons and to a lesser degree astroglia as cellular targets. The role of oligodendroglia, the myelin-forming cells in the central nervous system (CNS), in developmental neurotoxicity has been emphasized only in recent years. Oligodendrocytes originate from migratory, mitotic progenitors that mature progressively into postmitotic myelinating cells. During differentiation, oligodendroglial lineage cells pass through a series of distinct phenotypic stages that are characterized by different proliferative capacities and migratory abilities, as well as dramatic changes in morphology with sequential expression of unique developmental markers. In recent years, it has become appreciated that oligodendrocyte lineage cells have important functions other than those related to myelin formation and maintenance, including participation in neuronal survival and development, as well as neurotransmission and synaptic function. Substantial knowledge has accumulated on the control of oligodendroglial survival, migration, proliferation, and differentiation, as well as the cellular and molecular events involved in oligodendroglial development and myelin formation. Recently, studies have been initiated to address the role of oligodendrocyte lineage cells in neurotoxic processes. This article examines recent progress in oligodendroglial biology, focuses attention on the characteristic features of the oligodendrocyte developmental lineage as a model system for neurotoxicological studies, and explores the role of oligodendrocyte lineage cells in developmental neurotoxicity. The potential role of oligodendroglia in environmental lead neurotoxicity is presented to exemplify this thesis.

Directional guidance of oligodendroglial migration by class 3 semaphorins and netrin-1

Oligodendrocytes, the myelin-forming cells of the CNS, are generated from multiple foci distributed along the developing neural tube. Little is known about the endogenous guidance cues controlling the migration of oligodendrocyte precursor cells (OPCs) from their site of emergence toward their final destination, mainly the future white matter tracts. During embryonic development, the optic nerve is populated by OPCs originating in the diencephalon that migrate from the chiasm toward the retina. Here we show that OPCs migrating into the embryonic optic nerve express the semaphorin receptors neuropilin-1 and -2, as well as deleted in colorectal cancer (DCC) and, to a lesser extend unc5H1, two of the netrin-1 receptors. Using a functional migration assay, we provide evidence that Sema 3A and netrin-1 exert opposite chemotactic effects, repulsive or attractive, respectively, on embryonic OPCs. In addition, we show that Sema 3F has a dual effect, chemoattractive and mitogenic on embryonic OPCs. The localization of cells expressing Sema 3A, Sema 3F, and netrin-1 is consistent with a role for these ligands in the migration of OPCs in the embryonic optic nerve. Altogether, our results suggest that the migration of OPCs in the embryonic optic nerve is modulated by a balance of effects mediated by members of the semaphorin and netrin families.

Down-regulation of GAP-43 During Oligodendrocyte Development and Lack of Expression by Astrocytes In Vivo: Implications for Macroglial Differentiation

The discovery of molecular markers which are selectively expressed during the development of specific classes of rat central nervous system macroglia has greatly advanced our understanding of how these cells are related. In particular, it has been shown in tissue culture that oligodendrocytes and some astrocytes (type-2) may be derived from a common progenitor cell (O-2A progenitor). However, the existence of type-2 astrocytes in vivo has yet to be unequivocally established. Recently, it has been reported that the neural-specific growth-associated protein-43 (GAP-43, otherwise known as B-50, F1, pp46 and neuromodulin) may be expressed by cells of the O-2A lineage in vitro. We set out to examine the cellular specificity of GAP-43 in O-2A progenitors and their descendants in vitro and in vivo. Using a polyclonal antiserum against a GAP-43 fusion protein we have shown the presence of immunoreactive GAP-43 in the membranes of bipotential O-2A glial progenitor cells and type-2 astrocytes by Western blotting and immunocytochemistry of cells in culture. In contrast to previous studies, double labelling with mature oligodendrocyte markers showed that GAP-43 is down-regulated during oligodendrocyte differentiation in vitro. Immunohistochemical staining of sections of developing rat brain demonstrated the same developmental regulation of GAP-43, suggesting that oligodendrocytes only express GAP-43 at immature stages. In addition, normal and reactive astrocytes in tissue sections were not labelled with GAP-43.

Common developmental requirement for Olig function indicates a motor neuron/oligodendrocyte connection

The oligodendrocyte lineage genes Olig1 and Olig2 encode related bHLH proteins that are coexpressed in neural progenitors. Targeted disruption of these two genes sheds light on the ontogeny of oligodendroglia and genetic requirements for their development from multipotent CNS progenitors. Olig2 is required for oligodendrocyte and motor neuron specification in the spinal cord. Olig1 has roles in development and maturation of oligodendrocytes, evident especially within the brain. Both Olig genes contribute to neural pattern formation. Neither Olig gene is required for astrocytes. These findings, together with fate mapping analysis of Olig-expressing cells, indicate that oligodendrocytes are derived from Olig-specified progenitors that give rise also to neurons.

Architecture of the optic chiasm and the mechanisms that sculpt its development

At the optic chiasm the two optic nerves fuse, and fibers from each eye cross the midline or turn back and remain uncrossed. Having adopted their pathways the fibers separate to form the two optic tracts. Research into the architecture and development of the chiasm has become an area of increasing interest. Many of its mature features are complex and vary between different animal types. It is probable that numerous factors sculpt its development. The separate ganglion cell classes cross the midline at different locations along the length of the chiasm, reflecting their distinct periods of production as the chiasm develops in a caudo-rostral direction. In some mammals, uncrossed axons are mixed with crossed axons in each hemi-chiasm, whereas in others they remain segregated. These configurations are the product of different developmental mechanisms. The morphology of the chiasm changes significantly during development. Neurons, glia, and the signals they produce play a role in pathway selection. In some animals fiber-fiber interactions are also critical, but only where crossed and uncrossed pathways are mixed in each hemi-chiasm. The importance of the temporal dimension in chiasm development is emphasized by the fact that in some animals uncrossed ganglion cells are generated abnormally early in relation to their retinal location. Furthermore, in albinos, where many cells do not exit the cell cycle at normal times, there are systematic chiasmatic abnormalities in ganglion cell projections.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Timing of CNS cell generation: a programmed sequence of neuron and glial cell production from isolated murine cortical stem cells

Multipotent stem cells that generate both neurons and glia are widespread components of the early neuroepithelium. During CNS development, neurogenesis largely precedes gliogenesis: how is this timing achieved? Using clonal cell culture combined with long-term time-lapse video microscopy, we show that isolated stem cells from the embryonic mouse cerebral cortex exhibit a distinct order of cell-type production: neuroblasts first and glioblasts later. This is accompanied by changes in their capacity to make neurons versus glia and in their response to the mitogen EGF. Hence, multipotent stem cells alter their properties over time and undergo distinct phases of development that play a key role in scheduling production of diverse CNS cells.

Long-term culture of purified postnatal oligodendrocyte precursor cells. Evidence for an intrinsic maturation program that plays out over months

Oligodendrocytes myelinate axons in the vertebrate central nervous system (CNS). They develop from precursor cells (OPCs), some of which persist in the adult CNS. Adult OPCs differ in many of their properties from OPCs in the developing CNS. In this study we have purified OPCs from postnatal rat optic nerve and cultured them in serum-free medium containing platelet-derived growth factor (PDGF), the main mitogen for OPCs, but in the absence of thyroid hormone in order to inhibit their differentiation into oligodendrocytes. We find that many of the cells continue to proliferate for more than a year and progressively acquire a number of the characteristics of OPCs isolated from adult optic nerve. These findings suggest that OPCs have an intrinsic maturation program that progressively changes the cell's phenotype over many months. When we culture the postnatal OPCs in the same conditions but with the addition of basic fibroblast growth factor (bFGF), the cells acquire these mature characteristics much more slowly, suggesting that the combination of bFGF and PDGF, previously shown to inhibit OPC differentiation, also inhibits OPC maturation. The challenge now is to determine the molecular basis of such a protracted maturation program and how the program is restrained by bFGF.

Oligodendrocyte development and thyroid hormone

Thyroid hormone plays an important role in brain development and is essential to ensure a normal myelination. The effects of thyroid hormone are mediated by nuclear thyroid hormone receptors, which act as ligand-regulated transcription factors. There are several isoforms encoded by two genes, alpha and beta. Developmental studies have shown that alpha isoforms are widely expressed in the fetal brain, while beta isoforms expression is more restricted with a dramatic increase that begins at birth in the rat. Remarkably, receptor number reaches maximal levels by postnatal day 10, when serum thyroid hormone levels also peak and myelination is the most prominent event in the developing rat brain. Likewise, oligodendrocyte precursor cells express alpha isoforms and expression of the beta isoforms is confined to the differentiated oligodendrocytes, suggesting that these isoforms might mediate different thyroid hormone effects in the oligodendrocyte lineage. Thyroid hormone acts at multiple steps in the development of oligodendrocytes: (a) Early in development, it can function as an instructive signal for the generation of oligodendrocytes and enhance the proliferation of the committed preprecursor oligodendrocyte cells. (b) Thyroid hormone regulates the number of oligodendrocyte generated by directly promoting their differentiation. Since oligodendrocytes are produced in vitro after the same period in culture regardless of whether thyroid hormone was added to the cultures, it has been suggested that thyroid hormone is required for neither the timing nor the generation of oligodendrocytes, but is necessary to achieve adequate oligodendrocyte numbers. (c) Finally, thyroid hormone increases morphological and functional maturation of postmitoitic oligodendrocytes by stimulation of the expression of various myelin genes.

Purification and characterization of astrocyte precursor cells in the developing rat optic nerve

The signaling interactions that control oligodendrocyte generation from their precursor cells have been studied intensively. Much less is known about how astrocyte generation is normally controlled. Here we report the purification and characterization of astrocyte precursor cells (APCs) from the developing rat optic nerve. APCs are antigenically distinct from astrocytes. Both cell types are Pax2(+) and vimentin+, whereas astrocytes are GFAP+ and S100beta+, and the precursor cells are A2B5(+). In contrast to purified astrocytes, purified APCs rapidly die in serum-free culture but can be saved by basic fibroblast growth factor (bFGF) and glial growth factor 2 (GGF2). Unlike oligodendrocyte precursor cells, APCs do not differentiate by default; their differentiation into GFAP+ cells is induced by ciliary neurotrophic factor (CNTF) or by leukemia inhibitory factor (LIF). Finally, the survival, proliferation, and differentiation of APCs were promoted by coculture with other embryonic optic nerve cell types but not with purified embryonic retinal ganglion cells, indicating that interactions with non-neuronal cells are likely to play an important role in controlling astrocyte generation in the developing optic nerve.

Maturation-dependent apoptotic cell death of oligodendrocytes in myelin-deficient rats

Mutations in the proteolipid protein gene (PLP/plp), which encodes the major intrinsic membrane protein in central nervous system (CNS) myelin, cause inherited dysmyelination in mammals. One of these mutants, the myelin-deficient (md) rat, has severe dysmyelination that is associated with oligodendrocyte cell death. Using the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) assay, which labels apoptotic cells, we find that cell death is increased in multiple white matter tracts of md rats. The tracts that myelinate the earliest show the earliest increase in cell death, and cell death persists for at least 22 days, the lifespan of these mutant animals. In all tracts, and at all developmental ages examined, apoptotic cells expressed the markers of mature oligodendrocytes, such as myelin basic protein, myelin-associated glycoprotein, and the Rip antigen, but not chondroitin sulfate proteoglycan, a marker of oligodendrocyte precursors. Mature oligodendrocytes fail to accumulate in md brain because they die before they fully mature.

Purification and characterization of adult oligodendrocyte precursor cells from the rat optic nerve

Oligodendrocyte precursor cells (OPCs) persist in substantial numbers in the adult brain in a quiescent state suggesting that they may provide a source of new oligodendrocytes after injury. To determine whether adult OPCs have the capacity to divide rapidly, we have developed a method to highly purify OPCs from adult optic nerve and have directly compared their properties with their perinatal counterparts. When cultured in platelet-derived growth factor (PDGF), an astrocyte-derived mitogen, perinatal OPCs divided approximately once per day, whereas adult OPCs divided only once every 3 or 4 d. The proliferation rate of adult OPCs was not increased by addition of fibroblast growth factor (FGF) or of the neuregulin glial growth factor 2 (GGF2), two mitogens that are normally produced by retinal ganglion cells. cAMP elevation has been shown previously to be essential for Schwann cells to survive and divide in response to GGF2 and other mitogens. Similarly we found that when cAMP levels were elevated, GGF2 alone was sufficient to induce perinatal OPCs to divide slowly, approximately once every 4 d, but adult OPCs still did not divide. When PDGF was combined with GGF2 and cAMP elevation, however, the adult OPCs began to divide rapidly. These findings indicate that adult OPCs are intrinsically different than perinatal OPCs. They are not senescent cells, however, because they retain the capacity to divide rapidly. Thus, after demyelinating injuries, enhanced axonal release of GGF2 or a related neuregulin might collaborate with astrocyte-derived PDGF to induce rapid division of adult OPCs.

Cells of the oligodendrocyte lineage proliferate following cortical stab wounds: an in vitro analysis

We have previously shown that a cortical stab wound induces the proliferation of microglia and astrocytes in situ, but no evidence was obtained for proliferation of cells of the oligodendrocyte lineage (Amat et al., 1996). To study further the properties of cells involved in repair following brain injury, groups of adult rats received either sham operations or bilateral stab wounds. Proliferating cells were labeled in vivo 3 days later with [3H]-thymidine (Thy) and sacrificed the same day. Oligodendrocyte-enriched preparations were isolated, cultured, and analyzed. The fate and antigenic phenotype of the proliferating cells was analyzed using three-color immunofluorescence combined with autoradiography at 1, 2, 3, 5, and 10 days in vitro (DIV). Cells were immunostained for ganglioside GD3 (glial stem cells), O4 antigen (cells of the oligodendrocyte lineage), galactosyl ceramide (GC, differentiated oligodendrocytes), and GFAP (astrocytes). Thymidine-labeled O4+/GC- cells were found only in cultures from wounded animals and most of them differentiated in vitro as mature oligodendrocytes, but no Thy+/O4+/GC+ oligodendrocytes were seen at 1, 2, or 3 DIV. There was also a marked increase in the number of Thy+/GD3+ cells in the experimental cultures. In both experimental and control groups the total number of Thy+ and Thy- GD3+ cells declined with time in culture concomitant with an increase in total number of both Thy+ and Thy- GFAP+ astrocytes, and without any significant change in the Thy+ cell fraction of O4+ oligodendrocytes in the experimental cultures. Therefore most of the GD3+/O4- cells apparently differentiated as GFAP+ astrocytes, not as oligodendrocytes. We conclude that O4+/GC- oligodendrocyte precursor cells, but not differentiated oligodendrocytes, proliferate in response to brain injury. These cells proliferate slowly or not at all in normal adult animals and constitute a phenotypically and kinetically distinct group from the GD3+ glial precursors. This result is consistent with the existence within the adult CNS of a quiescent premyelinating oligodendrocyte. We propose that these immature committed oligodendrocytes are induced to proliferate at the wound site and serve as a source of new oligodendrocytes.

Glial ontogeny and glial neoplasia: the search for closure

The last ten years have seen rapid progress in both our understanding of the normal progression and control of gliogenesis and in the laboratory techniques necessary to sustain and study most glial cell types, including progenitor cells of both type-1 astrocyte (T1A) and oligodendrocyte-type-2 astrocyte (T2A) lineage. These studies have direct relevance for the lineage analysis of human gliomas, optimizing in vitro glioma models, and suggesting potentially fertile new grounds for glioma biology research. We do not yet known whether malignant transformation occurs only in mature glia that then 'de-differentiate' into cells with glial precursor phenotypes and behavior characteristics, whether neoplastic transformation occurs in O-2A progenitor cells, or whether both mechanisms may occur in different patients. However, preliminary results suggest that astrocytomas can arise from two different glial lineages, that oligodendrogliomas and mixed oligo-astrocytomas arise exclusively from the O-2A lineage, and that medulloblastomas may also have a connection with the O-2A lineage. An ontogeny-based glioma classification system may lead to better prognostic patient data and better predict patient response to treatment than currently available classification systems. Available data from the study of developmental glial biology raises serious doubts about the fidelity and relevance of in vitro glioma models that rely on culture media supplemented with animal serum and suggest that relying on chemically-defined media conditioned by astrocytes may be the better research strategy. Findings from the study of normal gliogenesis also suggest that growth factors are likely to act as much more than simple mitogens in glioma biology. Potentially fertile areas of research for glioma biology include studying the cooperative effect of multiple growth factors, potential growth factor effects as survival factors, inhibitors of differentiation, and differentiation inducers, and studying potential positive humoral feedback loops between glioma cells and normal glial cells, as well as normal non-glial cells, within and surrounding each glioma.

Cytokines which signal through the LIF receptor and their actions in the nervous system

A number of different cytokines, each initially characterized on the basis of very different biological activities, all have very similar signalling pathways and share a similar tertiary structure. These cytokines include leukaemia inhibitory factor, ciliary neuronotrophic factor, oncostatin M, growth-promoting activity and cardiotrophin 1. They all have been found to regulate a number of properties of cells of the developing and mature nervous system in vitro and thus are neuroregulatory cytokines. The actions of these cytokines include regulation of neurotransmitter phenotype, differentiation of neuronal precursor cells both in the peripheral nervous system and in the spinal cord, survival of differentiated neurons, and regulation of development of both astrocytes and oligodendrocytes. In addition, studies in animal models show that these factors can rescue sensory and motor neurons from axotomy-induced cell death, which suggests that they can act as trauma factors for injured neurons. Analysis of the expression patterns of the different neuroregulatory cytokines and their receptors reveals that the receptors are expressed throughout nervous system development and following trauma, whereas the cytokines show temporal and spatial specific expression patterns. This is consistent with the idea that specific cytokines have specific roles in neural development and repair, but that their signalling pathways are shared. The phenotypes of the receptor knockouts show clear deficits in nervous system development, indicating a crucial role for LIF receptor signalling. Knockouts of individual cytokines are less dramatic, but LIF and CNTF knockouts do reveal deficits in maintenance of motor neurons or following trauma. Thus, whereas LIF and CNTF have clear roles in maintenance and following trauma, it is unclear which of the cytokines is involved in nervous system development. In clinical terms, these findings add further support to the use of these cytokines in nervous system trauma and disease.

Gliogenesis in postnatal rat optic nerve: LC1 + microglia and S100-beta + astrocytes

Lipocortin 1 (LC1) and S100-beta, two Ca(2+)-binding proteins that serve as specific markers for microglia and astrocytes, respectively, have been used to study postnatal gliogenesis in the rat optic nerve. Computerized image analysis was used to quantify and map the stained and unstained glia in transverse sections (10 microns thick) taken 1-2 mm from the chiasm in optic nerves from rat pups at postnatal day 0 (P0), P7, P14, P21, P28, P38 and adults. The number of astrocytes was remarkably constant (100 per section) at all ages. Because the area of the nerve increases 10-fold from P0 to adult, the population density of astrocytes begins al > 5000 mm-2 and drops to 400 mm-2 in the mature nerve; however, because the nerve length increases two-fold, the number of astrocytes doubles over the same period. In contrast, the number of LC1 + cells per section initially is sparse (4 at P0), increases rapidly up to 36 at P21 and levels off at 49 in adults. The microglia population density is relatively stable throughout development (200-300 mm-2) except during the peak of oligodendroblast apoptosis (P21) when it rises to 450 mm-2. Neonatally, LC1 immunoreactivity predominantly labels spherical-ameboid cells; but by P28 they are replaced by mature ramified microglia. The number of unstained cells (putative oligodendrocytes) per section increases from 11 at P0 to a peak of 308 at P21, and declines slightly to 269 in adults. While generally confirming concepts of astrocyte and oligodendrocyte ontogeny from the literature, the present report adds considerable detail regarding microglia, which often have been ignored. Microglia identified by LC1 immunoreactivity comprise 12% of the glia in adult optic nerve near the chiasm.

Fate of developing astrocytes in the optic nerve of the myelin-deficient rat

There is considerable debate on the development of a glial cell line in the rat optic nerve, which is characterized by the specific expression of the A2B5 and HNK-1 epitopes. This cell line has been assumed to give rise to oligodendrocytes and so-called type 2 astrocytes. However, it is doubtful that the latter cell type really exists in vivo. In the present study, we have addressed this question by investigating the development of astrocytes in the myelin-deficient (md) rat, which is characterized by dysmyelination and loss of oligodendrocytes. Defective oligodendrocytes were observed by the third postnatal day, well before the generation of type 2 astrocytes. Consequently, the number of type 2 astrocytes was reduced in cultures prepared from optic nerves of md rats vs. controls. This finding was not paralleled in vivo; i.e., no dying astrocytes were observed in md sections by conventional electron microscopy. However, immunoreactivity against the HNK-1 epitope was enhanced in md compared to control sections. Ultrastructurally, HNK-1 immunoreactivity was detected predominantly on the axonal surface at astroaxonal contact sites, which were found only at the nodes of Ranvier within controls but extended to the whole axonal surface in md animals. Only a minor portion of the immunoreactivity derived from glial cells, presumably from oligodendrocytes at the paranodal region in controls. Thus, the HNK-1 epitope is not a useful antigen for distinguishing astrocytes in the rat optic nerve. Accordingly, our results do not provide evidence for the existence of specialized type 2 astrocytes in vivo. In vitro, these cells are probably only oligodendrocytes that mimic some astroglial features if grown in serum-containing media.

Glial lineages and myelination in the central nervous system

Oligodendrocytes, derived from stem cell precursors which arise in subventricular zones of the developing central nervous system, have as their specialist role the synthesis and maintenance of myelin. Astrocytes contribute to the cellular architecture of the central nervous system and act as a source of growth factors and cytokines; microglia are bone-marrow derived macrophages which function as primary immunocompetent cells in the central nervous system. Myelination depends on the establishment of stable relationships between each differentiated oligodendrocyte and short segments of several neighbouring axons. There is growing evidence, especially from studies of glial cell implantation, that oligodendrocyte precursors persist in the adult nervous system and provide a limited capacity for the restoration of structure and function in myelinated pathways damaged by injury or disease.

Retinal ganglion cell axons drive the proliferation of astrocytes in the developing rodent optic nerve

We show that the proliferation of astrocytes in the developing rodent optic nerve absolutely depends on axons and that this axonal influence depends on axonal transport but not on axonal electrical activity. We also show that purified retinal ganglion cells stimulate DNA synthesis in optic nerve astrocytes in culture and that the effect can be mimicked by fibroblast growth factor but not by neuregulins or several other growth factors. Taken together with previous findings, our present results indicate that axons promote glial cell proliferation and survival in the developing optic nerve by at least three distinct mechanisms.

Transplanting oligodendrocyte progenitors into the adult CNS

This review covers a number of aspects of the behaviour of oligodendrocyte progenitors following transplantation into the adult CNS. First, an account is given of the ability of transplanted oligodendrocyte progenitors, grown in tissue culture in the presence of PDGF and bFGF, to extensively remyelinate focal areas of persistent demyelination. Secondly, we describe how transplanted clonal cell lines of oligodendrocyte progenitors will differentiate into astrocytes as well oligodendrocytes following transplantation into pathological environments in which both oligodendrocytes and astrocytes are absent, thereby manifesting the bipotentially demonstrable in vitro but not during development. Finally, a series of studies examining the migratory behaviour of transplanted oligodendrocyte progenitors (modelled using the oliodendrocyte progenitor cell line CG4) are described. These show that CG4 cells do not survive (or migrate) when transplanted into the normal adult CNS. However, if they are transplanted into CNS tissue that has previously been exposed to 40 Gy of x-irradiation then transplanted CG4 cells survive, divide and migrate over large distances. Moreover, within an x-irradiated environment, migrating transplanted CG4 cells are able to enter remotely located foci of demyelination and contribute to the remyelination of the demyelinated axons within. These studies demonstrate that although the normal adult CNS does not appear to support survival and migration of the CG4 cell line, it is possible to manipulate the environment in such a way that these nonpermissive properties of the environment can be overcome.

The Lineages of Neuroglial Cells

The study of neuroglial cell lineages in the CNS identifies the time in development, when astrocytes and oligodendrocytes diverge from a common precursor cell. Recent studies using retroviral tracing show that the lineages for astrocytes and oligodendrocytes begin to diverge as early as embryonic day 13 (E13) in the cerebellum and as early as E15 in the forebrain. A very small percentage of glial precursor cells present in late pre- and postnatal development are pluripotential, but the vast majority of astrocytes and oligodendrocytes in the brain are derived from “committed” precursors. The precursors for these postmitotic astrocytes and oligodendrocytes are immature astrocytes and oligodendrocytes (progenitors) that express molecular properties unique to each of these cell types.

It is critical to distinguish glial lineage (thought of in terms of a glial cell's ancestry in normal development) from glial plasticity (the potential of a glial cell to alter its fate when its normal environment is changed). In tissue culture, a bipotential cell known as the O-2A cell generates oligodendrocytes under one set of culture conditions and retains plasticity to become a subtype of astrocyte under another set. Whether all cells phenotyped as O-2A cells in culture are bipotential or whether only a subset displays this capacity is still unclear.