Focal ventricular origin and migration of oligodendrocyte precursors into the chick optic nerve

The multiple dorsoventral origins and migratory pathway of tectal oligodendrocytes in the developing chick

Oligodendrocytes have been considered to originate in a restricted ventricular zone of the ventral neural tube and to migrate and mature in their final targets. However, recent studies indicate that oligodendrocytes arise from multiple distinct dorsoventral origins. In this study, we investigate oligodendrocyte lineage cells in the embryonic optic tectum of chick, which develops from the dorsal region of the neural tube and invasion of optic tract. Oligodendrocyte precursor cells (OPCs) first appeared bilaterally on either side of the floor plate at E5. With further development, OPCs increased and spread laterally and dorsally to populate the optic tectum. At E7, OPCs appeared in another site along the ventral midline of the third ventricle, just dorsal to the optic chiasm. To examine the migration routes of these ventrally derived OPCs, we used DiI tracing in the organic culture and retinal denervation. Our results reveal that OPCs dispersed bilaterally along the optic tract and then migrated to the optic tectum in the stratum opticum (SO). In addition to these extrinsic OPCs, OPCs intrinsic to the tectal ventricle zone were identified at E14 using a combination of immunohistochemistry and retroviral mediated lineage tracing studies. These data support stage-specific dorsoventral origins and distribution of oligodendrocytes populating the optic tectum.

Intrinsic and extrinsic inhibition of oligodendrocyte development by rat retina

Cell patterning in the vertebrate CNS reflects the combination of localized cell induction, migration and differentiation. A striking example of patterning is the myelination of visual system. In many species, retinal ganglion cell axons are myelinated in the optic nerve but are unmyelinated in the retina. Here, we confirm that rat and mouse retina lack oligodendrocytes and their precursors and identify multiple mechanisms that might contribute to their absence. Soluble cues from embryonic retina inhibit the induction of oligodendrocytes from neural stem cells and their differentiation from optic nerve precursors. This inhibition is mediated by retinal-derived BMPs. During development BMPs are expressed in the retina and addition of the BMP antagonist Noggin reversed retinal inhibition of oligodendrocyte development. The lack of retinal oligodendrocytes does not simply reflect expression of BMPs, since no oligodendrocytes or their precursors developed when embryonic retinal cells were grown in the presence of Noggin and/or inductive cues such as Shh and IGF-1. Similarly, injection of Noggin into the postnatal rat eye failed to induce oligodendrocyte differentiation. These data combined with the proposed inhibition of OPC migration by molecules selectively expressed at the nerve retina junction suggest that multiple mechanisms combine to suppress retinal myelination during development.

The molecular orchestra of the migration of oligodendrocyte precursors during development

During development of the central nervous system (CNS), postmitotic cells (including neurons and myelin-generating cells, the oligodendrocytes) migrate from the germinal areas of the neural tube where they originate to their final destination sites. The migration of neurons during development has been extensively studied and has been the topic of detailed reviews. The migration of oligodendrocyte precursor cells (OPCs) is also an extremely complex and precise event, with a widespread migration of OPCs across many regions to colonize the entire CNS. Different mechanisms have been shown to direct the migration of OPCs, among them contact-mediated mechanisms (adhesion molecules) and long-range cues (chemotropic molecules). This review provides a detailed overview and discussion of the cellular and molecular basis of OPCs migration during development. Because it has been shown that neuronal and oligodendroglial lineages share some of these mechanisms, we briefly summarize similarities and differences between these two types of neural cells. We also summarize the changes in the normal migration of OPCs during development that would be relevant for different neurological diseases (including demyelinating diseases, such as multiple sclerosis, and glial cancers). We also examine the relevance of these migratory properties of the oligondendrocytic cell lineage for the repair of neural damage.

Oligodendrocyte precursors on the move: mechanisms directing migration

Oligodendrocyte precursor cells traverse long distances to reach their axonal targets. The molecules that influence their migration include a combination of short-range attractants and repellents and long-range chemoattractants and chemorepellents. Here, the authors review mechanisms that direct oligodendrocyte precursor cells as they migrate throughout the developing CNS.

The nuclear orphan receptor COUP-TFI is important for differentiation of oligodendrocytes

We report here that a member of the nuclear hormone receptor superfamily, chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TFI), plays a critical role in glial cell development and subsequent central nervous system myelination. We demonstrate that COUP-TF1 is expressed in cells of oligodendrocyte lineage. Furthermore, we demonstrate that COUP-TFI null mutant mice exhibit delayed axon myelination and increased dysmyelination in the central nervous system. Using in vitro differentiation assays, we show that these myelination defects are due to delays in oligodendrocyte differentiation. Finally, in situ hybridization and transfection analysis suggests that COUP-TFI acts as an upstream regulator of SCIP/Oct-6/Tst-1, a transcription factor involved in axon myelination. Taken together, these results suggest that COUP-TFI is an important regulator of oligodendrocyte differentiation.

A role for the polysialic acid – neural cell adhesion molecule in PDGF-induced chemotaxis of oligodendrocyte precursor cells

Directed migration of oligodendrocyte precursor cells (OPCs) is important for myelin formation and repair but the mechanisms of directional control are poorly understood. Here we have tested the role of polysialic acid-neural cell adhesion molecule (PSA-NCAM) in the directional migration of OPCs towards platelet-derived growth factor (PDGF). Using a Boyden microchemotaxis chamber and the Dunn direct viewing chamber, we show that in concentration gradients of PDGF, PSA-positive OPCs polarize and efficiently migrate towards the source of PDGF (chemotaxis). The loss or inactivation of the polysialic tail of NCAM leads to an altered pattern of OPC migration in response to PDGF gradients. Cells under these conditions, while being polarized and migrating, show no bias of displacement towards the source of PDGF and make random turns. By contrast, directed migration of OPCs towards basic fibroblast growth factor was not affected by the removal of PSA. Moreover, inactivation of PSA does not interfere with the random migration pattern of cells in uniform concentrations of PDGF (chemokinesis). These results suggest that PSA-NCAM is specifically involved in establishing the directionality of OPC migration in response to the concentration gradient of PDGF, but it is not essential for cell motility per se.

Redox state as a central modulator of precursor cell function

In our attempts to understand how the balance between self-renewal and differentiation is regulated in dividing precursor cells, we have discovered that intracellular redox state appears to be a critical modulator of this balance in oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells. The intracellular redox state of freshly isolated progenitor cells allows prospective isolation of cells with different self-renewal characteristics, which can be further modulated in opposite directions by prooxidants and antioxidants. Redox state is itself modulated by cell-extrinsic signaling molecules that alter the balance between self-renewal and differentiation: growth factors that promote self-renewal cause progenitors to become more reduced, while exposure to signaling molecules that promote differentiation causes progenitors to become more oxidized. Moreover, pharmacological antagonists of the redox effects of these cell-extrinsic signaling molecules antagonize their effects on self-renewal and differentiation, further suggesting that cell-extrinsic signaling molecules that modulate this balance converge on redox modulation as a critical component of their effector mechanism. A further example of the potential relevance of intracellular redox state to development processes emerges from our attempts to understand why different central nervous system (CNS) regions exhibit different temporal patterns of oligodendrocyte generation and myelinogenesis. Characterization of O-2A progenitor cells (O-2A/OPCs) isolated from different regions indicates that these developmental patterns are consistent with properties of the specific O-2A/OPCs resident in each region. Marked differences were seen in self-renewal and differentiation characteristics of O-2A/OPCs isolated from cortex, optic nerve, and optic chiasm. In conditions where optic nerve-derived O-2A/OPCs generated oligodendrocytes within 2 days, oligodendrocytes arose from chiasm-derived cells after 5 days and from cortical O-2A/OPCs after only 7-10 days. These differences, which appear to be cell intrinsic, were manifested both in reduced percentages of clones producing oligodendrocytes and in a lesser representation of oligodendrocytes in individual clones. In addition, responsiveness of optic nerve-, chiasm-, and cortex-derived O-2A/OPCs to thyroid hormone (TH) and ciliary neurotrophic factor (CNTF), well-characterized inducers of oligodendrocyte generation, was inversely related to the extent of self-renewal observed in basal division conditions. These results demonstrate hitherto unrecognized complexities among the precursor cells thought to be the immediate ancestors of oligodendrocytes and suggest that the properties of these different populations may contribute to the diverse time courses of myelination in different CNS regions. Strikingly, O-2A/OPCs isolated from cortex and analyzed immediately upon isolation were more reduced in their redox state than were optic nerve-derived cells, precisely as would be predicted from our analysis of the role of redox state in modulating the balance between self-renewal and differentiation. Chiasm-derived cells, which exhibited self-renewal properties intermediate between cortex- and optic nerve-derived cells, were more reduced than optic nerve cells but more oxidized that cortical O-2A/OPCs.

Netrin 1 mediates spinal cord oligodendrocyte precursor dispersal

In spinal cord, oligodendrocyte precursors that give rise to myelin-forming cells originate in a restricted domain of the ventral ventricular zone. During development, these cells migrate widely throughout the spinal cord. Netrin 1 is expressed at the ventral ventricular zone during oligodendrocyte precursors emigration, and, in vitro, netrin 1 acts as chemorepellent and antagonizes platelet-derived growth factor (PDGF) chemoattraction. Oligodendrocyte precursors express the netrin receptors DCC and UNC5 and function-blocking anti-DCC antibody inhibits chemorepulsion of ventral spinal cord explants and netrin-secreting cells. In spinal cord slice preparations, addition of function-blocking anti-DCC antibody or netrin 1 dramatically inhibits oligodendrocyte precursor migration from the ventral ventricular zone. These data indicate the initial dispersal of oligodendrocyte precursors from their localized origin is guided by a chemorepellent response to netrin 1.

Spatiotemporal gradient of oligodendrocyte differentiation in chick optic tectum requires brain integrity and cell-cell interactions

The development of oligodendrocytes in the chicken optic tectum (OT) was studied in vivo and in vitro by analyzing expression of myelin-associated glycoprotein (MAG) with a monoclonal antibody. MAG(+) cells first appeared in the anterior OT on embryonic day (E) 12, were present throughout the anterior half on E15, and eventually filled the tectum on E17. This spatiotemporal appearance of MAG(+) oligodendrocytes resembled two streams of cells entering the OT along the afferent and efferent axonal layers. However, experiments determined that this appearance of MAG immunoreactivity was the result of a gradient of oligodendrocyte differentiation and was not cell migration. First, retroviral vector labeling of OT progenitors in vivo on E3 resulted in labeled oligodendrocytes in late embryos. In addition, pieces of OT from as early as E3 kept in culture for a week developed numerous MAG(+) oligodendrocytes. Pieces of both anterior and posterior E7 OT developed MAG(+) oligodendrocytes after 3 days in culture, well ahead of their normal schedule in vivo. BrdU incorporation studies revealed that these cells were not born in culture, but merely differentiated. Monolayer cultures made from dissociated E10 or later OT cells developed MAG(+) oligodendrocytes, but monolayers made from E7 OT cells did not. These experiments demonstrate that oligodendrocyte progenitors were present in the OT as early as E3, that they could differentiate precociously, and that their normal progressive differentiation in situ must be due to removal of inhibitory constraints rather than the onset of inductive factors. Also, certain cell-cell interactions occur between E7 and E10, which cannot be disrupted if oligodendrocyte differentiation is to occur.

Subpallial dlx2-expressing cells give rise to astrocytes and oligodendrocytes in the cerebral cortex and white matter

The precise origins of postnatal subventricular zone (SVZ) cells are not known. Furthermore, the gliogenic potential of progenitors expressing Dlx genes that migrate ventrodorsally from the ganglionic eminences has not been explored in vivo. Here, we identify the embryonic origins of two distinct populations of postnatal SVZ cells: SVZ border cells, which express Zebrin II, and migratory cells in the central SVZ, which are generally devoid of Zebrin II expression (Staugaitis et al., 2001). Zebrin II is expressed by all cells of the telencephalic primordium, with its expression becoming restricted to astrocytes in the mature telencephalon. As the neuroepithelium folds during corticostriatal sulcus formation (embryonic day 13-15), a wedge of Zebrin II+ cells is created at the presumptive site of the dorsolateral SVZ. At this time, Dlx2-expressing cells and their progeny begin to migrate ventrodorsally along a medial path from the ganglionic eminences. These migratory subpallial cells invade the wedge of Zebrin II+ cells to form the central region of the SVZ. We used a Dlx2/tauLacZ knock-in to perform a short-term lineage analysis of Dlx2-expressing cells throughout SVZ formation and the postnatal peak of gliogenesis. Dlx2/tauLacZ [beta-galactosidase (beta-gal)]-expressing cells populate the central SVZ, whereas Zebrin II-expressing cells form its borders. Furthermore, beta-gal expression demonstrates a lineage relationship between Dlx2-expressing cells and glia residing in the dorsal telencephalon. We propose a model for the formation of the postnatal SVZ and demonstrate that subpallium-derived Dlx2-expressing cells give rise to astrocytes and oligodendrocytes in the white matter and cerebral cortex.

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.

Induction of oligodendrocyte fate during the formation of the vertebrate neural tube

The development of the central nervous system (CNS) comprises a series of inductive and transforming events that includes rostro-caudal and dorso-ventral patterning, neuroglial specification and extensive cell migration. The patterning of the neural tube is also characterized by the transcription of specific genes, which encode for morphogens and transcription factors essential for cell fate specification. The generation of oligodendrocytes, the myelin forming glial cells in the CNS, appears to be restricted to specific domains localized in the ventral neuroepithelium. Signaling mediated by sonic hedgehog (Shh) seems to command the early phase of the specification of uncommitted neural stem cells into the oligodendroglial lineage. Once generated, oligodendrocyte progenitors have to follow a developmental program that involves changes in cell morphology, migratory capacity and sensitivity to extracellular trophic factors before becoming mature myelinating cells. This minireview aims to discuss molecular aspects of the early induction of oligodendroglial fate during the formation of the CNS.

Extracellular and intracellular regulation of oligodendrocyte development: roles of Sonic hedgehog and expression of E proteins

Recent advances in understanding oligodendrocyte development have revealed the importance of both extra- and intracellular molecules in regulating the induction, survival, and proliferation of early oligodendrocyte progenitors. The signaling molecule Sonic hedgehog (Shh) is critical for normal development of oligodendrocytes, although the precise influences of Shh on cells of the oligodendrocyte lineage are unclear. The present study shows that Shh increased the number of oligodendrocyte precursors in both pure cultures of oligodendrocyte precursors and mixed cultures from embryonic rat spinal cord. In pure precursor cultures Shh increased cell survival. In mixed cultures, Shh increased both the survival and proliferation of oligodendrocyte precursors in a concentration dependent manner. One intracellular consequence of exposure to Shh is the activation of transcription factors in oligodendrocyte lineage cells, which are critical for oligodendrocyte development, helix-loop-helix (HLH) transcription factors, Olig1 and 2. In many cases, HLH proteins such as Olig1 and Olig2 heterodimerize with other HLH proteins, such as members of the E subfamily, which are critical regulators of cell proliferation and differentiation. Immature (A2B5(+)) and more mature (O4(+)) rat oligodendrocyte precursors in dissociated cell culture expressed Olig1 as well as E proteins, HEB and E2A. Similarly, cells bearing the morphology of oligodendrocyte precursors expressed both Olig1 and HEB or E2A. We propose that E2A and/or HEB, possibly in combination with Olig1 and 2, are critical components of oligodendrogenesis and may regulate cell survival, proliferation, and fate decisions in the oligodendrocyte lineage.

Normal timing of oligodendrocyte development from genetically engineered, lineage-selectable mouse ES cells

Oligodendrocytes are post-mitotic cells that myelinate axons in the vertebrate central nervous system (CNS). They develop from proliferating oligodendrocyte precursor cells (OPCs), which arise in germinal zones, migrate throughout the developing white matter and divide a limited number of times before they terminally differentiate. Thus far, it has been possible to purify OPCs only from the rat optic nerve, but the purified cells cannot be obtained in large enough numbers for conventional biochemical analyses. Moreover, the CNS stem cells that give rise to OPCs have not been purified, limiting one's ability to study the earliest stages of commitment to the oligodendrocyte lineage. Pluripotent, mouse embryonic stem (ES) cells can be propagated indefinitely in culture and induced to differentiate into various cell types. We have genetically engineered ES cells both to positively select neuroepithelial stem cells and to eliminate undifferentiated ES cells. We have then used combinations of known signal molecules to promote the development of OPCs from selected, ES-cell-derived, neuroepithelial cells. We show that the earliest stages of oligodendrocyte development follow an ordered sequence that is remarkably similar to that observed in vivo, suggesting that the ES-cell-derived neuroepithelial cells follow a normal developmental pathway to produce oligodendrocytes. These engineered ES cells thus provide a powerful system to study both the mechanisms that direct CNS stem cells down the oligodendrocyte pathway and those that influence subsequent oligodendrocyte differentiation. This strategy may also be useful for producing human cells for therapy and drug screening.

Differentiation of proliferated NG2-positive glial progenitor cells in a remyelinating lesion

Cells that express the NG2 proteoglycan (NG2+ cells) constitute a large cell population in the adult mammalian central nervous system (CNS). They give rise to mature oligodendrocytes in culture and are thus considered to be oligodendrocyte progenitor cells (OPCs). They proliferate in response to a variety of insults to the CNS, but their ability to differentiate into oligodendrocytes in vivo has not been established. We used bromodeoxyuridine (BrdU) to trace the fate of NG2+ cells that proliferated in response to a chemically induced demyelinating lesion in the adult rat spinal cord. Cells that were proliferating 24 hr after lesioning were labeled by a single injection of BrdU, and their antigenic phenotype was examined at various times up to 28 days post-lesioning (28 dpl). Initially, at 2 dpl, NG2+/BrdU+ cells were found almost exclusively at the periphery of the lesion. At 7 dpl, the number of NG2+/BrdU+ cells increased in the lesion center and decreased from the surrounding areas. The number of NG2+/BrdU+ cells inside the lesion further decreased with time, concomitant with progression of remyelination and appearance of BrdU+ mature oligodendrocytes. Double labeling with (3)H-thymidine and BrdU combined with NG2 immunohistochemistry showed that some NG2+ cells in the lesion had undergone at least two rounds of cell division. These observations strongly suggest that NG2+/BrdU+ cells that appeared in response to the demyelinating insult gave rise to mature remyelinating oligodendrocytes, providing an in vivo evidence for the differentiation of NG2+ cells into oligodendrocytes.

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.

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.

The basic helix-loop-helix factor olig2 is essential for the development of motoneuron and oligodendrocyte lineages

Sonic hedgehog (Shh), an organizing signal from ventral midline structures, is essential for the induction and maintenance of many ventral cell types in the embryonic neural tube. Olig1 and Olig2 are related basic helix-loop-helix factors induced by Shh in the ventral neural tube. Although expression analyses and gain-of-function experiments suggested that these factors were involved in motoneuron and oligodendrocyte development, they do not clearly define the functional differences between Olig1 and Olig2. We generated mice with a homozygous inactivation of Olig2. These mice did not feed and died on the day of birth. In the spinal cord of the mutant mice, motoneurons are largely eliminated and oligodendrocytes are not produced. Olig2(-/-) neuroepithelial cells in the ventral spinal cord failed to differentiate into motoneurons or oligodendrocytes and expressed an astrocyte marker, S100beta, at the time of oligodendrogenesis. Olig1 or Olig3, other family members, were expressed in the descendent cells that should have expressed Olig2. We concluded that Olig2 is an essential transcriptional regulator in motoneuron and oligodendrocyte development. Our data provide the first evidence that a single gene mutation leads to the loss of two cell types, motoneuron and oligodendrocyte.

Oligodendrocyte precursor cells from different brain regions express divergent properties consistent with the differing time courses of myelination in these regions

Different CNS regions exhibit different temporal patterns of oligodendrocyte generation and myelinogenesis. Characterization of oligodendrocyte-type-2 astrocyte progenitor cells (here abbreviated as O-2A/OPCs) isolated from different regions indicates these developmental patterns are consistent with properties of the specific O-2A/OPCs resident in each region. Marked differences were seen in self-renewal and differentiation characteristics of O-2A/OPCs isolated from cortex, optic nerve and optic chiasm. In conditions where optic nerve-derived O-2A/OPCs generated oligodendrocytes within 2 days, oligodendrocytes arose from chiasm-derived cells after 5 days and from cortical O-2A/OPCs only after 7-10 days. These differences, which appear to be cell-intrinsic (and may be related to intracellular redox state), were manifested both in reduced percentages of clones producing oligodendrocytes and in a lesser representation of oligodendrocytes in individual clones. In addition, responsiveness of optic nerve-, chiasm- and cortex-derived O-2A/OPCs to thyroid hormone (TH) and ciliary neurotrophic factor (CNTF), well-characterized inducers of oligodendrocyte generation, was inversely related to the extent of self-renewal observed in basal division conditions. Our results demonstrate hitherto unrecognized complexities among the precursor cells thought to be the immediate ancestors of oligodendrocytes, and suggest that the properties of these different populations may contribute to the diverse time courses of myelination in different CNS regions.

Origin and molecular specification of oligodendrocytes in the telencephalon

Previous studies have demonstrated that oligodendrocytes in the spinal cord and brainstem originate from restricted domains of ventral neuroepithelium under the influence of sonic hedgehog protein (SHH). However, the origin of oligodendrocytes in the telencephalon, the most anterior part of the CNS, has not been established. A recent study has precisely mapped the origin of telencephalic oligodendrocytes to a small region of the ventral telencephalon in rodents, and has presented evidence that telencephalic oligodendrogenesis is, like that of the spinal cord, under the influence of SHH signaling.

Posttranscriptional regulation of p18 and p27 Cdk inhibitor proteins and the timing of oligodendrocyte differentiation

A cell-intrinsic timer helps control when rodent oligodendrocyte precursor cells (OPCs) exit the cell cycle and terminally differentiate when cultured in platelet-derived growth factor (PDGF) and thyroid hormone (TH). There is evidence that the cyclin-dependent kinase inhibitor (CKI) p27/Kip1 (p27) is a component of this TH-regulated timer, as it increases as OPCs proliferate and is required for the timer to operate accurately. Here, we provide evidence that another CKI, p18/INK (p18), may also be a component of the timer: it increases as OPCs proliferate, and its overexpression in OPCs accelerates the timer, causing the cells to differentiate prematurely. We also show that the overexpression of p27 accelerates the timer and that the increases in both p27 and p18 that occur in proliferating OPCs are controlled posttranscriptionally. By contrast, we show that the overexpression of either p18 or p27 in OPCs proliferating in PDGF and the absence of TH greatly slows the cell cycle but fails to accelerate the spontaneous differentiation that normally occurs independently of TH.

Glial cell migration directed by axon guidance cues

Widespread myelination by oligodendrocytes is essential for the normal functioning of the vertebrate CNS. Oligodendrocyte precursors initially arise in restricted regions of the neuroepithelium and migrate relatively long distances to their final destinations. The signals that guide this migration have remained poorly understood, but recent studies suggest that glial precursors use similar molecular cues to those that guide axons through the complex terrain of the developing CNS. For example, in the developing optic nerve, glial-precursor migration from the brain towards the retina is guided by netrin-1 and semaphorin 3a. These studies suggest a novel mechanism governing glial precursor migration and provide new insights into development and the potential to direct CNS injury repair.

Sonic hedgehog is a potent inducer of rat oligodendrocyte development from cortical precursors in vitro

Sonic Hedgehog (Shh) induces oligodendrocyte development in the ventral neural tube and telencephalon but its role in oligodendrocyte generation in dorsal telencephalon is debated. Transcripts for Shh and its receptor complex were detected in subventricular zone and neocortex from E17 to birth. As Shh is not yet expressed in E15 neocortex, we grew E15 cortical precursors (CP) into neurospheres in the presence of recombinant Octyl-Shh (O-Shh). After sphere adhesion and removal of O-Shh, enhanced neurite outgrowth and cell migration were already observed at 3 h. Three days after O-Shh treatment, oligodendrocyte progenitors (OP) emerged and continued to increase in number for 7 days while the ratio of neuronal cells decreased compared to control. Shh selectively triggered mitosis of OP but not neuronal progenitors and enhanced growth of neonatal OP. Thus Shh in E15-17 embryonic neocortex can signal CP to adopt an oligodendrocyte fate and favors expansion of this lineage.

Haematopoietic progenitor cells from adult bone marrow differentiate into cells that express oligodendroglial antigens in the neonatal mouse brain

Stem cells are self-renewable, pluripotent cells that, in adult life, proliferate by a characteristic asymmetric division in which one daughter cell is committed to differentiation whereas the other remains a stem cell. These cells are also characterized by their ability to differentiate into various cell types under heterotopic environmental influences. In the present study, we have explored the potential of adult haematopoietic bone marrow cells to differentiate into cells of oligodendroglial lineage under physiological, active myelinating conditions. We present evidence of generation of cells expressing oligodendroglial specific markers from a bone marrow subpopulation enriched on adult haematopoietic progenitor cells (CD117+) in vivo after intracerebral transplantation into the neonatal mouse brain. Our results suggest that adult bone marrow cells have the capacity to undergo differentiation from haematopoietic to oligodendroglial cells and add support the validity of bone marrow transplants as an alternative treatment for demyelinating diseases of the CNS including Multiple Sclerosis.

Loss of polysialic residues accelerates CNS neural precursor differentiation in pathological conditions

Using the model of lysolecithin-induced demyelination of the corpus callosum in wild-type, NCAM-deficient, and endoneuraminidase-injected mice, we have analyzed the consequences of the loss of expression of NCAM or PSA residues on the migration and proliferation capacities of neural precursors of the subventricular zone (SVZ). We showed that the absence of PSA or NCAM delayed migration of neural precursors to the olfactory bulb and consequently enhanced their recruitment at the lesion site. Moreover, after demyelination, the lack of NCAM but not PSA promoted proliferation in the SVZ and the lesion while the lack of PSA favored the differentiation of the traced cells into the oligodendroglial fate both in the SVZ and in the lesion. As previously demonstrated in vitro (L. Decker et al., 2000, Mol. Cell. Neurosci. 16, 422-439), these data illustrate the involvement of PSA and NCAM in neural precursor motility and differentiation in the normal and injured central nervous system, suggesting distinct roles for these two molecules under pathophysiological conditions.

Temporal control of glial cell migration in the Drosophila eye requires gilgamesh, hedgehog, and eye specification genes

In the Drosophila visual system, photoreceptor neurons (R cells) extend axons towards glial cells located at the posterior edge of the eye disc. In gilgamesh (gish) mutants, glial cells invade anterior regions of the eye disc prior to R cell differentiation and R cell axons extend anteriorly along these cells. gish encodes casein kinase Igamma. gish, sine oculis, eyeless, and hedgehog (hh) act in the posterior region of the eye disc to prevent precocious glial cell migration. Targeted expression of Hh in this region rescues the gish phenotype, though the glial cells do not require the canonical Hh signaling pathway to respond. We propose that the spatiotemporal control of glial cell migration plays a critical role in determining the directionality of R cell axon outgrowth.

3-Phosphoglycerate dehydrogenase, a key enzyme for l-serine biosynthesis, is preferentially expressed in the radial glia/astrocyte lineage and olfactory ensheathing glia in the mouse brain

l-Serine is synthesized from glycolytic intermediate 3-phosphoglycerate and is an indispensable precursor for the synthesis of proteins, membrane lipids, nucleotides, and neuroactive amino acids d-serine and glycine. We have recently shown that l-serine and its interconvertible glycine act as Bergmann glia-derived trophic factors for cerebellar Purkinje cells. To investigate whether such a metabolic neuron-glial relationship is fundamental to the developing and adult brain, we examined by in situ hybridization and immunohistochemistry the cellular expression of 3-phosphoglycerate dehydrogenase (3PGDH), the initial step enzyme for de novo l-serine biosynthesis in animal cells. At early stages when the neural wall consists exclusively of the ventricular zone, neuroepithelial stem cells expressed 3PGDH strongly and homogeneously. Thereafter, 3PGDH expression was downregulated and eventually disappeared in neuronal populations, whereas its high expression was transmitted to the radial glia and later to astrocytes in the gray and white matters. In addition, 3PGDH was highly expressed throughout development in the olfactory ensheathing glia, a specialized supporting cell that thoroughly ensheathes olfactory nerves. These results establish a fundamental link of the radial glia/astrocyte lineage and olfactory ensheathing glia to l-serine biosynthesis in the brain. We discuss this finding in the context of the hypothesis that 3PGDH expression in these glia cells contributes to energy metabolism in differentiating and differentiated neurons and other glia cells, which are known to be vulnerable to energy loss.

Guidance of glial precursor cell migration by secreted cues in the developing optic nerve

Oligodendrocyte precursors are produced in restricted foci of the germinative neuroepithelium in embryo brains and migrate to their sites of function, while astrocytes are produced in a wider area in the neuroepithelium. We investigated the guidance mechanisms of glial precursor (GP) cell migration in the optic nerve. GP cell migration in newborn rat optic nerve was monitored by the UV-thymine-dimer (TD) method. A double labeling study using NG2 and TD revealed that many of these in vivo migrating cells were NG2 positive, while some of them with large TD-positive nuclei were NG2 negative. An in vitro cell migration study using optic nerve with chiasma and/or eyeball tissue revealed that the GP cells migrated under the guidance of repulsive cues secreted from the optic chiasma. We detected the expression of netrin 1 and Sema3a in the optic chiasma, and that of Unc5h1 and neuropilin 1 in the optic nerve. Co-culture experiments of the optic nerve with cell clusters expressing guidance cues revealed that the migrating GP cells in the optic nerve were heterogeneous. Netrin 1 repelled a subtype of NG2-positive and PLP-positive GP cells with small nuclei. Sema3a repelled a subtype of GP cells with large nuclei.

Revisiting the function of PSA-NCAM in the nervous system

Since its first description the polysialylated form of NCAM (PSA-NCAM) is thought to be a major regulator of cell-cell interactions in the nervous system. Over the past few years many crucial questions have been answered concerning PSA biosynthesis and function. Among these are the identification and cloning of the key enzymes that are responsible for its synthesis and the fact that expression of PSA is not restricted to developmental stages but maintained in the adult nervous system. In the adult, PSA has been shown to be not only a marker of structural plasticity but seems to be a major player in these processes. Originally suggested to be a purely anti-adhesive factor, modulating cell-cell interactions in general and by this allowing plasticity, there is now increasing evidence that this might not be the whole story. Instead, it appears possible that PSA-NCAM interacts with secreted signaling molecules and by this fulfills a more instructive function in brain plasticity.

Migration and differentiation of Nkx-2.2+ oligodendrocyte progenitors in embryonic chicken retina

During early neural development, the Nkx-2.2 homeobox gene is initially induced in the ventral ventricular zone of the central nervous system (CNS). At later stages, Nkx-2.2+ cells migrate away into the surrounding regions and co-express several oligodendrocyte marker genes. Here we report that Nkx-2.2+ cells also migrate into the avian retina from the optic nerve. These Nkx-2.2+ cells are specifically labeled by the early oligodendrocyte markers PDGFR alpha, O4 and GalC, but not the microglial marker QH1 or neuron-specific marker MAP-2. These observations provide further evidence that Nkx-2.2 is selectively expressed in migratory oligodendrocyte progenitor cells and that retina oligodendrocytes originate from the ventral diencephalon.

Control of oligodendrocyte differentiation by the Nkx2.2 homeodomain transcription factor

Oligodendrocytes are derived from glial precursors that arise from the ventral neural tube early in development. In the developing chicken CNS, oligodendrocyte progenitors selectively express Nkx2.2 homeodomain transcription factor, raising the possibility that Nkx2.2 may directly regulate oligogliogenesis. In this study, we have examined Nkx2.2 expression in rodent glial precursors and studied the effect of a loss of Nkx2.2 on oligodendrocyte and astrocyte differentiation. We show that Nkx2.2 is also expressed in mammalian oligodendrocyte progenitors and that the differentiation of MBP-positive and PLP-DM20-positive oligodendrocytes is dramatically retarded in Nkx2.2-null mutants along the entire rostrocaudal axis. In contrast, no effect is seen on astrocytic differentiation. Interestingly, absence of Nkx2.2 expression leads to a ventral expansion of the Olig1/Olig2 expression in neuroepithelial cells into the Nkx2.2 domain and a consequent increase in the production of Olig1/Olig2-positive and platelet-derived growth factor receptor α-positive oligodendrocyte progenitors. These results strongly suggest that Nkx2.2 regulates the differentiation and/or maturation, but not the initial specification, of oligodendrocyte progenitors. Consistent with this suggestion, overproduction of Nkx2.2 protein in fibroblast cells can induce gene expression from the proteolipid protein promoter.

Roles, regulation, and mechanism of polysialic acid function during neural development

The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) appeared during the evolution of vertebrates as a new mechanism for regulation of cell interactions. This large and abundant glycoprotein can exert steric effects at the cell surface that lead to the attenuation of cell-cell bonds mediated not only by NCAM but also a variety of other adhesion receptors. PSA-NCAM expression changes both as a result of developmental programs and physiological inputs. This global modulation of cell-cell attachment has been shown to facilitate cell migration, axon pathfinding and targeting, and plastic changes in the embryonic and adult nervous system.

Hedgehog-dependent oligodendrocyte lineage specification in the telencephalon

In the caudal neural tube, oligodendrocyte progenitors (OLPs) originate in the ventral neuroepithelium under the influence of Sonic hedgehog (SHH), then migrate throughout the spinal cord and brainstem before differentiating into myelin-forming cells. We present evidence that oligodendrogenesis in the anterior neural tube follows a similar pattern. We show that OLPs in the embryonic mouse forebrain express platelet-derived growth factor alpha-receptors (PDGFRA), as they do in more caudal regions. They first appear within a region of anterior hypothalamic neuroepithelium that co-expresses mRNA encoding SHH, its receptor PTC1 (PTCH) and the transcription factors OLIG1, OLIG2 and SOX10. Pdgfra-positive progenitors later spread through the forebrain into areas where Shh is not expressed, including the cerebral cortex. Cyclopamine inhibited OLP development in cultures of mouse basal forebrain, suggesting that hedgehog (HH) signalling is obligatory for oligodendrogenesis in the ventral telencephalon. Moreover, Pdgfra-positive progenitors did not appear on schedule in the ventral forebrains of Nkx2.1 null mice, which lack the telencephalic domain of Shh expression. However, OLPs did develop in cultures of Nkx2.1−/− basal forebrain and this was blocked by cyclopamine. OLPs also developed in neocortical cultures, even though Shh transcripts could not be detected in the embryonic cortex. Here, too, the appearance of OLPs was suppressed by cyclopamine. In keeping with these findings, we detected mRNA encoding SHH and Indian hedgehog (IHH) in both Nkx2.1−/− basal forebrain cultures and neocortical cultures. Overall, the data are consistent with the idea that OLPs in the telencephalon, possibly even some of those in the cortex, develop under the influence of SHH in the ventral forebrain.

Oligodendrocyte development in the spinal cord and telencephalon: common themes and new perspectives

There are clear parallels between oligodendrocyte development in the spinal cord and forebrain. However, there is new evidence that in both of these regions oligodendrocyte lineage development may be more complex than we earlier thought. This stems from the recent identification of three new transcription factor genes, Olig1, Olig2 and Sox10, that are expressed from the early stages of oligodendrocyte lineage development. In this article, we highlight the common themes underlying specification and early development of oligodendrocytes in the spinal cord and telencephalon. Then, we discuss recent studies of Sox10 and the Olig genes and their implications for oligodendrocyte specification. We conclude that although the mechanisms of oligodendrogenesis appear to be fundamentally similar at different rostro-caudal levels of the neuraxis, there are still many unanswered questions about the details of oligodendrocyte specification.

Monofocal origin of telencephalic oligodendrocytes in the anterior entopeduncular area of the chick embryo

Oligodendrocytes are the myelin-forming cells in the central nervous system. In the brain, oligodendrocyte precursors arise in multiple restricted foci, distributed along the caudorostral axis of the ventricular neuroepithelium. In chick embryonic hind-, mid- and caudal forebrain, oligodendrocytes have a basoventral origin, while in the rostral fore-brain oligodendrocytes emerge from alar territories (Perez Villegas, E. M., Olivier, C., Spassky, N., Poncet, C., Cochard, P., Zalc, B., Thomas, J. L. and Martinez, S. (1999) Dev. Biol. 216, 98–113). To investigate the respective territories colonized by oligodendrocyte progenitor cells that originate from either the basoventral or alar foci, we have created a series of quail-chick chimeras. Homotopic chimeras demonstrate clearly that, during embryonic development, oligodendrocyte progenitors that emerge from the alar anterior entopeduncular area migrate tangentially to invade the entire telencephalon, whereas those from the basal rhombomeric foci show a restricted rostrocaudal distribution and colonize only their rhombomere of origin. Heterotopic chimeras indicate that differences in the migratory properties of oligodendroglial cells do not depend on their basoventral or alar ventricular origin. Irrespective of their origin (basal or alar), oligodendrocytes migrate only short distances in the hindbrain and long distances in the prosencephalon. Furthermore, we provide evidence that, in the developing chick brain, all telencephalic oligodendrocytes originate from the anterior entopeduncular area and that the prominent role of anterior entopeduncular area in telencephalic oligodendrogenesis is conserved between birds and mammals.

Distinct sites of origin of oligodendrocytes and somatic motoneurons in the chick spinal cord: oligodendrocytes arise from Nkx2.2-expressing progenitors by a Shh-dependent mechanism

In the vertebrate spinal cord, oligodendrocytes arise from the ventral part of the neuroepithelium, a region also known to generate somatic motoneurons. The emergence of oligodendrocytes, like that of motoneurons, depends on an inductive signal mediated by Sonic hedgehog. We have defined the precise timing of oligodendrocyte progenitor specification in the cervico-brachial spinal cord of the chick embryo. We show that ventral neuroepithelial explants, isolated at various development stages, are unable to generate oligodendrocytes in culture until E5 but become able to do so in an autonomous way from E5.5. This indicates that the induction of oligodendrocyte precursors is a late event that occurs between E5 and E5.5, precisely at the time when the ventral neuroepithelium stops producing somatic motoneurons. Analysis of the spatial restriction of oligodendrocyte progenitors, evidenced by their expression of O4 or PDGFR(α), indicate that they always lie within the most ventral Nkx2.2-expressing domain of the neuroepithelium, and not in the adjacent domain characterized by Pax6 expression from which somatic motoneurons emerge. We then confirm that Shh is necessary between E5 and E5.5 to specify oligodendrocyte precursors but is no longer required beyond this stage to maintain ongoing oligodendrocyte production. Furthermore, Shh is sufficient to induce oligodendrocyte formation from ventral neuroepithelial explants dissected at E5. Newly induced oligodendrocytes expressed Nkx2.2 but not Pax6, correlating with the in vivo observation. Altogether, our results show that, in the chick spinal cord, oligodendrocytes originate from Nkx2.2-expressing progenitors.

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.

Extensive proliferation of oligodendrocyte precursors in the parenchyma of the embryonic chick central nervous system

The proliferation of oligodendrocyte lineage cells in the chick embryo central nervous system (CNS) was examined by double-immunolabeling with a lineage marker monoclonal antibody (mAb) O4 or mAb O1 and 5-bromo-3'-deoxyuridine (BrdU). In all regions examined, the first O4-positive (O4+) cells appeared in restricted regions of the ventricular zone (VZ), regarded as a site of oligodendrocyte origin. Within the O4+ focus, less than 20% of the O4+ cells incorporated BrdU. In contrast, O4+ cells in the parenchyma were mitotically active; for example, 40-50% of early O4+ cells were labeled with BrdU. Some of these were unipolar in shape, indicative of migratory precursor cells. The frequency of O4+/BrdU+ cell appearance decreased to less than 20% with further development. O1+ oligodendrocytes were largely mitotically inactive, with only approximately 5% of O1+ cells incorporating BrdU. These results clearly demonstrated that the VZ generates relatively few precursor cells and that these oligodendrocyte precursors actively generate their cohort in the parenchyma of the CNS.

Selective expression of Nkx-2.2 transcription factor in chicken oligodendrocyte progenitors and implications for the embryonic origin of oligodendrocytes

Recent studies have demonstrated that oligodendrocytes originate from the ventral region of the developing spinal cord. However, the precise neuroepithelial origin of oligodendrocytes remains controversial, and the transcriptional control of oligodendrocyte lineage specification is largely unknown. Here we present evidence that oligodendrocytes in the embryonic chicken spinal cord can be generated from neuroepithelial cells that express the Nkx-2.2 homeodomain transcription factor. Nkx-2.2 expression is initially confined to a narrow stripe of neuroepithelium flanking the floor plate. Later, Nkx-2.2+ cells migrate ventrally and dorsolaterally into the surrounding gray and white matter regions where they undergo rapid proliferation. Double labeling experiments revealed that Nkx-2.2+ cells coexpress markers specific for oligodendrocyte progenitors, e.g., PDGFRalpha+, O4, and R-mAb antigens. In the brain, the Nkx-2.2 cells are also highly migratory and can generate oligodendrocytes. The persistent expression of the Nkx-2.2 homeodomain transcription factor in the oligodendrocyte lineage suggests its important role in the control of oligodendrocyte development.

Oligodendrocyte precursor migration and differentiation: combined effects of PSA residues, growth factors, and substrates

Using the oligosphere strategy (V. Avellana-Adalid et al., 1996, J. Neurosci. Res. 45, 558-570), we compared the migratory behavior of oligodendrocyte preprogenitors (OPP) that expressed the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) and of GD3-positive oligodendrocyte progenitors (OP). To study the role of PSA in OPP migration, we used endoneuraminidase-N, which specifically cleaves PSA from NCAM. Kinetic data showed that (i) migration velocity decreased with time and was favored on polyornithine compared to Matrigel; (ii) cells emerging from spheres enriched in PSA-NCAM+ OPP migrated farther than those from spheres enriched in GD3+ OP, their migration being enhanced by the addition of growth factors; (iii) removal of PSA from NCAM moderately reduced OPP migration and induced their differentiation in GD3+ OP and GFAP+ astrocytes; (iv) blocking integrins reduced their migration, suggesting an alternative mechanism of migration. Altogether these data illustrate that motility and differentiation of OPP involve the combinatorial action of PSA-NCAM, molecules of the ECM and their receptors, and growth factors.

Gliogenesis in the central nervous system

Multipotential neuroepithelial stem cells are thought to give rise to all the differentiated cells of the central nervous system (CNS). The developmental potential of these multipotent stem cells becomes more restricted as they differentiate into progressively more committed cells and ultimately into mature neurons and glia. In studying gliogenesis, the optic nerve and spinal cord have become invaluable models and the progressive stages of differentiation are being clarified. Multiple classes of glial precursors termed glial restricted precursors (GRP), oligospheres, oligodendrocyte-type2 astrocyte (O-2A) and astrocyte precursor cells (APC) have been identified. Similar classes of precursor cells can be isolated from human neural stem cell cultures and from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. In this review, we discuss gliogenesis, glial stem cells, putative relationships of these cells to each other, factors implicated in gliogenesis, and therapeutic applications of glial precursors.

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.

Consequences of neural cell adhesion molecule deficiency on cell migration in the rostral migratory stream of the mouse

In vertebrates, interneurons of the olfactory bulb (OB) are generated postnatally and throughout life at the subventricular zone of the forebrain. The neuronal precursors migrate tangentially through the forebrain using a well defined pathway, the rostral migratory stream (RMS), and a particular mode of migration in a chain-like organization. A severe size reduction of the OB represents the most striking morphological phenotype in neural cell adhesion molecule (NCAM)-deficient mice. This defect has been traced back to a migration deficit of the precursors in the RMS and linked to the lack of the polysialylated form of NCAM. In this study we investigate the morphological alterations and functional properties of the RMS in mice totally devoid of all isoforms of NCAM and polysialic acid (PSA). We show that a morphologically altered, but defined and continuous pathway exists in mutants, and we present in vivo and in vitro evidence that PSA-NCAM in the RMS is not essential for the formation and migration of chains. Instead, we find a massive gliosis associated with the formation of membrane specializations in a heterotypic manner, linking precursors to astrocytes. This finding and the over-representation and defasciculation of axons in the pathway suggest that important interactions between migrating cells and their stationary environment are perturbed in the mutants. Finally, we used transplantation experiments to demonstrate that lack of PSA-NCAM leads to a decrease but not a total blockade of migration and demonstrate that the mutant RMS is functional in transporting normal neuronal precursors to the OB.

Spatiotemporal development of oligodendrocytes in the embryonic brain

In the central nervous system (CNS), oligodendrocytes have long been considered to be the last cell type to be generated during development. In rodents, the progenitor cells that give rise to oligodendrocytes have been reported to originate in the subventricular zone. Here, we review recent data demonstrating the existence of oligodendrocyte precursor cells in the ventricular layer of the neural tube that emerge prior to the progenitor stage. Oligodendrocyte precursors arise in restricted foci that are distributed along the rostrocaudal axis of the neural tube, for the most part ventrally. The generation of oligodendrocyte precursor cells occurs either simultaneously with, or follows closely upon the emergence of the first neurons. Experiments with quail-chick chimeras provide evidence that oligodendrocyte progenitors derived from ventricular precursors migrate either tangentially or radially to colonize extensive or segmentally restricted territories of the brain. The choice depends on their site of origin. Finally, we discuss the possibility that oligodendrocytes could be a mosaic population that originates from at least two types of precursor cells.

Local control of oligodendrocyte development in isolated dorsal mouse spinal cord

The earliest oligodendrocyte precursors have been proposed to arise in the ventral ventricular zone of the embryonic thoraco-lumbar spinal cord and subsequently migrate to populate dorsal spinal cord. Using the expression of O4 immunoreactivity to define cells of the oligodendrocyte lineage, the development of oligodendrocytes in different regions of the mouse spinal cord was assayed. Consistent with earlier studies in other species, isolated explants of E11 ventral but not dorsal mouse spinal cord developed oligodendrocytes after 7 days in vitro. In contrast, in cultures derived from E13 embryos O4(+) oligodendrocytes developed in both ventral and dorsal cultures after 5 days in vitro. These data are consistent with a ventral to dorsal migration of committed oligodendrocyte progenitors occurring between E11 and E13. Although isolated early embryonic dorsal spinal cord does not give rise to oligodendrocytes in short term cultures, in long term cultures O4(+) cells develop in a subset of dorsal explants. After 10 days in vitro approximately 25% of both cervical and thoraco-lumbar E11 derived dorsal explants contained significant numbers of O4(+) cells. The molecular requirements for the dorsally-derived oligodendrocytes was similar to that in ventral cord. The appearance of O4(+) cells was dependent on sonic hedgehog and enhanced by neuregulin. These data suggest that early embryonic dorsal mouse spinal cord has an independent potential to generate oligodendrocytes under appropriate conditions. Whether this potential is realized during normal spinal cord development is currently unknown.

Oligodendrocyte lineage and the motor neuron connection

One of the more surprising recent discoveries in glial biology has been that oligodendrocytes (OLs) originate from very restricted regions of the embryonic neural tube. This was surprising because myelinating OLs are widespread in the mature central nervous system, so there was no reason to suspect that their precursors should be restricted. What we now know about early OL development suggests that they might have as much (or more) in common with ventral neurons-specifically motor neurons (MNs)-as with other types of glia. This has implications for the way we think about glial development, function, and evolution. In this article we review the evidence for a shared MN-OL lineage and debate whether this is the only lineage that generates OLs. We decide in favour of a single embryonic lineage with regional variations along the anterior-posterior neuraxis.

Development of macroglial cells in the embryonic chick optic nerve

Macroglia development in the embryonic chick optic nerve was immunohistochemically examined. The astrocytes with glial fibrillary acidic protein immunoreactivity were initially restricted to the retinal end of the optic nerve at stage 40, but had widely dispersed within the optic nerve in an out-side-in manner by stage 44. Oligodendrocytes with myelin basic protein immunoreactivity appeared at stage 38, and were widely distributed at stage 40. Electron microscopic observation confirmed the presence of intermediate filaments in the glial fibers and developing myelin at stages 40-41. The present results suggest that oligodendrocyte precursors undergo terminal differentiation slightly earlier than astrocyte precursors with respect to the expression of marker proteins.

Early specification of oligodendrocytes in the chick embryonic brain

Oligodendrocytes are the myelin-forming cells in the central nervous system of vertebrates. In the rodent embryo, these cells have been shown to emerge from restricted territories of the neuroepithelium. However, a comprehensive view of the development of oligodendroglial populations from their ventricular sources remains to be established. As a first step toward this aim, we have examined in vivo the spatiotemporal emergence of oligodendrocytes in the chick embryonic brain. We have detailed the patterns of expression of three early markers of the oligodendroglial lineage: the plp/dm-20 and PDGFRalpha transcripts and the O4-reactive antigen. During embryonic development, these molecules showed a similar segmental pattern of expression. However, plp/dm-20(+) cells were already observed, in the ventricular layer, at E2.5, i.e., 2 days before the appearance of O4(+) and PDGFRalpha(+) cells, suggesting that oligodendrocyte precursors arise nearly simultaneously with neurons. In the chick embryonic brain, the onset of expression of plp/dm-20 appears therefore to be the earliest event indicative of oligodendroglial specification and we propose, based on the expression of plp/dm-20 transcript, a ventricular map of the foci at which oligodendrocytes originate. In addition, we document the precocious segregation, from E5, of plp/dm-20(+) and PDGFRalpha(+) oligodendroglial cells in the subventricular and mantle layers of the brain.

Progenitor cells of the adult mouse subventricular zone proliferate, migrate and differentiate into oligodendrocytes after demyelination

Identifying a source of cells with the capacity to generate oligodendrocytes in the adult CNS would help in the development of strategies to promote remyelination. In the present study, we examined the ability of the precursor cells of the adult mouse subventricular zone (SVZ) to differentiate into remyelinating oligodendrocytes. After lysolecithin-induced demyelination of the corpus callosum, progenitors of the rostral SVZ (SVZa) and the rostral migratory pathway (RMS), expressing the embryonic polysialylated form of the neural cell adhesion molecule (PSA-NCAM), increased progressively with a maximal expansion occurring after 2 weeks. This observation correlated with an increase in the proliferation activity of the neural progenitors located in the SVZa and RMS. Moreover, polysialic acid (PSA)-NCAM-immunoreactive cells arizing from the SVZa were detected in the lesioned corpus callosum and within the lesion. Tracing of the constitutively cycling cells of the adult SVZ and RMS with 3H-thymidine labelling showed their migration toward the lesion and their differentiation into oligodendrocytes and astrocytes but not neurons. These data indicate that, in addition to the resident population of quiescent oligodendrocyte progenitors of the adult CNS, neural precursors from the adult SVZ constitute a source of oligodendrocytes for myelin repair.