Developmental appearance of oligodendrocytes in the embryonic chick retina

Models for evaluating glioblastoma invasion along white matter tracts

White matter tracts (WMs) are one of the main invasion paths of glioblastoma multiforme (GBM). The lack of ideal research models hinders our understanding of the details and mechanisms of GBM invasion along WMs. To date, many potential in vitro models have been reported; nerve fiber culture models and nanomaterial models are biocompatible, and the former have electrically active neurons. Brain slice culture models, organoid models, and microfluidic chip models can simulate the real brain and tumor microenvironment (TME), which contains a variety of cell types. These models are closer to the real in vivo environment and are helpful for further studying not only invasion along WMs by GBM, but also perineural invasion and brain metastasis by solid tumors.

Structural and functional identification of two distinct inspiratory neuronal populations at the level of the phrenic nucleus in the rat cervical spinal cord

The diaphragm is driven by phrenic motoneurons that are located in the cervical spinal cord. Although the anatomical location of the phrenic nucleus and the function of phrenic motoneurons at a single cellular level have been extensively analyzed, the spatiotemporal dynamics of phrenic motoneuron group activity have not been fully elucidated. In the present study, we analyzed the functional and structural characteristics of respiratory neuron population in the cervical spinal cord at the level of the phrenic nucleus by voltage imaging, together with histological analysis of neuronal and astrocytic distribution in the cervical spinal cord. We found spatially distinct two cellular populations that exhibited synchronized inspiratory activity on the transversely cut plane at C4–C5 levels and on the ventral surface of the mid cervical spinal cord in the isolated brainstem–spinal cord preparation of the neonatal rat. Inspiratory activity of one group emerged in the central portion of the ventral horn that corresponded to the central motor column, and the other appeared in the medial portion of the ventral horn that corresponded to the medial motor column. We identified by retrogradely labeling study that the anatomical distributions of phrenic and scalene motoneurons coincided with optically detected central and medial motor regions, respectively. Furthermore, we anatomically demonstrated closely located features of putative motoneurons, interneurons and astrocytes in these regions. Collectively, we report that phrenic and scalene motoneuron populations show synchronized inspiratory activities with distinct anatomical locations in the mid cervical spinal cord.

Origin of Oligodendrocytes in the Vertebrate Optic Nerve: A Review

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

Expression of myelin genes in the developing chick retina

In submammalian animals including chicks, the retina contains oligodendrocytes (OLs), and axons in the optic fiber layer are wrapped with compact myelin within the retina; however, the expression of myelin genes in the chick retina has not been demonstrated yet. In the present study, we examined the expression of three myelin genes (proteolipid protein, PLP; myelin basic protein, MBP; cyclic nucleotide phosphodiesterase, CNP) and PLP in the developing chick retina, in comparison to the localization of Mueller cells. In situ hybridization demonstrated that all three myelin genes began to be expressed at E14 in the chick embryo retina. They are mostly restricted to the ganglion cell layer and the optic fiber layer, with a few exceptions in the inner nuclear layer where Mueller cells reside; however, PLP mRNA+ cells do not express glutamine synthetase, or vice versa. The present results elucidate that myelin genes are expressed only by OLs that are mostly localized in the innermost layer of the developing chick retina.

Myelin oligodendrocyte-specific protein is expressed in Müller cells of myelinated vertebrate retinas

The retina of nonmammalian vertebrates has a loose myelin that enwraps the large axons of the ganglion cells in all areas, whereas that of mammals lacks myelin, with some exceptions, such as the rabbit retina, which shows compact myelin restricted to the myelinated streak. Electron microscopy studies in chicken retina showed processes of Müller cells (MCs) and oligodendrocytes enwrapping ganglion cell axons. How each of these cells contributes to chicken retina myelination and whether the MC of other myelinated retinas is involved in myelination remain unknown. By immunohistochemistry, with a monoclonal antibody against myelin oligodendrocyte-specific protein (MOSP), we show that MOSP is intensely expressed in the MC and the optic-fiber layer (OFL) in myelinated but not in unmyelinated retinas. By immunocytochemistry with isolated MCs from the chick and rabbit retinas, we show that MOSP is concentrated in the innermost domain of the vitread processes. By immunoblotting, we show that protein extracts from myelinated retinas, but not those from unmyelinated retinas, presented a single band labelled with anti-MOSP of molecular weight similar to that of brain MOSP. In addition, we show that the MC of the embryonic chicken retina starts to express MOSP just before myelination starts. Our results agree with those of electron microscopy studies showing myelin in chick retina formed by MC processes and with those of immunohistochemistry studies in rabbit and human retinas showing expression of other myelin molecules in the MC. Altogether, our results suggest that the MC in myelinated retinas might contribute MOSP to myelin.

A coculture assay to visualize and monitor interactions between migrating glioma cells and nerve fibers

Glioma-cell migration is usually assessed in dissociated cell cultures, spheroid cultures, acute brain slices and intracranial implantation models. However, the interactions between migrating glioma cells and neuronal tracts remain poorly understood. We describe here a protocol for the coculture of glioma cells with myelinated axons in vitro. Unlike other methods, this protocol allows the creation of in vitro conditions that largely mimic the complex in vivo environment. First, long retinal axons from embryonic chicken are formed in an organotypic culture. Glioma cells are then positioned in the vicinity of the explants to allow them to contact the axons, interact with them and eventually migrate along them. High-resolution video microscopy and confocal microscopy can be used to monitor the migratory behavior. This protocol, which takes about 5 days to complete, could be applied to different types of tumor cells that interact with neurites, and is suitable for pharmacological and genetic approaches aimed at elucidating mechanisms underlying tumor migration.

ROCKs are expressed in brain tumors and are required for glioma-cell migration on myelinated axons

The interactions between migrating glioma cells and myelinated fiber tracts are poorly understood. We identified that C6 glioma cells can migrate along myelinated chicken retinal axons in a novel coculture, thereby expressing small GTPases of the Rho family and serine/threonine Rho-associated kinases (ROCKs). We found that the ROCK1 isoform is also highly expressed in native human high-grade gliomas. Glioma cells migrated faster in vitro along myelinated axons than on laminin-1, with the former but not the latter being specifically and reversibly blocked by the ROCK inhibitor Y27632. These data suggest that the mechanisms underlying the migration of glioma cells on myelinated axons differ from those underlying the migration on extracellular matrix molecules such as laminin-1.

Retroviral misexpression of cVax disturbs retinal ganglion cell axon fasciculation and intraretinal pathfinding in vivo and guidance of nasal ganglion cell axons in vivo

The transcription factor cVax (Vax2) is expressed in the ventral neural retina and restricted expression is a prerequisite for at least three prominent aspects of retinal dorsal-ventral patterning: polarized expression of EphB/B-ephrin molecules, the retinotectal projection and the distribution of rod photoreceptors across the retina. In the chick retina, the fasciculation pattern of ganglion cell axons also differs between the dorsal and ventral eye. To investigate the molecular mechanisms involved, the nerve fiber layer was analyzed after retroviral misexpression of several factors known to regulate the positional specification of retinal ganglion cells. Forced cVax expression ventralized the fasciculation pattern and caused axon pathfinding errors near the optic disc. Ectopic expression of different ephrin molecules indicated that axon fasciculation is, at least in part, mediated by the EphB system. Finally, we report that retroviral misexpression of cVax increased the pool of EphA4 receptors phosphorylated on tyrosine residues and altered the guidance preference of nasal axons in vitro. These results identify novel functions for cVax in intraretinal axon fasciculation and pathfinding as well as suggest a mechanism to explain how restricted cVax expression may influence map formation along the dorso-ventral and antero-posterior axes of the optic tectum.

S100 immunoreactive glial cells in the forebrain and midbrain of the lizard Gallotia galloti during ontogeny

We identified S100 immunoreactive cells in the brain of the lizard Gallotia galloti during ontogeny using immunohistochemical techniques for light and electron microscopy. In double labeling experiments with antibodies specific for S100A1 and S100B (anti-S100) and proliferative cell nuclear antigen (anti-PCNA), myelin basic protein (anti-MBP), phosphorylated neurofilaments (SMI-31), glial fibrillary acidic protein (anti-GFAP), or glutamine synthetase (anti-GS), we detected S100-like immunoreactivity in glial cells but never in neurons. Restricted areas of the ventricular zone were stained in the hypothalamus from E32 to postnatal stages, and in the telencephalon at E35, E36, and in adults. S100 immunoreactivity was observed predominantly in scattered PCNA-negative cells that increased in number from E35 to the adult stage in the myelinated tracts of the brain and had the appearance of oligodendrocytes. Quantitative analysis revealed that all of the S100-positive glial cells were GFAP-negative, whereas most of the S100-positive glial cells were GS-positive. Ultrastructurally, most of these S100-positive/GS-positive glial cells resembled oligodendrocytes of light and medium electron density. In adult lizards, a small subpopulation of astrocyte-like cells was also stained in the pretectum. We conclude that in the lizard S100 can be considered a marker of a subpopulation of oligodendrocytes rather than of astrocytes, as is the case in mammals. The S100-positive subpopulation of oligodendrocytes in the lizard could represent cells actively involved in the process of myelination during development and in the maintenance of myelin sheaths in the adult.

Exogenous growth factors stimulate the regeneration of ganglion cells in the chicken retina

Recent reports have found that the posthatch chicken retina has the capacity for neuronal regeneration. The purpose of this study was to test whether the types of cells destroyed by neurotoxic lesions influence the types of cells that are regenerated, and whether exogenous growth factors stimulate neural regeneration in the chicken retina. N-methyl-D-aspartate (NMDA) was used to destroy amacrine and bipolar cells; kainate was used to destroy bipolar, amacrine, and ganglion cells; colchicine was used to selectively destroy ganglion cells. Following toxin-induced damage, bromo-deoxyuridine was used to label proliferating cells. In some animals, growth factors were injected into the vitreous chamber of the eye. We found that the proliferation of cells within the retina was stimulated by toxin-induced cell loss, and by insulin and FGF2. After either kainate- or colchicine-induced retinal damage, some of the newly generated cells expressed markers and had the morphology of ganglion cells. The combination of insulin and FGF2 stimulated the regeneration of ganglion cells in kainate- and colchicine-treated retinas. We conclude that exogenous growth factors can be used to stimulate neural regeneration in the retina. We propose that the type of neuron destroyed in the retina may allow or promote the regeneration of that neuronal type.

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.

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.

Localization of the neuronal class III beta-tubulin in oligodendrogliomas: comparison with Ki-67 proliferative index and 1p/19q status

The class III beta-tubulin isotype (betaIII) is widely regarded as a neuronal marker in development and neoplasia. Whereas the expression of betaIII in neuronal/neuroblastic tumors is differentiation-dependent, the aberrant expression of this cytoskeletal protein in astrocytomas is associated with an ascending gradient of malignancy. To test the generality of this observation we have compared the immunoreactivity (IR) profiles of the betaIII isotype with the Ki-67 nuclear antigen proliferative index in 41 archival, surgically excised oligodendrogliomas (32 classical [WHO grade II] and 9 anaplastic [WHO grade III]). Seventeen of 41 tumors were examined by quantitative microsatellite analysis for loss of 1p and/or 19q. Minimal deletion regions were defined on 1p (D1S468, D1S214) and 19q (D19S408, D19S867). Three of 10 classical oligodendrogliomas had combined 1p/19q loss, while 2 exhibited loss of either 1p or 19q. Three of 7 anaplastic tumors had combined 1p/19q loss. BetaIII IR was present in all tumors, but was significantly greater in the anaplastic (median labeling index [MLI] 61%, interquartile range [IQR] 55%-64%) as compared with the classical variants (MLI, 19%, IQR, 11-36%) (p < 0.0001). A highly significant relationship was found to exist between betaIII and Ki-67 LIs (betaIII, p < 0.0001 and Ki-67, p < 0.0001. r = 0.809). BetaIII localization delineated hitherto understated unipolar or bipolar tumor phenotypes with growth cones and leading cell processes resembling migrating oligodendrocyte progenitor cells. Codistribution of betaIII and GFAP IR was present in "gliofibrillary" tumor areas. Synaptophysin IR was detected in rare tumor cells (mean LI, 0.7%), and only in 4/41 samples (10%), denoting a lack of relationship between betaIII and synaptophysin expression. No significant differences in betaIII LIs were observed in tumors with 1p and/or 19q loss as compared to those with 1p/19q intact status. Increased betaIII IR in oligodendrogliomas is associated with an ascending degree of malignancy and thus is a potentially useful tumor marker. However, the significance of high betaIII LIs in low-grade oligodendrogliomas with respect to prognostic and predictive value requires further evaluation. Class III beta-tubulin expression in oligodendrogliomas should not be construed as a priori evidence of divergent neuronal differentiation.

Arrested oligodendrocyte lineage progression during human cerebral white matter development: dissociation between the timing of progenitor differentiation and myelinogenesis

Immature oligodendrocytes (OLs) derive from a large pool of late OL progenitors that populate human cerebral white matter throughout the latter half of gestation. We recently reported that a minor population of immature OLs are present in human cerebral white matter for at least 3 months before these cells commit to myelinogenesis around 30 wk postconceptional age. Since this finding supports dissociation between the events that regulate human immature OL maturation and their commitment to myelinogenesis, we characterized here the cellular sequence of events that characterize immature OLs during the transition from a premyelinating to a myelinating state. Commitment of immature OLs to myelinogenesis in human cerebral white matter correlated with the longitudinal extension of specialized processes, designated "pioneer processes," that made multiple types of apparent contacts with axons. This event coincided with the appearance of 3 distinct populations of sheaths that varied in their labeling for myelin basic protein (MBP). Since few axons initially labeled for MBP, this supported the occurrence in vivo of O4-negative, O1-positive premyelin sheaths that precede MBP-positive compacted myelin. These observations identify 3 sequential stages of early myelinogenesis: 1) the initial ensheathment of axons by premyelin sheaths generated by immature OLs; 2) the initial insertion of MBP into transitional sheaths; and 3) the generation of MBP-rich mature myelin.

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.

Oligodendroglia in the avian retina: immunocytochemical demonstration in the adult bird

Immunohistochemical techniques were used in conjunction with an avian-specific probe for oligodendrocyte (OLG) marker, the antibody for transferrin binding protein (TfBP), to study the characteristics and distribution of OLGs in the retina of chickens and quails. For comparison, other antibodies such as myelin basic protein, Rip, and those for labeling Müller cells and microglia were used. A large population of OLGs was found to be distributed throughout the retina, with the distinct pattern of a central-to-peripheral gradient. It was possible to detect a spectrum of OLG morphology that bore a resemblance to the subtype of the mammalian central nervous system. In addition to these mature OLGs, limited numbers of TfBP-positive (TfBP(+)) cells with the morphology of immature OLGs were found in the immediate vicinity of the optic head. The majority of OLGs appeared in the ganglion cell layer throughout the retina, whereas OLGs in the nerve fiber layer were seen mainly in the central zone of the retina, near the optic nerve head. Double-labeling experiments showed that OLGs were associated with myelin only in the central region, where the majority of retinal OLGs occurred, but not toward the periphery of the retina. The present study is the first comprehensive analysis of the morphological features and spatial distribution of OLGs in the adult avian retina and provides in vivo evidence for the existence of a substantial population of both mature and immature OLGs in the retina of adult birds. The putative functions of TfBP(+) OLGs including myelination and the tropic role of the ganglion cells are discussed in conjunction with the physical properties of TfBP and structural characteristics of the avascular retina of birds.

Fine structure of the retino-optic nerve junction in the chicken

The aim of this study is to investigate a fine structure of the retino-optic nerve junction in the chicken. We especially focused on the myelin sheaths and astrocytes in the intraocular optic nerve (ION) and its adjoining parts. A part of the axons of retinal nerve fiber layer (NFL) were myelinated. Ganglion cell axons were ensheathed by loose myelin in the NFL and by a compact one in the ION and optic nerve (ON). Myelin structure changed from loose type to a compact one within the very narrow NFL-ION junction. Loose myelin forming cells are dark type of oligodendrocytes in the retina. From the most peripheral ON to the choroidal part of ION, astrocytes contained abundant microtubules. The optic nerve around the lamina cribrosa is exposed to mechanical force during eye movement. It is suggested that these microtubules may perform the cytoskeletal function. Astrocytes in the retinal part of ION had longer processes filled with abundant gliofilaments. They may provide the mechanical support for the ganglion cell axons, which are exposed directly to intraocular pressure. Although astrocytes in the retinal level of ION extended their processes into the retina, their soma was never found in the retina.

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.

Glial cells in the bird retina: immunochemical detection

The avian retina is remarkably different from its mammalian counterpart in macroglial cell appearance. First, it is completely devoid of astrocytes. Thus, Müller cells constitute the only astrocytic-like cell population in avian retinae, whereas mammalian retinae also contain astrocytes in close association with blood vessels. Second, axons in the optic nerve layer of the retina of birds are myelinated, unlike those found in most mammalian species, with the exception of the rabbit, in which the medullary rays of the retina are myelinated by oligodendrocytes. Recent studies have revealed evidence that bird retinae contain a large number of oligodendrocytes, but which glial cell type myelinates axons intraretinally is still controversial. Apart from macroglial appearance, microglia in the bird retina show a very similar pattern of distribution to that of mammalian counterparts. This article reviews the existing data, including our new observations, and discusses the issues that remain to be resolved.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

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.