Radial glial cells are present in the velum medullare of adult monkeys

As a rule, in thin mammalian CNS tissues such as the median eminence and the retina, radial glia remains the dominating macroglia in adulthood, whereas in most other regions of the brain radial glial is substituted by multipolar macroglia i.e. astroglia. The Velum medullare is another thin CNS tissue but there are no reports on the dominating macroglia forms of this structure. Thus, Golgi-impregnated sections of adult monkey brains were studied for the presence of radial glial cells. Indeed, this structure was found to be transversed by many radial glial fibres terminating with pial endfeet whereas in adjacent thick brain tissues the glia limitans was formed by marginal astrocytes. It is concluded that fibrous radial glia may dominate in adult mammalian and even primate CNS tissues with a thickness of up to 1 mm.     

The distribution of glial fibrillary acidic protein and vimentin in postnatal marmoset (Callithrix jacchus) brain

Antisera to glial fibrillary acidic protein (GFAP) and vimentin were used to elucidate the distribution of these intermediate filament proteins in postnatal marmoset brains of various ages. The ependyma of the lateral ventricles was unique in being equally immunoreactive for both GFAP and vimentin at all ages. Vimentin alone was consistently demonstrated in endothelial and leptomeningeal cells at all ages. In neonates, vimentin immunoreactivity greatly exceeded that of GFAP and was located primarily in radial glia in the subependymal plate of the anterior cerebrum. Their vimentin-positive processes formed thick fascicles in the corpus callosum but separated into fine fibres on entering the cortex. GFAP immunoreactivity in these cells and processes was very limited. With age, GFAP-positive cells increased in number and displayed the typical stellate appearance of astroglia. The vimentin-positive radial glial population decreased considerably during this period and by 6 months had virtually disappeared. The GFAP reaction in adult brain was even more widespread, largely due to the increased number of positive astrocytes in the white matter. Vimentin immunoreactivity in the adult was greatly diminished and positive radial glia were not detectable. A major change in intermediate filament protein expression, therefore, occurs in the early postnatal period and probably reflects phases in the differentiation of radial glial precursors into astrocytes.     

Radial glia phenotype: origin, regulation, and transdifferentiation

Radial glial cells play a major guidance role for migrating neurons during central nervous system (CNS) histogenesis but also play many other crucial roles in early brain development. Being among the earliest cells to differentiate in the early CNS, they provide support for neuronal migration during embryonic brain development; provide instructive and neurotrophic signals required for the survival, proliferation, and differentiation of neurons; and may be multipotential progenitor cells that give rise to various cell types, including neurons. Radial glial cells constitute a major cell type of the developing brain in numerous nonmammalian and mammalian vertebrates, increasing in complexity in parallel with the organization of the nervous tissue they help to build. In mammalian species, these cells transdifferentiate into astrocytes when neuronal migration is completed, whereas, in nonmammalian species, they persist into adulthood as a radial component of astroglia. Thus, our perception of radial glia may have to change from that of path-defining cells to that of specialized precursor cells transiently fulfilling a guidance role during brain histogenesis. In that respect, their apparent change of phenotype from radial fiber to astrocyte probably constitutes one of the most common transdifferentiation events in mammalian development.     

Development of glial cells in the cerebral wall of ferrets: direct tracing of their transformation from radial glia into astrocytes

Coronal sections of the cerebral wall from developing ferrets (newborn to adult) were double-stained with antibodies to vimentin and glial fibrillary acidic protein (GFAP). At birth, the dominant glial population was radial glia and these cells labeled only for vimentin. A small population of immature astrocytes in the cortical plate was double labeled for GFAP and vimentin. In successive days, the number of vimentin-positive radial glia gradually decreased and they disappeared entirely at about 21 days. During this same period, the double-stained astrocytes increased in number and were distributed throughout the cortical plate and intermediate zone. After 6 weeks of age the astrocytes were mostly confined to the developing white matter. Around this time they gradually lost their vimentin staining, and in the adult no vimentin-positive elements were seen except at the ependymal surface. In newborn ferrets single radial glial cells were also visualized by applying the carbocyanine dye DiI onto the pial surface of fixed brains. While most radial glia extended from the ventricular zone to the pial surface, a substantial fraction of them had lost their contact to the ventricular zone. Their somata were displaced into the subventricular zone and lower portion of the intermediate zone. The possibility that radial glia transform into astrocytes was directly tested by injecting fluorescent dyes under the pial surface of newborn ferrets at a time when virtually no GFAP-positive astrocytes are present. The tracer, which was taken up in the upper portion of the cortical plate, stained the radial glial cell somata in the ventricular zone in a similar way as the dye DiI did in the fixed brains. As the radial glial cells disappeared at successively longer survival times, the tracer was ultimately found within newly formed GFAP-positive astrocytes. These results provide strong support for the hypothesis that radial glia cells are the immature form of astrocytes (Choi and Lapham: Brain Res. 148:295-311, '78; Schmechel and Rakic: Anat. Embryol. (Berl.) 156:115-152, '79), and also show that, at least in the ferret cortex, the transformation is accompanied by a change in the expression of intermediate filament protein.     

Astroglial changes in the cerebral cortex of AIDS brains: a morphometric and immunohistochemical investigation

Astroglial changes in the cerebral cortex of AIDS brains were analysed by means of morphometry. Astrocytes with and without immunoreactivity for glial acidic protein (GFAP) were counted and their size was measured. In the two investigated cortical areas (frontal and parietal), a similar reaction pattern of astroglia was observed. The total number of astrocytes (i.e. GFAP-positive and GFAP-negative astrocytes) did not differ between control and AIDS brains. However, the number of GFAP-positive astrocytes was significantly increased in AIDS brains, while the number of GFAP-negative cells was significantly reduced. Nuclear size of GFAP-negative and GFAP-positive astrocytes was significantly increased. The reaction pattern of cortical astrocytes in AIDS seems to be characterized by GFAP production in protoplasmic astroglia as well as by hypertrophy of all astrocytes.     

Ganglioside GD3 in radial glia and astrocytes in situ in brains of young and adult mice

Ganglioside GD3 occurs in immature cells in the neuroectoderm. However, with regard to particular cellular locations of GD3, rat brain has received more attention than mouse brain. In brains from neonatal mice the most intense GD3 immunostaining appears to occur in structures that differ from those that immunostain the most intensely in brains from neonatal rats (Cammer and Zhang: J Histochem Cytochem 44: 143–149, 1996). In the present study epifluorescence and confocal microscopy were used for the purpose of identifying the types of GD3-immunopositive structures in brains of neonatal, 2-week-old, and adult mice. Vibratome sections from mouse brains were double immunostained for GD3 and respective markers for macrophages, microglia, and cells belonging to the oligodendrocyte lineage. Surprisingly, none of those marker antigens immunostained intensely in the same respective structures as GD3. The GD3-positive structures, however, did resemble protoplasmic astrocytes and radial glia, some with GD3-positive end-feet at the glia limitans; however, we did not rule out the possibility that there might be some GD3 on the surfaces of prooligodendroblasts. The scarcity of glial fibrillary acidic protein (GFAP)-positive cells in brains of neonatal mice made it impractical to look for GD3+/GFAP+ structures that might belong to the astrocyte lineage. The Mu subunit of glutathione-S-transferase (Mu) was shown to label radial glia and the few GFAP-positive cells in brains of neonatal mice. Subsequently, confocal microscopy showed Mu and GD3 to be colocalized in radial glia and protoplasmic astrocytes in the neonate. In brains from mice ≥2 weeks of age GD3 immunostaining was demonstrated in GFAP-positive astrocytes, including reactive astrocytes. Much of the GD3 appeared to occur at the tips of astrocyte processes. It is suggested that GD3 in radial glia and astrocytes may function as a ligand enabling recognition of those structures by neurons or as a precursor of more complex gangliosides in neurons. © 1996 Wiley-Liss, Inc.     

Plasticity of astroglia: evidence supporting process elongation by "stretch"

The structural plasticity of cerebral astroglia was investigated in vivo by implantation experiments. Immunocytochemical markers for glia filament protein were used to identify the astrocytes. First it was established that implanted nitrocellulose filters provided a substrate for astrocytes from different brain regions of young rats. Astrocytes attached to the filter and projected fine processes into it. Longer implantation times increased the density and length of glial processes within filter spaces. Astrocytes that penetrated the filters implanted in the tectum exhibited more processes than those in the cortex, suggesting regional differences of astrocyte distributions. Second it was observed that astrocytes that attached to the filter formed elongated processes when they were tethered within an expanding matrix. This was shown by implanting the nitrocellulose filter together with PC12 cells that continued to grow. The implantation of neither PC12 cells without filters nor nitrocellulose filters alone induced the formation of elongated astroglia with parallel aligned processes, resembling radial glia. Such glial forms only occurred in the filter/PC12 cell cografts. This indicates that processes of astrocytes adherent to nitrocellulose filters could be stretched in response to expansion of the surrounding tissue.     

Embryonic cerebellar astroglia in vitro

Three types of astroglia appear during cerebellar development--radial glia and Bergmann glia, which are thought to facilitate neuronal migration, and astrocytes, which are thought to compartmentalize mature granule neurons. Cells resembling Bergmann glia and astrocytes have been described in cultures of cerebellar cells harvested from early postnatal cerebellum. In this study, we have used cell-type specific antisera to visualize embryonic forms of cerebellar astroglia and their interaction with embryonic neurons in vitro. When cells were dissociated from mouse cerebellum on the thirteenth embryonic day (E13), 3 forms of cells were stained with antisera raised against purified glial filament protein ( AbGF ), all of which had more elongated processes and less complex shapes than astroglia from postnatal day 7. The vast majority of embryonic cerebellar neurons did not contact these immature forms of astroglia.     

Prenatal exposure to ethanol alters the postnatal development and transformation of radial glia to astrocytes in the cortex

Postmitotic neurons migrate from a zone(s) near the ventricles to the neocortex. During this migration, neurons associate with radial glia. After serving their role as guides for neuronal migration, the radial glia transform into astrocytes. Prenatal exposure to ethanol causes abnormal neuronal migration. We examined the effects of gestational exposure to ethanol on radial glia and astrocytes. Radial glia were stained immunohistochemically with the antibody RAT-401, and astrocytes were labeled with an antibody directed against glial-fibrillary acidic protein (GFAP). The subjects were the offspring of rats fed an ethanol-containing liquid. diet (Et), pair-fed a liquid control diet (Ct), or fed chow and water (Ch). During the first postnatal week, radial glial fibers (in Et-treated rats and controls) stretched from the ventricular surface through the developing. cerebral wall to the pial surface. In the Et-treated rats, the radial processes were less dense and more poorly fasciculated than they were in the Ch-and Ct-treated rats. Moreover, by postnatal day (P) 5, there was a significant reduction in RAT-401 immunostaining in the Et-treated rats, particularly in the superficial cortex. A similar reduction in control rats did not begin until P10. In all three treatment groups, GFAP-immunoreactive astrocytes were in the cortex throughout the period from P1 to P45. In neonates, GFAP-positive cells were distributed in the marginal zone (layer I) and the intermediate zone (the white matter). The number of GFAP-positive cells in the cortical plate increased steadily with time so that, by P26, GFAP-immunoreactive astrocytes were distributed evenly through all cortical laminae. Interestingly, between P5 and P12, the number of astrocytes was significantly greater in Et-treated rats than in controls. Thus prenatal exposure to ethanol induces the premature loss of RAT-401-positive processes and the precocious increase in GFAP immunostaining. These ethanol-induced changes in glial development indicate that ethanol accelerates the transformation of radial glia into astrocytes. Moreover, the ethanol-induced premature degradation of the network of radial glial fibers may underlie the migration of late-generated neurons to ectopic sites. © 1993 Wiley-Liss, Inc.     

Developmental potential of radial glia investigated by transplantation into the developing rat ventricular system in utero

During development there is a clear correlation between position of dividing progenitor cells, mode of division and developmental potential, suggesting that the local environment of progenitor cells may influence their cell fate [ 17 (6), 639-647]. The contribution of these conditions was investigated here by transplantation of radial glial progenitor cells into isotopic, isochronic, heterotopic and heterochronic environment conditions. Neuronal cells were removed from E14 spinal cords using negative immunoselection. The remaining radial glia were transplanted into the ventricular system of host embryos and pups. Distance of migration as well as morphological and antigenic phenotype of transplanted radial glia was examined after various survival times post transplantation. Host age clearly influenced migration and differentiation of transplant cells, with transplant cells migrating further in younger hosts and differentiating earlier in older aged host environments. Evidence is presented showing that most transplanted spinal cord radial glia give rise to astrocytes. In addition some transplanted radial glia were shown to give rise to neurons in spinal cord regions. Radial glia did not appear to generate neurons in the brains of host animals until postnatal ages, perhaps because transplanted radial glia were isolated from spinal cord and thus may not have been influenced to behave as endogenous radial glia in the brain which commonly produce neurons.     

Astroglia and microglia in cerebellar neuronal migration disturbances

Between the neuronal and glial cells there is a close relationship conditioning a tight morphological correlation and proper functional interplay. Disturbed interaction between glial and neuronal components leads to inappropriate neural circuits. The reflection of the failure of neural circuit organisation is the picture of morphological changes of neurons and glia. The appearance of microglia and astroglia was analysed in a defectively formed cellular network due to cerebellar neuronal migration disturbances. Focal disruption of neuron migration leads to their differentiation in an abnormal position manifested as heterotopias and cortical anomalies. Neurons that had lost their proper migratory way and heterotopically settled in the white matter were encircled by GFAP-positive astrocytes, with morphology appropriate for surrounding white matter. The microglial cells infiltrated the parenchyma within the heterotopic neurons playing a role in their elimination. In the cerebellar cortical malformations astrocytes were grouped near the Purkinje cells. In the minimal cortical dysplasia the increased number of astrocytes supported the neurons. Impaired morphological components of the glial-pial barrier were observed. In the massive cortical malformations a few degenerated astrocytes followed the disarranged Purkinje cells, while microglia and Bergmann glia fibres were not present. Absence of cells supporting and organizing the cerebellar cortex had an effect on loss of Purkinje cell shape, their disorientation and abnormal position. The appearance and localisation of the astroglia and microglia in the abnormal cerebellar circuitry due to migration disturbances is dependent on the pathomechanism of the anomalies.     

Radial glial cells as neuronal precursors: a new perspective on the correlation of morphology and lineage restriction in the developing cerebral cortex of mice

Radial glia is a ubiquitous cell type in the developing central nervous system (CNS) of vertebrates, characterized by radial processes extending through the wall of the neural tube which serve as guiding cables for migrating neurons. Radial glial cells were considered as glial precursor cells due to their astroglial traits and later transformation into astrocytes in the mammalian CNS. Accordingly, a hypothetical morphologically distinct type of precursor was attributed the role of neurogenesis. Recent evidence obtained in vitro and in vivo, however, revealed that a large subset of radial glia generates neurons. We further demonstrate here that the progeny of radial glial cells does not differ from the progeny of precursors labeled from the ventricular surface, implying that there is no obvious relation between precursor morphology and neuron-glia lineage decisions in the developing cerebral cortex of mice. Moreover, we show that many radial glial cells seem to maintain their process during cell division and discuss the implications of this observation for the orientation of cell division. These new data are then related to radial glial cells in other non-mammalian vertebrates persisting into adulthood and suggest that radial glia are not only neurogenic during development, but also in adulthood.     

GFAP-positive progenitor cells produce neurons and oligodendrocytes throughout the CNS

Once thought to merely act as scaffolds in neuronal migration, recent evidence suggests that radial glia may serve as progenitors for the majority of neurons in the CNS. Cre/loxP fate-mapping experiments were carried out using a fragment of a glial-specific promoter (glial fibrillary acidic protein; GFAP) to drive expression of Cre recombinase. We show that GFAP+ progenitor cells give rise to neurons and oligodendrocytes throughout the CNS. We find very little regional heterogeneity in the neurogenic potential of radial glia between dorsal and ventral telencephalon. Additionally, radial glia serve as precursors for subpopulations of interneurons in the ventral telencephalon. Interestingly, the human GFAP promoter but not the mouse GFAP promoter is active in oligodendrocyte progenitor cells. We also demonstrate that the most commonly used Cre reporter lines are very inefficient in detecting Cre-dependent recombination in astrocytes and describe a new Cre reporter line for assessing recombination in astrocytes.     

Role of activated astrocytes in neuronal damage: Potential links to HIV-1-associated dementia

HIV-1-associated dementia (HAD) is an important complication of HIV-1 infection. Reactive astrogliosis is a key pathological feature in HAD brains and in other central nervous system (CNS) diseases. Activated astroglia may play a critical role in CNS inflammatory diseases such as HAD. In order to test the hypothesis that activated astrocytes cause neuronal injury, we stimulated primary human fetal astrocytes with HAD-relevant pro-inflammatory cytokine IL-1beta. IL-1beta-activated astrocytes induced apoptosis and significant changes in metabolic activity in primary human neurons. An FITC-conjugated pan-caspase inhibitor peptide FITC-VAD-FMK was used for confirming caspase activation in neurons. IL-1beta activation enhanced the expression of death protein FasL in astrocytes, suggesting that FasL is one of the potential factors responsible for neurotoxicity observed in HAD and other CNS diseases involving glial inflammation. Our data presented here add to the developing picture of role of activated glia in HAD pathogenesis.     

Peripapillary glial cells in the chick retina: A special glial cell type expressing astrocyte, radial glia, neuron, and oligodendrocyte markers throughout development

Peripapillary glial cells of the chick are a special type of glia, not only because of their position, forming a boundary between the retina on one side and the optic nerve head (ONH) and the pecten on the other, but also because although they have the same orientation and similar shape as the retinal Müller cell (a type of radial glia) and express common markers for these cells and astrocytes, they do not express glutamine synthetase (GS) or carbonic anhydrase C (CA-C), enzymes intensely expressed by Müller cells and astrocytes. In this study, we present further molecular characterization of these cells, using immunohistochemistry techniques. We show that peripapillary glial cells express a novel neuron antigen, 3BA8, that in the adult retina is located only in one neuron type (the amacrine cell) and in the inner plexiform layer (IPL). They also express an antigen specific to myelin and oligodendrocytes, MOSP, and a glial antigen, 3CB2, expressed by radial glia and astrocytes throughout the CNS. The study of the developmental expression of these three antigens in the peripapillary glial cell territory shows different spatiotemporal labeling patterns: 3CB2 and 3BA8 are expressed much earlier (embryonic days E3 and E5, respectively) than MOSP (E12), and during a developmental window (E6-E10) 3BA8 labels the peripapillary glial cells intensely and does not label the ONH or the optic nerve (ON), which are labeled later. The expression of 3CB2 is much more intense in the peripapillary glial cells than in Müller cells from early stages of development up to E16, and the expression of MOSP starts earlier in the peripapillary glial cells than in the Müller cells and is maintained with much higher intensity in the peripapillary glial cells throughout development. These findings show that Müller and peripapillary glial cells follow independent courses of differentiation, which together with the fact that the peripapillary glial cells express molecules typical of neurons, oligodendrocytes, radial glia, and astrocytes are evidence that peripapillary glial cells are a unique type of glia in the CNS.     

Neuronal and glial coexpression of argininosuccinate synthetase and inducible nitric oxide synthase in Alzheimer disease

The enzyme argininosuccinate synthetase (ASS) is the rate limiting enzyme in the metabolic pathway leading from L-citrulline to L-arginine, the physiological substrate of all isoforms of nitric oxide synthases (NOS). ASS and inducible NOS (iNOS) expression in neurons and glia was investigated by immunohistochemistry in brains of Alzheimer disease (AD) patients and nondemented, age-matched controls. In 3 areas examined (hippocampus, frontal, and entorhinal cortex), a marked increase in neuronal ASS and iNOS expression was observed in AD brains. GFAP-positive astrocytes expressing ASS were not increased in AD brains versus controls, whereas the number of iNOS expressing GFAP-positive astrocytes was significantly higher in AD brains. Density measurements revealed that ASS expression levels were significantly higher in glial cells of AD brains. Colocalization of ASS and iNOS immunoreactivity was detectable in neurons and glia. Occasionally, both ASS-and iNOS expression was detectable in CD 68-positive activated microglia cells in close proximity to senile plaques. These results suggest that neurons and astrocytes express ASS in human brain constitutively, whereas neuronal and glial ASS expression increases parallel to iNOS expression in AD. Because an adequate supply of L-arginine is indispensable for prolonged NO generation, coinduction of ASS enables cells to sustain NO generation during AD by replenishing necessary supply of L-arginine.     

Radial glia in the zebrafish brain: Functional,structural, and physiological comparison with the mammalian glia

The neuroscience community has witnessed a tremendous expansion of glia research. Glial cells are now on center stage with leading roles in the development, maturation, and physiology of brain circuits. Over the course of evolution, glia have highly diversified and include the radial glia, astroglia or astrocytes, microglia, oligodendrocytes, and ependymal cells, each having dedicated functions in the brain. The zebrafish, a small teleost fish, is no exception to this and recent evidences point to evolutionarily conserved roles for glia in the development and physiology of its nervous system. Due to its small size, transparency, and genetic amenability, the zebrafish has become an increasingly prominent animal model for brain research. It has enabled the study of neural circuits from individual cells to entire brains, with a precision unmatched in other vertebrate models. Moreover, its high neurogenic and regenerative potential has attracted a lot of attention from the research community focusing on neural stem cells and neurodegenerative diseases. Hence, studies using zebrafish have the potential to provide fundamental insights about brain development and function, and also elucidate neural and molecular mechanisms of neurological diseases. We will discuss here recent discoveries on the diverse roles of radial glia and astroglia in neurogenesis, in modulating neuronal activity and in regulating brain homeostasis at the brain barriers. By comparing insights made in various animal models, particularly mammals and zebrafish, our goal is to highlight the similarities and differences in glia biology among species, which could set new paradigms relevant to humans.     

Radial Glia-Like Cells in the Supraoptic Nucleus of the Adult Rat

Conventional light and confocal microscopy of thick vibratome sections of the hypothalamus of adult male and female rats immunostained for the astrocytic marker glial fibrillary acidic protein (GFAP) revealed that the supraoptic nucleus (SON) contains two morphologically distinct types of astrocytes. One has a stellate form, similar to that of most astrocytes in the adult CMS. The other has a morphology reminiscent of radial glia in the developing CNS: from their cell bodies, located along the ventral glia lamina (VGL), arise one long thick process that spans the SON in the coronal plane, several horizontally-oriented processes that form a dense network in the VGL, and a short process oriented towards the pia. The latter astrocytes are immunoreactive for vimentin, an intermediate filament protein of immature glial cells and a marker for radial glia. The stellate astrocytes showed no vimentin immunoreactivity. The functional significance of each type of supraoptic astrocyte is at present unknown but the presence of radial glia-like cells in this hypothalamic region suggests that the SON retains a certain degree of immaturity during adulthood, that may be linked to its well known capacity to undergo neuronal-glial plasticity under physiological and experimental stimulation.     

Nerve cell loss in the thalamic mediodorsal nucleus in Huntington’s disease

We estimated the total neurone number, glial number, and glial index (ratio glial cells/neurone) in the thalamic mediodorsal nucleus (MD) in seven patients suffering from Huntington’s disease (HD; four males, three females, mean age 52.4 ± 13.6 years) and age- and sex-matched controls (four males, three females, mean age 53.6 ± 12.1 years) by means of a stereological protocol. The mean total neurone number (N_Tˉ) in the MD of controls was 2,985,188 ± 174,710, the mean glial number (G_Tˉ; astrocytes, oligodendrocytes) 21,785,008 ± 2,986,678, and the glial index 7.29 ± 0.88. In HD, the average neurone number was decreased by 23.8% to 2,275,321 ± 247,162 (Mann-Whitney U-test P < 0.05), the mean glial number by 29.7 % to 15,318,895 ± 1,722,524 (Mann-Whitney U-test P < 0.05), the glial index was slightly reduced to 6.81 ± 1.06. Gallyas’ impregnation for the demonstration of fibrous astroglia gave strongly positive results in all cases with HD and negative results in the controls. The morpho-functional correlation of the results is complicated because individual variability, presence of segregated and parallel neuronal circuits, and plasticity of the adult human CNS must be considered. Received: 9 September 1997 / Revised: 24 August, 12 November 1998 / Accepted: 13 November 1998     

Morphology and differentiation of radial glia in the developing rat spinal cord

Important events underlying the proper functioning of the central nervous system (CNS) include the production, assembly, and differentiation of appropriate types and numbers of cells during development. The mechanisms that control these events are difficult to unravel because of displacement of cells from their sites of origin to their permanent locations and because of the diverse cellular composition of the CNS. As in other regions of the mammalian CNS, the two major classes of neuroglial cells in the rat spinal cord are oligodendrocytes and astrocytes. In the developing spinal cord, radial glia are prominent. In this study, radial glia in the cervical region of the spinal cord were analysed. 1,1'Dioctadecyl-3,3,3'-tetramethylindocarbocyanine perchlorate (DiI) was used to determine the morphology and distribution of radial glia during spinal cord development. The DiI labelling technique enabled locating glial precursor cells during spinal cord development. Radial fibres that extended from the central canal to the pial surface were present at embryonic days 14, 16, and 18 in the developing spinal cord. Their distribution was restricted with increasing development, and by embryonic day 20 the only remaining evidence of radial glia were short radial processes in the white matter.