RADIAL GLIAL CELLS: key organisers in CNS development

Denis S. Barry*, Janelle M.P. Pakan† and Kieran W. McDermott†

*Department of Anatomy, Trinity College Dublin, Dublin, Ireland.

†Department of Anatomy and Neuroscience, University College Cork, Cork, Ireland.

*Corresponding author

Denis BarryDepartment of Anatomy Trinity College DublinDublinIreland

Email: debarry@tcd.ie

Phone: + 353 1 8961793

Abstract

Radial glia are elongated bipolar cells present in the CNS during development. Our

understanding of the unique roles these cells play has significantly expanded in the last

decade. Historically, radial glial cells were primarily thought to provide an architectural

framework for neuronal migration. Recent research reveals that radial glia play a more

dynamic and integrated role in the development of the brain and spinal cord. They represent a

major progenitor pool during early development and can give rise to a small population of

multipotent cells in neurogenic niches of the adult CNS. Radial glial cells are a

heterogeneous population, with divergent and often poorly understood roles across different

brain and spinal cord regions during development; this heterogeneity extends to specialized

adult subtypes, such as tanycytes, Müller glial cells and Bergman glial cells which possess

morphological similarities to radial glial but play distinct functional roles in the CNS.

Keywords: Radial glia, neurodevelopment, neuronal migration, glioma

Cell Facts

Radial glial cells differentiate from neuroepithelial cells in the developing CNS.

Radial glial cells possess an elongated radial process spanning the CNS from the

ventricular zone to the pial surface.

Radial glial cells have multifunctional roles; they provide structural support during axon

growth, they act as a scaffold for neuronal migration.

Radial glial cells are important progenitor cells contributing to gliogenesis in all regions

and additionally to neurogenesis in the mammalian cerebral cortex.

Radial glial cells are a transient cell type present mainly during development although a

few exceptional radial glial-like cell populations exist into adulthood.

Alterations in the radial glial cell network during development may lead to disorganized

CNS tissues resulting in different neurodevelopmental disorders.

Introduction

Radial glial cells have been the subject of much interest during the on-going efforts to

understand CNS formation. This cell type, with its long radial process, was originally

discovered spanning the foetal spinal cord by Camillo Golgi in 1885 (Rakic, 2003). Ramon

Cajal originally suggested a glial identity for these cells by demonstrating their

morphological similarities with astrocytes, but their glial phenotype was ultimately confirmed

60 years later when immunohistochemistry and electron microscopy showed that radial glial

cells contain glycogen granules and GFAP, which are intracellular characteristics found only

in glia (Choi, 1981; Levitt & Rakic, 1980). Today, radial glial cells are recognised as

morphologically, biochemically and functionally distinct from other neural cell types. They

display an apical - basal polarity possessing a periventricular cell body and an elongated

process extending from a ventricular attachment to an end-foot anchored to the opposing pial

surface (Fig. 1A, B). Some radial glia are thought to persist into adult life, albeit in very

limited numbers, and some in specific neurogenic niches. Radial glia subtypes have also been

identified, including tanycytes around the ventricles, Müller glia in the retina and Bergmann

glia in the cerebellum (Fig. 1C-E). Notably, both Müller and Bergmann glia preserve their

radial morphology postnatally (Guo et al, 2013; Surzenko et al, 2013).

The classical role of radial glia is in neuronal migration (Rakic, 1972), acting as

guidance cables aiding the migration of new-born neurons; however, modern cell fate

determination and imaging methodologies have revealed that radial glia are also

multifunctional neural stem cells that structurally orchestrate CNS organization and generate

different cells types, conforming to the needs of the different neural compartments they

occupy.

Cell origin and plasticity

Neuroepithelial cells arise from the ectoderm early in development, divide symmetrically and

generate the neural plate. This invaginates to form the neural tube, the width of which is

occupied by a polarised pseudostratified neuroepithelium. Neuroepithelial cells possess a

basal side attached to the pial surface and an apical side that contacts the lumen of the neural

tube. As development proceeds, they multiply and form the CNS germinal zones, namely the

ventricle zone and subventricular zones. Here, neuroepithelial cells, most of which by now

have a radial morphology and are termed radial neuroepithelial cells, serve as neural stem

cells, expanding the CNS by self-renewing and populating it by generating neurons and glia.

After completing the early phases of neurogenesis, they begin their transformation into radial

glial cells. This is initiated by the down regulation of epithelial features, such as tight

junctions (Aaku-Saraste et al, 1996) and completed by the up regulation of glial specific

identifiers such as the membrane proteins GLAST and BLBP (Morest & Silver, 2003) and

the appearance of cytoplasmic glycogen granules (Gadisseux & Evrard, 1985). Radial glial

cells and radial neuroepithelial cells express the intermediate filament nestin (Hartfuss et al,

2001) and both undergo interkinetic neuronal migration (LaMonica et al, 2013; Tsai et al,

2010). While this developmental sequence of events appears to be consistent for many CNS

radial glia, in the cerebral cortex they retain the capacity to generate large numbers of

neurons. In regions other than the cerebral cortex, the contribution of differentiated radial glia

to on-going neurogenesis is unclear. In the spinal cord and cerebellum, their appearance

temporally precedes the first appearance of astrocytes and oligodendrocytes suggesting that

here most radial glia bypass neurogenesis, and adopt a glial fate after fulfilling their guidance

(Rakic, 2003) and boundary forming functions (Barry et al, 2013).

Cell functions

Radial glia and their subtypes show extraordinary adaptability facilitating the

formation of the cerebellum, hypothalamus, cerebral cortex, and spinal cord. While their roles

in different CNS regions can overlap, their functions seem to depend on the developmental

status of the brain region they occupy. Their longest established and best characterised

function is in neuronal migration, intricately guiding new-born neurons from germinal zones

to their target destinations in the correct lamina of the cerebral cortex (Fig. 2A, B) (Rakic,

1972; Xu et al, 2013). It is not clear whether or not radial glia support neuronal migration in

the developing spinal cord, but it seems unlikely as neurogenesis and neuronal migration

largely precede the differentiation of radial glia from the neuroepithelium (Barry &

McDermott, 2005).

Radial glia are now considered key progenitor cells, comprising the majority of

mitotically active cells in brain ventricular zones (Lui et al, 2011; Malatesta & Gotz, 2013;

Pilz et al, 2013). Subsequently, many clearly differentiate into astrocytes later in development

(Noctor et al, 2004). Moreover, recent in vivo genetic fate mapping experiments have

revealed that some radial glia in the cerebral cortex are lineage restricted to generating upper

layer neurons, implicating them in human brain evolution (Franco et al, 2012). These stem

cell roles for radial glia have transformed the heretofore prevailing view of separate

neuroepithelial lineages for neurons and glia. However, as much of this evidence has been

obtained using different experimental techniques, in different regions and across different

species, it seems premature to assume that all radial glia generate neurons in all regions of the

CNS.

Apart from their stem cell and neuronal migration roles, radial glia cells also facilitate

the formation and compartmentalisation of the white matter (Steindler, 1993). For example,

axons forming the corpus callosum grow within a transient glial ‘sling’, which is most likely

composed of radial glia, that disappears around the perinatal period (Silver et al, 1982). In the

spinal cord, finely organised radial glial processes also create boundaries which separate

nascent axon tracts in the emerging dorsal and lateral white matter (Fig. 2C, D) (Barry et al,

2013). These recent observations demonstrate new temporally separated roles for spinal cord

radial glia; firstly, organising axonogenesis and then, when axon tracts have matured,

generating glial cells.

Recent data have also implicated radial glia in the regulation of cerebral cortical

vascularisation via modulation of canonical Wnt signalling (Ma et al, 2013). Indeed,

inhibition of radial glial cell division in developing cerebral cortex of Orc3 knockout mice

leads to neonatal cerebral haemorrhage resulting in major reductions in vessel density and

branch point frequency (Ma et al, 2013). This implicates radial glia in the pathogenesis of

new-born cerebrovascular diseases, such as perinatal haemorrhagic stroke.

Associated Pathologies

As described, radial glia give rise to nearly all cortical neurons and glia and serve as

neuronal migration conduits. Therefore, dysfunction of radial glial cell cycle has catastrophic

consequences for brain lamination. Lissencephaly and micro-lissencephaly (smooth brain) are

neurodevelopmental diseases caused by defects in neurogenesis and neural migration and

result in reduced brain volume and a lack of cortical sulci and gyri at birth. Affected patients

may experience mental retardation, motor and speech dysfunction, balance problems, and

epilepsy. More extreme cases result in death within a few months of life (Wu & Wang,

2012). Microlissencephaly and lissencephaly are associated with the radial glial intracellular

scaffold protein’s Lis1 and its binding partner Nde1 (Alkuraya et al, 2011; Reiner et al,

1993), which aid in microtubule organisation and play an essential role in radial glial

differentiation (Pawlisz & Feng, 2011). Animal models lacking Lis1 and Nde1 show severe

neurogenesis and neuronal migration abnormalities (Pawlisz & Feng, 2011). Further insight

into the precise mechanisms underlying how radial glia mediate neuronal migration are

essential when considering the pathologies resulting from lissencephaly and other

developmental disorders that cause abnormal migration, such as foetal alcohol syndrome.

Gliomas are the most common adult brain tumours and often the most lethal. Some

are thought to originate during development (Vick et al, 1977) and oncogenic cells have been

linked to progenitor cell populations resident not only during development, but also in the

adult (Sanai et al, 2005; Wu & Wang, 2012). It is not unsurprising, therefore, that radial glia

are recognised as potential targets of oncogenic inducers and have high potential for

malignant transformation (De Rosa et al, 2012; Wu & Wang, 2012). A full understanding of

the lifecycle of a neural stem cell will be central to our understanding of embryonic brain

tumorigenesis and related developmental diseases.

Acknowledgements

The authors wish to acknowledge funding from the following sources: The Health

Research Board of Ireland, Programme for Research in Third Level Institutions, The Irish

Research Council.

Figure Legends

Figure 1. Radial glial cells and radial glial-like subtypes in the embryonic and adult CNS. A)

Radial glial cell bodies are present in the ventricular zone (VZ) of the developing spinal cord.

Their processes extend from the central canal (CC) through developing white matter (WM) to

the pial surface. B) Radial glia in the developing cerebral cortex also have cell bodies in the

VZ and their processes extend though the sub-ventricular zone (SVZ) and the developing

cortical plate (CP) to the pial surface. C) Tanycytes in the adult hypothalamus have cell

bodies adjacent to the third ventricle (3V) and processes that extend to the pial surface. D) In

the adult cerebellar cortex Bergmann glia have cell bodies in the Purkinje cell layer (PCL)

and extend highly branched processes through the molecular layer (ML) to the pial surface.

E) Müller glia in the adult retina are large elongated cells extending from the photoreceptors

(PR) to the ganglion cell layer (GCL). Median eminence (ME), arcuate nucleus (AN),

ventromedial hypothalamus (VMH), granule cell layer (GL), inner and outer nuclear layer

(INL and ONL), inner and outer plexiform layer (IPL and OPL).

Figure 2. Radial glial cell structure and function in the developing CNS. A) Progenitor cell

potential of neuroepithelial cells and radial glia during development. Neuroepithelial cells

proliferate and generate neuroblasts and immature neurons. They then differentiate into radial

glia which proliferate and elongate. Radial glia in the cortex contribute to neurogenesis

directly or via immediate neuronal precursor cells (nIPC). Cortical and spinal cord radial glia

contribute to gliogenesis by producing astrocytes (light blue) and possibly oligodendrocytes.

Some radial glia may also differentiate into ependymal cells which line the ventricles of the

adult CNS. B) Radial glia proliferate at the apical surface of the VZ and serve as scaffolds for

newly formed neurons to migrate through the SVZ and into the developing CP. This process

may also facilitate the migration of radial glial cell derived astrocytes. C) BLBP-expressing

radial glial processes form structural boundaries in the spinal cord, delineating the putative

dorsal columns cord (inset). D) A 3-dimensional cross-section view of the embryonic spinal

cord (see red inset schematic) showing BLBP-expressing radial glia forming distinct

corridors (arrows) in the white matter, through which axons may grow. Scale bars = 50 µm.

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