Radial glial cells (RGCs), or radial glial progenitor cells (RGPs), are bipolar-shaped progenitor cells that are responsible for producing all of the neurons in the cerebral cortex. RGPs also produce certain lineages of glia, including astrocytes and oligodendrocytes. Their cell bodies (somata) reside in the embryonic ventricular zone, which lies next to the developing ventricular system. During development, newborn neurons use radial glia as scaffolds, traveling along the radial glial fibers in order to reach their final destinations. Despite the various possible fates of the radial glial population, it has been demonstrated through clonal analysis that most radial glia have restricted, unipotent or multipotent, fates. Radial glia can be found during the neurogenic phase in all vertebrates (studied to date). The term "radial glia" refers to the morphological characteristics of these cells that were first observed: namely, their radial processes and their similarity to astrocytes, another member of the glial cell family.
Structure
Müller glia Müller glia are radial glial cells that are present in the developing, as well as the adult, retina. As in the cortex, Müller glia have long processes that span the entire width of the retina, from the basal cell layer to the apical layer. However, unlike cortical radial glia, Müller glia do not appear in the retina until after the first rounds of neurogenesis have occurred. Studies suggest that Müller glia can dedifferentiate into readily dividing neural progenitors in response to injury.
The characteristics that truly set Müller glia apart from radial glia in other areas of the brain is their possession of optical properties. The majority of the retina is actually largely light scattering, suggesting that Müller glia serve as the main fiber responsible for the relay of light to the photoreceptors in the rear of the retina. Properties that help Müller glia achieve this function include a limited number mitochondria (which are very light scattering), as well as a specialized arrangement of internal protein filaments. Müller glia are the predominant type of macroglia in the retina, so they take on many of the supportive functions that astrocytes and oligodendrocytes usually handle in the rest of the central nervous system.
Bergmann glia
Bergmann glia (also known as radial epithelial cells, Golgi epithelial cells, or radial astrocytes) are unipolar astrocytes derived from radial glia that are intimately associated with Purkinje cells in the cerebellum. Since Bergmann glia appear to persist in the cerebellum, and perform many of the roles characteristic of astrocytes, they have also been called "specialized astrocytes." Bergmann glia have multiple radial processes that extend across the molecular layer of the cerebellar cortex and terminate at the pial surface as a bulbous endfoot. Bergmann glial cells assist with the migration of granule cells, guiding the small neurons from the external granular layer down to the internal granular layer along their extensive radial processes. Besides their role in early development of the cerebellum, Bergmann glia are also required for synaptic pruning. Following Purkinje cell death induced by CNS injury, Bergmann glia undergo extensive proliferative changes so as to replace lost or damaged tissue in a process known as gliosis.
Development Radial glial cells originate from the transformation of neuroepithelial cells that form the neural plate during neurogenesis in early embryonic development. This process is mediated through the down-regulation of epithelium-related protein expression (such as tight junctions) and an up-regulation of glial-specific features such as glycogen granules, the astrocyte glutamate aspartate transporter (GLAST), the intermediate filament vimentin, and, in some instances, including humans, glial fibrillary acidic protein (GFAP). After this transition, radial glia retain many of the original characteristics of neuroepithelial cells including: their apical-basal polarity, their position along the lateral ventricles of the developing cortex, and the phasic migration of their nuclei depending on their location with the cell cycle (termed "interkinetic nuclear migration").
Function
Progenitors
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