The radial glia was first revealed by the Golgi silver impregnation at the turn of the twentieth century and later described by Rakic in the early 1970s. They have a highly organized palisading cytoarchitecture, in relation to their role in neuronal migration from the ventricular zone to the cortical layers. In the developing CNS, cell bodies of the radial glia lose their ventricular insertion and migrate toward the brain surface. In the monkey, these glial cells appear during the first third of gestation, first in the spinal cord and then in the diencephalon, the telencephalon, and the cerebellum. During the peak of neurogenesis, there are two distinct classes of proliferative cells: those expressing GFAP that will give rise to radial glia and those not expressing this marker (i.e., neuroblasts). These GFAP+ and GFAP- cells are intermixed in the ventricular and subventricular zones, which illustrates that glial and neuronal cell lines coexist within the early fetal proliferative zones and that the onset of glial phenotypic expression occurs prior to the last division of neuroblasts. Expression of embryonic radial glial identity requires the presence of extrinsic soluble signals is in embryonic forebrain, including neuronal factors.
By using a Golgi technique, Schmechel and Rakic showed in the monkey brain that radial glia transforms into astrocytes. Radial glial cells assume a variety of transitional forms during the process of this transformation into mature astrocytes. This transformation occurs in different areas of the CNS at a specific embryonic age and is initiated after neuronal migration has begun to subside. The bipolar radial cell becomes transformed into the astrocytic multipolar form; this may occur directly or with a monopolar radial form as an intermediate state of transformation (Fig. 2). The number of astroglial cells increases at an accelerated pace after neurogenesis is complete.
A recent discovery suggests that in the developing CNS, radial glial cells may have the potential to self-renew and to generate both neurons and astrocytes; these data led to the concept that a lineage relationship between these two cell types underlies the radial organization of the neocortex.
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