Organization of radial glial cells during the development of the rat dentate gyrus.

The temporal and spatial patterns of development of radial glial processes in the rat dentate gyrus have been studied in immunohistochemical preparations stained for the presence of either the glial fibrillary acidic protein (GFAP) or the vimentin-associated antigen R4. Additional electron microscopic (EM) observations were made from material prepared either immunohistochemically or by the Golgi method. R4 immunoreactive radial fibers were observed in the incipient dentate gyrus as early as E13 and by E14 the density of stained fibers was clearly higher in the anlage of the dentate gyrus than in the adjacent hippocampus. By E15 it was possible to identify in the EM the endfeet of radial glial cells that contained numerous glycogen particles. GFAP-positive radial processes were first observed on E17; these processes tended to be of larger diameter than those stained with the R4 antibody, suggesting that they were among the more mature processes. The orientation of both the R4- and GFAP-positive glial processes changed throughout the last week of embryonic life and by the end of the first postnatal week they formed a complex meshwork of intertwined processes. The distribution of their cell bodies also changed with time; initially their perikarya were located in the neuroepithelium at the lateral margin of the hippocampal primordium; later they were found mainly beneath the granule cell layer. Dividing cells that contained GFAP were observed along the trajectory of the migrating granule cell precursors and in the hilus of the dentate gyrus; at later stages some GFAP-positive mitotic figures were seen within and immediately below the granule cell layer. On the basis of these observations, we have attempted to reconstruct the role that radial glial processes play in the morphogenesis of the dentate gyrus. First, radial processes extend from the neuroepithelium to the pial surface prior to the migration of neurons that will form the dentate gyrus. These early generated glia appear to form the boundaries of the developing dentate gyrus and provide an internal lattice that may guide the initial wave of migrating progenitor cells. As the dentate gyrus enlarges, these early formed processes maintain their contacts along the hippocampal fissure and along the pial surface of the dentate anlage. Thus, with time they become increasingly distorted and are ultimately compressed into two bundles; one lies deep to the hippocampal fissure parallel to the granule cell layer and the other is located at the fimbriodentate juncture. Second, with subsequent gliogenesis newly formed glial processes are inserted radially with respect to the incipient granule cell layer along a transverse gradient (suprapyramidalinfrapyramidal). This pattern of radial dprocess accretion is correlated with the wave of migration of granule cells and granule cell precursors from the incipient hilus of the dentate gyrus to the granule cell layer. The cascade of radial process insertion from the suprapyramidal to infrapyramidal limb of the dentate gyrus appears to bias granule cell migration toward the growing tip of the granule cell layer. Third, thinner, and presumably younger, radial glial processes are often oriented perpendicular to thicker processes. Since neuronal migration always follows the paths outlined by the thinner processes, one possible explanation for this observation is that as radial glial processes age, they lose their “attractiveness” to neurons as migratory guides.