The granule cell layer of the adult dentate gyrus possesses two characteristics of an immature nervous system. The first is that granule cells continue to be generated in the innermost region of the granule cell layer, and newly generated and developing granule cells in the adult express highly polysialylated neural cell adhesion molecule (PSA-NCAM). PSA-NCAM-expressing apical dendrites have dynamically unstable processes such as irregular shafts and many stick-like or fan-shaped fine processes. The second is that radial glia-like cells expressing glial fibrillary acidic protein (GFAP) remain in a similar region of the granular layer. The numbers of PSA-NCAM-expressing granule cells and GFAP-expressing radial glia-like cells show a parallel age-dependent decrease during aging. Moreover, by using confocal laser scanning microscopy and immunoelectron microscopy, we demonstrated that PSA-NCAM-expressing dendrites and GFAP-expressing radial processes are partly in contact with each other, and occasionally the radial glial processes envelop the PSA-NCAM-positive dendritic processes. The temporal and spatial relationship between the two immature elements suggests that the processes of the radial glia-like cells are closely associated with the dendritic growth of the newly generated granule cells in the adult dentate gyrus and that these two immature features of neurons and glia in the dentate gyrus diminish with age.