Glial Tubes in the Rostral Migratory Stream of the Adult Rat

In the central nervous system cell migration is usually restricted to developmental periods and occurs mainly radially, following the radial glia. Nevertheless, in the subependymal layer of the adult rodent forebrain tangential migration of newly generated neuronal precursors directed to the olfactory bulb, which follow a well-defined pathway without dispersion, has been recently demonstrated. In the present study, by using light microscopic immunocytochemistry for glia-associated antigens (glial fibrillary acidic protein, S-100 and vimentin), and conventional electron microscopy, we observed a dense mesh-work of astrocytic cells and processes throughout the subependymal layer of the adult rat. These cells were organized to form tangentially oriented glial tubes in the subependymal layer of the lateral ventricle and in its rostral extension to the olfactory bulb. Glial tubes were particularly evident within the rostral extension and were widely intercommunicating. Using markers for the proliferating/ migrating cells of the rostral migratory stream (5-bromo-2'-deoxyuridine, PSA-NCAM, class III beta-tubulin), we provide evidence that long chains of PSA-NCAM/beta-tubulin-positive, newly generated cells are consistently observed inside the glial tubes. These results demonstrate the existence of a peculiar glial organization within the subependymal layer of the adult rat, consisting of long astrocytic tubes that likely represent a new type of glial guidance, accounting for the tangential migration of a high number of cells along their restricted pathway, to the olfactory bulb.