In the early postnatal subventricular zone (SVZ), two seemingly unrelated events occur simultaneously: a massive tangential migration of neuroblasts towards the olfactory bulb, known as the rostral migratory stream (RMS), and the outward movement of radial glia (RG) undergoing astrocytic transformation. Because of the orthogonal arrangement between these two sets of cells, little, if any, relevance has been ascribed for their possible interactions. By depositing DiI at the pial surface we have studied RG transformation within the SVZ/RMS, from birth up to the end of the first postnatal week. While still within the SVZ/RMS, RG morphology changed from simple bipolar to highly complex branched profiles, attaining their highest degree of complexity at the interface of the SVZ with the overlying white matter. At this interface cell bodies of radial glia accumulate and their processes run tangentially, surrounding the SVZ/RMS. Processes of RG surrounding the SVZ/RMS could also be observed by immunostaining for vimentin, GFAP, and nestin. In contrast, in the white matter all DiI-labeled RG presented a simple bipolar profile. These results indicate that the outward radial migration of the transforming RG does not occur uniformly. Instead, the different morphologies and cell densities that RG assume when they cross the SVZ/RMS and overlying white matter imply different migratory behaviors. Finally, our data suggest that RG provide a cellular scaffold to the early postnatal SVZ/RMS, much in the same way as astrocytes in the adult RMS.
New neurons are constantly added to the olfactory bulb of rodents from birth to adulthood. This accretion is not only dependent on sustained neurogenesis, but also on the migration of neuroblasts and immature neurons from the cortical and striatal subventricular zone (SVZ) to the olfactory bulb. Migration along this long tangential pathway, known as the rostral migratory stream (RMS), is in many ways opposite to the classical radial migration of immature neurons: it is faster, spans a longer distance, does not require radial glial guidance, and is not limited to postmitotic neurons. In recent years many molecules have been found to be expressed specifically in this pathway and to directly affect this migration. Soluble factors with inhibitory, attractive and inductive roles in migration have been described, as well as molecules mediating cell-to-cell and cell-substrate interactions. However, it is still unclear how the various molecules and cells interact to account for the special migratory behavior in the RMS. Here we will propose some candidate mechanisms for roles in initiating and stopping SVZ/RMS migration.
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