Adult subventricular zone (SVZ) neuroblasts migrate in the rostral migratory stream to the olfactory bulbs. Brain lesions generally increase SVZ neurogenesis or gliogenesis and cause SVZ cell emigration to ectopic locations. We showed previously that glia emigrate from the SVZ toward mechanical injuries of the somatosensory cerebral cortex in mice. Here we tested the hypotheses that SVZ neurogenesis increases, that neuroblasts emigrate, and that epidermal growth factor expression increases after cortical injuries. Using immunohistochemistry for phenotypic markers and BrdU, we show that newborn doublecortin-positive SVZ neuroblasts emigrated toward cerebral cortex lesions. However, the number of doublecortin-positive cells in the olfactory bulbs remained constant, suggesting that dorsal emigration was not at the expense of rostral migration. Although newborn neuroblasts emigrated, rates of SVZ neurogenesis did not increase after cortical lesions. Finally, we examined molecules that may regulate emigration and neurogenesis after cortical lesions and found that epidermal growth factor was increased in the SVZ, corpus callosum, and cerebral cortex. These results suggest that after injuries to the cerebral cortex, neuroblasts emigrate from the SVZ, that emigration does not depend either on redirection of SVZ cells or on increased neurogenesis, and that epidermal growth factor may induce SVZ emigration.

Doublecortin (Dcx) is a microtubule-associated protein expressed by migrating neuroblasts in the embryo and in the adult subventricular zone (SVZ). The adult SVZ contains neuroblasts that migrate in the rostral migratory stream (RMS) to the olfactory bulbs. We have examined the distribution and phenotype of Dcx-positive cells in the adult mouse SVZ and surrounding regions. Chains of Dcx-positive cells in the SVZ were distributed in a tight dorsal population contiguous with the RMS, with a separate ventral population comprised of discontinuous chains. Unexpectedly, Dcx-positive cells were also found outside of the SVZ: dorsally in the corpus callosum, and ventrally in the nucleus accumbens, ventromedial striatum, ventrolateral septum, and bed nucleus of the stria terminalis. Dcx-positive cells outside the SVZ had the morphology of migrating cells, occurred as individual cells or in chain-like clusters, and were more numerous anteriorly. Of the Dcx-positive cells found outside of the SVZ, 47% expressed the immature neuronal protein class III beta-tubulin, 8% expressed NeuN, a marker of mature neurons. Dcx-positive cells did not express molecules found in astrocytes, oligodendrocytes, or microglia. Structural and immunoelectron microscopy revealed that cells with the ultrastructural features of neuroblasts in the SVZ were Dcx+, and that clusters of neuroblasts emanated ventrally from the SVZ into the parenchyma. Our results suggest that the distribution of cells comprising the walls of the lateral ventricle are more heterogeneous than was thought previously, that SVZ cells may migrate dorsally and ventrally away from the SVZ, and that some emigrated cells express a neuronal phenotype.

During development radial glia (RG) are neurogenic, provide a substrate for migration, and transform into astrocytes. Cells in the RG lineage are functionally and biochemically heterogeneous in subregions of the brain. In the subventricular zone (SVZ) of the adult, astrocyte-like cells exhibit stem cell properties. During examination of the response of SVZ astrocytes to brain injury in adult mice, we serendipitously found a population of cells in the walls of the ventral lateral ventricle (LV) that were morphologically similar to RG. The cells expressed vimentin, glial fibrillary acidic protein (GFAP), intermediate filament proteins expressed by neural progenitor cells, RG and astrocytes. These RG-like cells had long processes extending ventrally into the nucleus accumbens, ventromedial striatum, ventrolateral septum, and the bed nucleus of the stria terminalis. The RG-like cell processes were associated with a high density of doublecortin-positive cells. Lesioning the cerebral cortex did not change the expression of vimentin and GFAP in RG-like cells, nor did it alter their morphology. To study the ontogeny of these cells, we examined the expression of molecules associated with RG during development: vimentin, astrocyte-specific glutamate transporter (GLAST), and brain lipid-binding protein (BLBP). As expected, vimentin was expressed in RG in the ventral LV embryonically (E16, E19) and during the first postnatal week (P0, P7). At P14, P21, P28 as well as in the adult (8-12 weeks), the ventral portion of the LV retained vimentin immunopositive RG-like cells, whereas RG largely disappeared in the dorsal two-thirds of the LV. GLAST and BLBP were expressed in RG of the ventral LV embryonically and through P7. In contrast to vimentin, at later stages BLBP and GLAST were found in RG-like cell somata but not in their processes. Our results show that cells expressing vimentin and GFAP (in the radial glia-astrocyte lineage) are heterogeneous dorsoventrally in the walls of the LV. The results suggest that not all RG in the ventral LV complete the transformation into astrocytes and that the ventral SVZ may be functionally dissimilar from the rest of the SVZ.