Migration patterns of subventricular zone cells in adult mice change after cerebral cortex injury

The subventricular zone (SVZ) generates the largest number of migratory cells in the adult brain. SVZ neuroblasts migrate to the olfactory bulbs (OB) in the adult, whereas during development, SVZ cells migrate into many adjacent nuclei. Previously, we showed that cerebral cortex injury in the adult causes molecular and cellular changes which may recapitulate the developmental migratory directions. Consistent with this, growth factors, as well as models of illness or injury can cause adult SVZ cells to migrate into non-olfactory bulb nuclei. Here, we tested the hypothesis that cerebral cortex injury in the adult mouse induces changes in migration, by labeling adult SVZ cells with a retroviral vector and examining the distribution of cells 4 days and 3 weeks later. Four days after cortical lesions, disproportionately fewer retrovirally-labeled cells had migrated to the olfactory bulb in lesioned mice than in controls. Conversely, the number of cells found in non-olfactory bulb regions (primarily the area of the lesion and the corpus callosum) was increased in lesioned mice. The morphology of these emigrated cells suggested that they were differentiating into glial cells. Three weeks after cortical injury, the majority of retrovirally-labeled cells in both groups of mice had migrated into the granule and periglomerular layers of the olfactory bulb. At 3 weeks, we still observed retrovirally-labeled glial cells in the corpus callosum and in the area of the injury in lesioned mice. These results suggest that cortical lesions cause a transient change in migration patterns of SVZ progeny, which is characterized by decreases in migration to the olfactory bulb but increased migration towards the injury. Our studies also suggest that cortical lesions induce the production of new glial cells which survive for at least 3 weeks after injury. The data support the concept that in the adult, SVZ cells can generate progeny that migrate towards injured areas and thus potentially be harnessed for neural repair.