MIGRATION OF NEWLY GENERATED NEURONS UPON EPENDYMALLY DERIVED RADIAL GUIDE CELLS IN EXPLANT CULTURES OF THE ADULT SONGBIRD FOREBRAIN

The adult songbird forebrain undergoes neuronal production throughout adulthood, with the production of new neurons in discrete regions of t he neostriatal ventricular zone. Upon mitogenesis, these new neurons m igrate into the subjacent brain parenchyma along radially directed gui de fibers. In long-term ventricular zone explant cultures, derived fro m the higher vocal center of the adult canary, newly migratory neurons were found to associate preferentially with a characteristic substrat e cell type. These small, parvonuclear substrate cells formed tightly packed epithelioid sheets, in which ciliated ependymal cells were comm on, as recognized by both live observation and electron microscopy. A subpopulation of these cells was immunostained by monoclonal antibody 3A7, which preferentially stains the guide fiber network of the adult avian brain. These 3A7+ cells included ependymal cells and bipolar rad ial cells, as well as morphologically defined astrocytes. As they matu red in vitro, the 3A7+ bipolar radial cells extended long, unbranching fibers, which ultimately traversed the culture substrate. Like ependy mal cells, they supported neuronal migration. These cells were likely homologous to radial guide cells in vivo. Thus, neuronal migration in adult avian forebrain culture occurred upon guide cels of ependymal de rivation.