Neuronal development and migration in explant cultures of the adult canary forebrain

The vocal control nucleus (HVc) of the songbird forebrain undergoes neurogenesis in adulthood, as ventricular zone precursor cells divide, and their daughter cells migrate into the subjacent forebrain and differentiate into neurons. A procedure is now described by which the migration and development of these new neurons can be directly observed in vitro, in explant cultures of the adult canary forebrain. Cultured explants have yielded extensive outgrowth of new neurons from 2 neurogenic regions of the adult songbird brain: the HVc and its adjacent dorsomediocaudal neostriatum. Immunocytochemically identified neurons were determined by 3H-thymidine autoradiography to have been newly generated in vivo, in the days preceding explantation. These cells were immunoreactive for a variety of neuron-selective antigens, including MAP-1B, MAP-2, neuron-specific enolase, synaptic vesicle protein-2, N-CAM, and the tetanus toxin and A2B5 ligands. Combined immunostaining and autoradiography of cultures derived from HVcs exposed to 3H-thymidine in vivo revealed a population of newly generated, MAP-2/3H-thymidine-positive neurons. The neurons identified in these adult explant outgrowths comprised new, recently generated postmitotic cells which had arisen in the neostriatal ventricular zone and were in the early stages of their parenchymal migration at the time of tissue explantation. Accordingly, nonneurogenic control regions, including the anterior hyperstriatum, cerebellum, and optic tectum, did not display neuronal outgrowth in culture. Only forebrain regions exhibiting neurogenesis in vivo manifested neuronal outgrowth and maturation in vitro.