Wandering neuronal migration in the postnatal vertebrate forebrain.

Most non-mammalian vertebrate species add new neurons to existing brain circuits throughout life, a process thought to be essential for tissue maintenance, repair, and learning. How these new neurons migrate through the mature brain and which cues trigger their integration within a functioning circuit is not known. To address these questions, we used two-photon microscopy to image the addition of genetically labeled newly generated neurons into the brain of juvenile zebra finches. Time-lapse in vivo imaging revealed that the majority of migratory new neurons exhibited a multipolar morphology and moved in a nonlinear manner for hundreds of micrometers. Young neurons did not use radial glia or blood vessels as a migratory scaffold; instead, cells extended several motile processes in different directions and moved by somal translocation along an existing process. Neurons were observed migrating for ∼2 weeks after labeling injection. New neurons were observed to integrate in close proximity to the soma of mature neurons, a behavior that may explain the emergence of clusters of neuronal cell bodies in the adult songbird brain. These results provide direct, in vivo evidence for a wandering form of neuronal migration involved in the addition of new neurons in the postnatal brain.