Subventricular zone-derived neuronal progenitors migrate into the subcortical forebrain of postnatal mice

The presence of a germinal layer and the capacity to generate neurons, once thought restricted to the embryonic brain, persists in the forebrain of both postnatal and adult mammals. The two regions in which this phenomenon has been extensively demonstrated are the hippocampal dentate gyrus and the lateral ventricle subventricular zone (SVZ). SVZ-derived cells migrate along the rostral migratory stream into the olfactory bulb, where they differentiate into local interneurons. In this study, using tracer injections into the SVZ at different postnatal ages, we investigated the occurrence of secondary migratory pathways in the mouse subcortical forebrain. During the course of the first week postnatal, in addition to the well-characterized rostral migratory stream, SVZ-derived progenitors migrate in a ventral migratory mass across the nucleus accumbens into the basal forebrain and along a ventrocaudal migratory stream originating at the elbow between the vertical and horizontal limbs of the rostral migratory stream. These cells give rise to granule neurons in the Islands of Calleja and olfactory tubercle pyramidal layer, respectively. In adult, a very small number of cells continue to migrate along the ventrocaudal migratory stream, whereas no migration was observed across the nucleus accumbens. These data demonstrate that in early postnatal and, to a minor extent in adult mice, SVZ-derived cells contribute new neurons to the subcortical forebrain.