Postnatal subventricular zone of the neocortex contributes GFAP+ cells to the rostral migratory stream under the control of Sip1

► Postnatal dorsal neocortical SVZ contains a heterogeneous pool of glial progenitors. ► Dorsal SVZ (dSVZ) generates BLBP+ and GFAP+ cells of the early postnatal RMS. ► The dSVZ is expanded on conditional deletion of Sip1 in postmitotic cortical neurons. ► Sip1-conditional knockouts show increased GFAP expression within the RMS. ► Sip1-conditional knockouts show denser intercalation of GFAP+ cells and neuroblasts.