Early fate diversification of radial glial progenitors during corticogenesis

Radial glial progenitors (RGPs) generate all projection neurons (PNs) in the cerebral cortex through incompletely understood processes. We combined Mosaic Analysis with Double Markers at embryonic stages (E)12.5 and E13.5 with early postnatal callosal tracing to dissect RGP lineage progression. We find that multipotent RGPs generate all extra-telencephalic (ET) and intra-telencephalic (IT) PNs via parallel sublineages that emerge simultaneously at neurogenesis onset. ET-PN production progresses exclusively via small, self-consuming lineages; IT-PN lineages feature RGPs generating large translaminar outputs. The early emergence of IT-PN-fated RGPs, coinciding with a switch to direct neurogenesis, contributes to the stereotyped population-level progression of the multipotent lineage. We also identify POU3F transcription factors as candidate regulators of IT-PN fate via noncanonical mitotic chromatin binding. The results support a model whereby IT- and ET-PNs arise from an early bifurcation and parallel specification within the multipotent RGP lineage.