Figure 7: Schematic model for NSC progression.

Neuroectodermal cells yield the earliest NE cells of the CNS by launching Notch activation and HES5 expression, while non-CNS neuroectodermal cells lack this activation. Under proliferation conditions, HES5+ NE cells yield consecutive radial glial progenitor cell types and their corresponding neuronal and glial progeny, hence considered as primary NSCs generating CNS neural diversity. Following mitogen withdrawal, HES5+ NE cells exert their competence towards deep-layer-specific neuronal types (RELN, TBR1 but also FEZF2 and CTIP2; blue-to-red wave, bottom panel) and do so in a Notch-dependent manner. In addition, they also upregulate SVZ progenitor markers such as TBR2 and CUX1, CUX2 at the RNA level (Red font, light brown early wave; bottom panel) and in a Notch-dependent manner, implying on their future potential to generate these progenitors at later stages. In contrast to NE cells, HES5+ E-RG cells are already committed to early dorsocaudal cortical identity, based on their elongated polarized cell morphology, rosette formation capacity and FEZF2 and EMX2 expression. Hence, they exhibit competence towards deep layer neurons (CTIP2, FEZF2; blue-to-red wave, bottom panel). M-RG stage cells are characterized by lower HES5 percentages, reduced rosette organization, substantial accumulation of HES5+-derived HES5− progenitors expressing CUX1, CUX2 and TBR2 at the protein level, and competence for yielding upper layer neuronal fates (CUX1, CUX2, SATB2; light brown second wave, bottom panel) in a Notch-independent manner. HES5+ L-RG cells are able to give rise to astrocytes in a Notch-dependent manner (GFAP; light blue wave, bottom panel), yet both HES5+ and HES5− cells at that stage continue to contribute to neurogenesis. Ultimately, L-RG cells transform to long-term progenitors (LNP) associated with adult NSC progeny (purple wave, bottom panel). Horizontal green arrows mark transition in a Notch-dependent manner. Diagonal green and black arrows mark HES5+ and HES5− cells, respectively, subjected to differentiation following FACS-based separation. When indicated, Notch active pathways were confirmed by DAPT (red bar-headed lines). Top panel shows cell types and developmental potential. Bottom panel shows temporal phases of neuronal and glial markers derived by the stages indicated above. BP, basal progenitors.