Analysis of public in situ hybridization (ISH) gene expression database (Allen Developing Mouse ISH Atlas, 2008 http://developingmouse.brain-map.org/) defines early regional boundaries, cell types, and relationship to Notch signaling changes during subsequent development and morphogenic steps of DG formation at E11.5, E13.5, E15.5 and P28 stages. (A) Regional boundaries during early DG formation: Lef1 marks cortical ventricular zone, Wnt2b marks hem/fimbria, Msx1 marks choroid plexus, Cxcl12 marks meningeal mesenchyme: red arrow marks the dentate neuroepithelial (DNe) stem zone. (B) Cell-type specific markers: Sox2 marks NSCs, Eomes marks IPs, Neurod1 marks CA field neurons, Trp73 marks Cajal-Retzius cells (CRs). Increased Eomes+ IPs are already evident in the early E11.5 DNe, with numerous IPs arising from the ventricular surface and accumulating in outer basal regions, demarking the nascent SVZ, compared to other cell type markers: note a few hem-derived CRs are evident with transient co-expression of Emoes and Trp73, which continues to mark differentiated CR cells that also eventually express Reln (not shown) and migrate subpially towards the hippocampal fissure underneath Cxcl12 expressing meninges distinct from DNe pioneering Emoes expressing IPs, as is evident from E11.5-E13.5-E15.5 images. (C) High combinatorial Notch expression defines early DG formation: At E11.5, Notch1/3, Hes1, Hey1 and especially Lfng and Hes5 are strongly expressed in the early DNe, whereas Notch2 marks the choroid plexus and hem region, and Hey2 is not evident. By E13.5, increased Hes5 expression continues to mark the DNe, whereas Hey1 is progressively downregulated as Hey2 is transiently upregulated. Similarly, while Notch1 expression remains constant, Notch3 expression in the VZ is gradually replaced by increased Notch2 expression through E15.5: note Notch expressing NSCs appear in the DMS after Eomes and Dll1 expressing IPs. By P28 adult stages, Notch1 and especially Notch2 mark NSCs in the SGZ, along with Lfng and Hes5. (D) Summary of relative spatial-temporal expression levels during DG formation. Early DG formation is defined by increased IP production and high combinatorial Notch signaling transitions in NSCs, which follow IP migrations through transient outer zones to establish the adult SGZ neurogenic niche.