Fig. 7. Suppression of proneural gene expression in slowly dividing NPCs.

a RT and real-time PCR analysis of Ascl1 mRNA in NPCs isolated from the LGE of Rosa-rtTA;TRE-mCMV-H2B-GFP embryos at E16.5 and sorted on the basis of H2B-GFP retention after exposure to 9TB-Dox at E9.5. Data were normalized by the amount of β-actin mRNA, are expressed relative to the corresponding value for GFP⁺ cells, and are means ± SEM (n = 3 independent experiments), two-tailed paired t test. b, c RT and real-time PCR analysis of Neurog2 and Tbr2 mRNAs in cells positive for CD133 and GFP and negative for CD24 isolated by FACS from the neocortex of E17.5 embryos that had been subjected to in utero electroporation at E14.5 with plasmids for GFP alone (control) or together with p57 (b) or p18 (c). Data were normalized by the amount of β-actin mRNA, are expressed relative to the corresponding value for control, and are means ± SEM (n = 3 independent experiments), two-tailed paired t test. d Immunohistochemical analysis with antibodies to Ascl1 and to Sox2 for sections of the brain of control and Hey1 knockout mice at E14.5. Nuclei were stained with Hoechst 33342. Scale bar, 50 μm. Dashed lines indicate the ventricular surface. e Quantification of the proportion of Ascl1⁺ cells among Sox2⁺ cells in the dLGE for sections as in d. Data are means ± SEM (n = 4 embryos), two-tailed Student’s t test. f Model for the division of labor of Notch downstream effectors that can select the fate of “rapidly dividing” (oscillating Hes1/Hes5) versus “slowly dividing” (tonic Hey1) pools of embryonic NPCs, respectively.