(A–H) Immunofluorescence on sections of human (A–C, 19 gestational week) and mouse (D–H) fetal brains. Arrowheads indicate the regions that are enlarged in B and C and show pERK immunoreactivity in Pax6-positive aRG (B) and Hopx/Sox2 doubled-labeled bRG (C); the boxed areas in D are enlarged in the inset; the dashed line demarcates the ventricle; arrowheads point to robust pERK staining in the ventricular zone (VZ); the unfilled arrowhead in E indicates the area enlarged in G and H. Note that pERK signals are increased at the apical surface of VZ from E16.5 to E17.5 (F). Arrows indicate Hopx/Fabp7/Sox2 triple-positive cells (G) and Hopx/pERK double-labeled cells (H) on E18.5 cortical sections. Nuclei are stained with Hoechst 33342. (I) Bar charts showing the percentage of Hopx/pERK double-labeled cells relative to the total number of counted pERK (n = 642) or Hopx (n = 488) cells. Abbreviations: cp, cortical plate; ISVZ, inner subventricular zone; OSVZ, outer subventricular zone; Pia, pial surface; SP, cortical subplate. Scale bars: 20 µm (B, C, G, H and F), 200 µm (D), and 100 µm (E).