Specification of individual cortical layer neurons. Coronal (E18.5) sections were double labeled with the Cux2 riboprobe and the Tbr1 antibody. (a, a') Cux2+ and Tbr1+ cell populations are separated from each other, and the generation of intra-/supragranular layers seems preserved in Sp8 mutants. ISH of layer specific marker genes on (c, c', f, f', j, j') E18.5 coronal (medial is to the right) and (b, b', d, d', e, e', h, h', k, k') sagittal (rostral is to the left) forebrain sections. (g, g', l, l') Detection of Tbr1/2 protein on E18.5 coronal sections (medial is to the right). A reduced expression of Cux2 (d, d'), Lhx2 (e, e'), Robo1 (h, h') and Tbr1 (k, k') is visible in mutant cortices. In Sp8cKO, the subpopulations of Cux1+ (c, c'), RzR-β+ (f, f'), and ER81+ (j, j') cortical neurons are not molecularly specified. Tbr2 immunoreactive progenitors are diminished in the proliferative compartment of the Sp8 mutant cortex (box in (g, g')). Immunohistochemistry reveals that some Tbr1+ cells ectopically populated the upper CP and MZ in Sp8 mutants (arrow in (l')).