D/V patterning defects at the medial pallial-subpallial boundary (mPSB). ISH on E12.5 (coronal) forebrain sections. Images show the right brain hemisphere. (c-k') Blow-up images of the medial/ventral part of each boxed section. In cKO, the pallial markers (a, a', c, c') Emx2, (b, b', d, d') Pax6, and (e, e') Ngn2 expand into the ventral midline. Conversely, ventral markers (g, g') Dlx1, (h, h') Gsh2, and (j, j') Mash1 are not expressed in the dorsal septum. (f, f') Nkx2.1 activity is lost in the septum and rostral MGE of Sp8 mutants. (k, k') The Nkx6.2+ domain reflects a rudimentary MGE territory in cKO and contacts the Emx2+, Pax6+ and Ngn2+ midline. Note the (l, l') expansion of the Ngn2+ domain (arrows) and the (m, m') reduction of the Dlx1+ domain (arrows) around the midline of the Sp8 deficient telencephalon at E15.5. (n, n') Nkx2.1 (arrow) remains absent in the septum and rostral MGE of cKO at this stage. (o, o') Depletion of (Gad67+) interneurons in the Sp8 mutant cerebral cortex (arrow) at E18.5. For a better visualization of the shift in gene expression pattern, the arrowheads in (e, e') point at the constriction between the septum anlage and the LGE. ISH for Emx2/Pax6 (a, a', b, b', c, c', d, d') and Ngn2/Mash1 riboprobes (e, e', j, j') were performed on adjacent sections.