Fig. 9. The topography of thalamocortical projections is shifted in Dlx1/2^-/- embryos. Coronal (A-L) or horizontal (M-Q) hemisections of E16.5 Dlx1/2 heterozygous (left) and homozygous (right) mutant brains where DiI crystals (A-I) or DiI and DiA crystals (J-Q) were introduced in the occipital neocortex (A-C), the parietal neocortex (D-I), the occipital and parietal neocortex (J-L), or the putative dorsolateral geniculate nucleus (dLGN) and ventrobasal (VB) complex (M-Q). Schematic diagrams indicate the position of DiI and DiA crystals (A,D,G,J,M) and stars indicate their actual position in B,C. (A-L) Morphological landmarks, including the pial surface of the thalamus (broken line) and the retroflexus tract (white arrowhead), are used to position presumptive thalamic nuclei. In controls, injections in the occipital neocortex label cells in the putative dLGN (B,K) and injections in the parietal neocortex label cells in the VB complex (E,H,K). Open arrowheads indicate the medial boundary of the thalamic domain stained in wild-type embryos (B,E,H,K). This boundary is indicated in Dlx1/2^-/- embryos and shows that thalamic domains labeled by occipital and parietal injections are shifted medially (C,F). Note that the number of cells labeled is reduced in homozygous mutant embryos (compare B with C and E with F). In some cases, the region containing labeled cells was broader in mutant embryos, partially including the domain labeled in controls as well as a more medial domain (H,I). Similarly, double occipital and parietal injections show that the labeled domains in mutant embryos are medially displaced (K,L). In this case there is some overlap in the regions labeled by each dye (arrow in L). (M-Q) Horizontal sections at ventral levels (N,O) and more dorsal levels (P,Q) of brains after a thalamic double injection in the presumptive dLGN and VB. Even though the tracer crystals were relatively small, a large number of axons were stained in our experiments because of the small size of the thalamus. In wild-type animals, putative dLGN axons (red) and VB axons (green) turn into the striatum and remain as two separate bundles in the caudal (open arrowheads in N and P) and intermediate regions of the internal capsule, respectively (N,P). In Dlx1/2^-/- mutant embryos, dLGN axons are primarily detected ventrally (open arrowhead in O), and are mixed with VB axons. In more dorsal sections (Q), very few dLGN axons are visible (open arrowhead), indicating that they remain in ventral regions. On the contrary, a large number of VB axons are detected in a caudal region where normally dLGN axons travel (compare P with Q). dTH, dorsal thalamus; Fr, frontal neocortex; IC, internal capsule; Ncx, neocortex; Occ, occipital neocortex; Par, parietal neocortex; Str, striatum.