Patterns of NRG expression and distribution of ErbB4-expressing INs are complementary at E12.5. (A-E) In situ hybridization on coronal sections through E12.5 mouse forebrain for expression of ErbB4 (A), marking primarily INs generated in the MGE and migrating to the cortex, and the neuregulins Nrg3 (B) and the type III isoform of Nrg1 (D); (C, E) merged images shown in (A, B), and (A, D), respectively. (A) ErbB4-expressing cells are detected in the ventral telencephalon and cortical hem (arrowhead). Nrg3 (B) and Nrg1-type III (D) are robustly expressed in the mantle zone of the vTel. Most of the ErbB4-expressing INs within the vTel are found at locations with low or undetectable levels of Nrg3 and Nrg1-type III expression, as shown on merged adjacent sections (C, E). Numbered arrows indicate the different migrating streams of ErbB4-expressing cells at matching sites on all sections. CTX, cerebral cortex; LGE, lateral ganglionic eminence; MGE, medial ganglionic eminence. Scale bar: 500 μm.