Tangential migration of ErbB4-expressing INs from MGE to cortex is blocked by NRGs ectopically expressed within their subpallial migratory pathway. (A-C) In utero electroporation was performed at E12.5 to ectopically express the functional NRG constructs Nrg1α-EGF (A) and Nrg1β-EGF (B) in the ventral lateral forebrain. At E17.5, ectopic NRG expression domains were visualized by in situ hybridization with probes for Nrg1α (A, in green) and Nrg1β (B, in green) and MGE-derived INs were visualized by in situ hybridizations with ErbB4 probe (A', B', C, in red) on coronal sections through the transfected brains. In Nrg1-transfected brains, ErbB4-expressing INs do not enter the ectopic Nrg1 expression domain (arrows in (A, B)) and accumulate ventral to it (domains outlined by white line in (A, A', B, B')). In the control brains with similar transfected domains where only an eGFP expression vector was transfected, the ErbB4-expressing INs migrate through the transfected domain (arrowheads in (C)). Within the cortex posterior to the transfected domains, the number of ErbB4-expressing cells is drastically decreased in Nrg1-transfected cerebral cortex (A'', A''', B'', B''')" compared to control (C'', C'''). Arrowheads in (A'', B'', C") shown that the migration of ErbB4-expressing INs derived from the caudal ganglionic eminence that populate the hippocampus and caudomedial cortex is not affected by transfections into the migrational paths of INs from the MGE. (A''', B''', C''') Higher magnification images of the region indicated with arrows in (A'', B'', C''). CP, cortical plate; CTX, cerebral cortex; HC, hippocampus; IZ, intermediate zone; MZ, marginal zone; STR, striatum; TH, thalamus. Scale bars: 1 mm (A, A', B, B', C); 1 mm (A'', B'', C''); 0.5 mm (A''', B''', C''').