Patterns of NRG expression and distribution of ErbB4-expressing INs are complementary within neocortex of WT mice and are degraded in ErbB4-deficient mice. In situ hybridization on coronal sections through the cortical wall of littermates of E14.5 WT and ErbB4-/- HER4heart mice for expression of ErbB4, marking INs generated in the MGE and migrating within the marginal zone (MZ) and intermediate zone (IZ) of the cortex, and the neuregulins Nrg3 and the type I isoform of Nrg1. (A-D) In WT neocortex, ErbB4 expressing, MGE-derived INs are distributed in the MZ and the interface of IZ and subventricular zone (SVZ) (A), whereas Nrg3 is expressed in the cortical plate (CP) (B) and Nrg1-type I is expressed in the ventricular zone (VZ) and SVZ (C). The MGE-derived INs are largely found outside of the expression domains of Nrg1-type I and Nrg3 (D). (E-H) In the neocortex of ErbB4-/- HER4heart mice, MGE-derived INs are more broadly distributed than in WT, with a larger proportion found in the expression domains of Nrg1-type I in the VZ/SVZ and Nrg3 in the MZ (H). Scale bar: 200 μm.