Cells migrating from MGE explants on live forebrain slices avoid NRG expression domains in an ErbB4-dependent manner. (A) Pseudo-colored merged images of in situ hybridizations on coronal sections through E14.5 mouse forebrain for expression of Nrg3 (green) in the cortical plate (cp) and Nrg1-type III (violet) in the mantle zone of the vTel and in the ventricular zone (vz)/SVZ in the dTel. An explant from the medial ganglionic eminence (MGE) from E14.5 WT or ErbB4-/- HER4heart mice crossed to an eGFP reporter line was placed on a living coronal slice through E14.5 WT mouse forebrain, positioned in the mantle zone; the white outline (labeled Ex) marks the explant shown in (B), and approximates explant position in all cases. (B-C') MGE explants and cells migrating from them are marked with an eGFP reporter (white label). (B, B') MGE explant from WT eGFP mouse; (B') is higher power. After 48 hours in culture, eGFP-marked cells migrating from MGE explants exhibited little overlap with NRG expression domains, similar to in vivo WT (Figures 1, 2, and 3). The migrating cells abut the Nrg3 expression domain in the cp (arrowheads), although a few cells do enter it (arrow in (B, B'); inset in (B')), and also largely avoid domains of robust Nrg1-type III expression; for example, few labeled cells enter the domains dorsal (d) and ventral (v) to the MGE explant. Dual arrows mark the ganglionic eminence abutting the lateral ventricle. (C, C') MGE explant from ErbB4-/- HER4heart eGPF mice; (C') is higher power of (C). Cells deficient for ErbB4 migrating from eGFP-marked MGE explants from E14.5 ErbB4-/- HER4heart; eGFP mice, are distributed in a radial pattern distinct from that exhibited by WT MGE explants, and overlapping more with NRG expression domains than do WT cells. Scale bars: 500 μm (A-C); 250 μm (B', C').