Fig. 1. The intervening zone (IZ) displays intrinsic mediolateral differences and is shaped by antagonistic activities from neurogenesis-promoting signals and the IsO. Whole-mount in situ hybridization at the 3-somite stage (A,D-F,K,L) (dorsal views, anterior to the top) and 24 hours post-fertilization (hpf) (B,C,G-J) (sagittal views, anterior to the left) with the markers indicated (bottom left, color-coded). (A-C) Intervening zone (IZ) location in wild-type (wt) embryos. At the 3-somite stage, the IZ separates the ventro-caudal cluster (vcc) from the r2 motor (r2MN) and sensory neurons (r2S), and encompasses most of the MH primordium, as revealed by pax2.1 expression. By 24 hpf, the IZ (bracket in B,C) has narrowed to a stripe at the MHB. (D-F) Intrinsic differences between the neurogenic capacities of lateral versus medial domains of the IZ. Upon injection of 25 pg ngn1 mRNA at the 1-cell stage (ngn1-inj, E), ectopic neurogenesis is induced within the neural plate outside proneural clusters (arrows in E) including the basal domain of the IZ (bracket), while the IZ remains neurons-free in lateral regions. 125 pg ngn1 (F) are necessary to force neurogenesis within the IZ alar domain (arrow in F; location of the IZ in F is indicated by the bracket). This phenotype is correlated with the loss of expression of the MH markers pax2.1 and wnt1 at 24 hpf (G-J, red arrows). Note that the profile of pax2.1 and wnt1 expression is otherwise unaltered (pax2.1: optic chiasm, hindbrain interneurons: G,H, black arrows. wnt1: midbrain dorsal midline, rhombic lips: I,J, black arrows). (K,L) The anterior-to-posterior extent of the IZ correlates with IsO activity, and is enlarged in hdl mutants, which overactivate Wnt signaling (L compared with K, bracket). Scale bars: 0.1 mm. IZ, intervening zone; vcc, ventrocaudal cluster; r2MN, rhombomere 2 motorneurons; r4MN, rhombomere 4 motorneurons; r2S, rhombomere 2 sensory neurons; delA, delta A; ngn1, neurogenin 1.