Chuzhoi mutants exhibit a failure of closure 1 and a broadened midline. (A, B) Dorsal views of 6-somite E8.5 wild-type (A) and chuzhoi mutant (B) embryos demonstrating initiation of neural tube closure in the wild-type (arrow, A) but failure to initiate neural tube closure in the mutant. (C, D) Lateral views of E9.0 wild-type (C) and chuzhoi mutant (D) embryos; arrow marks the most posterior region of closed neural tube. The wild-type embryo has completed closure in the head and only the posterior neuropore remains open in the spine, whereas the chuzhoi embryos exhibits closure in the forebrain but remains open from the midbrain along the length of the spinal neural tube. (E, F) Transverse sections stained with haematoxylin and eosin through the caudal end of 5-somite wild-type (E) and chuzhoi mutant (F) embryos, revealing a compact ventral midline hinge point in wild-type (arrowhead, E) but a broadened ventral region and split median hinge point in mutants (arrowheads, F). (G, H) Wholemount in situ hybridisation for brachyury expression in E8.5 wild-type (G) and chuzhoi mutant (H) embryos demonstrates broadened ventral midline in mutants, particularly in posterior regions (arrow, H). (I-L) Wholemount in situ hybridisation for nodal expression in E8.0 (head-fold stage) (I, J) and E7.5 (K, L) wild-type (I, K) and chuzhoi mutant (J, L) embryos demonstrates shortened and broadened node area in chuzhoi mutants at headfold stage (J), while the shape of the node is not obviously altered at E7.5.