Fig. 2. Neurulation in the developing vertebrate embryo. (A) Neurulation begins with a unified layer of ectoderm, underneath which lies the endoderm. A single ectomere is shown in yellow. Ectomeres are discrete regions of superficial ectoderm that exhibit a segmented pattern of gene expression. Fate-mapping experiments suggest that, together with neural crest and neuroectoderm, they define a larger developmental unit (Couly and Le Douarin, 1999). Later, these tissues act on signaling centers in the facial prominences (Hu et al., 2003). (B) The ectoderm begins to fold upwards, giving rise to the neural folds. During this process, interactions between signaling molecules begin to delineate the medial ectoderm as being neural (green) and the lateral regions of ectoderm as being non-neural (blue). The prechordal plate mesendoderm (pcp) and the buccopharyngeal membrane (bpm) become evident at this stage. (C) The neural tube forms upon fusion of the neural folds, giving rise to discrete neuroectoderm (green) and surface ectoderm (blue). Around the same time, the border region between the neuroectoderm and surface ectoderm gives rise to neural crest cells. The surface ectoderm and neuroectoderm of single ectomeres remain aligned during this process. (D) Neurulation completes upon formation of the neural tube, and neural crest cells (nc) lie sandwiched between the facial (surface) ectoderm and the neuroectoderm. Again, the individual neuroectoderm and surface ectoderm components of the ectomere remain in register. (E) Sagittal section through neural tube of a stage 15 chick embryo, showing neural crest (nc) located between surface ectoderm (se) and neuroectoderm (ne). L, lateral; M, medial. (E) Unpublished data from J.A.H.'s laboratory.