Click here to close
Hello! We notice that you are using Internet Explorer, which is not supported by Xenbase and may cause the site to display incorrectly.
We suggest using a current version of Chrome,
FireFox, or Safari.
???displayArticle.abstract???
The identification of genes in Xenopus, chick and zebrafish expressed early in prospective neural crest (NC) cells has challenged the previous view that the NC is induced during the closure of the neural tube. We compare here the early inductive molecular mechanisms in different organisms and, despite observed differences, propose a general common model for NC induction.
Fig. 1. Temporal analysis of NC induction. Neural induction is compared in three different animal models: (a) Xenopus laevis, (b) Danio rerio (zebrafish) and (c) chick. The expression of early NC markers (green) and the analysis of NC competence (yellow) are used to propose the course of NC induction (red). In Xenopus and zebrafish, this progressive process is highly correlative with a decreased level of BMPs (blue) and with neural induction (purple). For BMPs, the dark blue represents the threshold values required for early NC induction. No early analysis of NC induction has been reported in the chick model. The dashed lines represent processes or expression patterns that have yet to be studied and analyzed. Early NC induction at the open neural tube stage, required for the initial specification of the crest, can be distinguished from late NC induction, which takes place at the neural fold/neural tube stage and is required for the maintenance of the NC cells. ↓G, onset of gastrulation.
Fig. 2. A double gradient model of NC induction. (a) A medio-lateral gradient of BMPs (red) is established in the ectoderm, specifying the neural plate border as anterior neural fold (pale purple) at a threshold concentration. (b) Posteriorizing signals (green curve), which correspond to the activities of Wnts, FGFs and RA, transform the most posterior part of the neural plate border into prospective NC cells (dark purple). These signals are generated in a gradient-like manner, with higher levels in the posterior part of the ectoderm and lower levels in the anterior region. These lower levels are also kept low by anti-posteriorizing signals, such as Dickkopf and Cerberus (green curve), produced by the anterior region of the embryo. (c) Once the neural tube is closed, some of the initial inductive signals are expressed in dorsal regions of the embryo, such as the epidermis, the NC and the dorsal neural tube. These signals are required to maintain the specification of NC cells in a process known as the late step of NC induction. The expression of BMPs and Wnts has been described for this step and there is some evidence to suggest that RA is expressed in the hindbrain region, but no expression of this molecule has been reported in the NC. A, anterior; P, posterior; M, medio; L, lateral; D, dorsal; V, ventral.
Fig. 3. Hypothetical model of anterior–posterior patterning of NC cells. The NC produces different populations of cells along the anterior–posterior axis, 1 being the most anterior and 5 the most posterior NC. Each of these cell types has a specific fate during development and originates from a specific location within the neural folds. We propose here that the antero-posterior differences of the NC are initiated early during development, at the neural plate stage, by posteriorizing signals (green curve) via a mechanism similar to that which distinguishes the prospective forebrain from the prospective NC cells. A, anterior; P, posterior; M, medio; L, lateral.
