May 1, 2014;
Setting appropriate boundaries: fate, patterning and competence at the neural plate border.
The neural crest
and craniofacial placodes are two distinct progenitor populations that arise at the border of the vertebrate neural plate
. This border region develops through a series of inductive interactions that begins before gastrulation and progressively divide embryonic ectoderm
into neural and non-neural regions, followed by the emergence of neural crest
and placodal progenitors. In this review, we describe how a limited repertoire of inductive signals-principally FGFs, Wnts and BMPs-set up domains of transcription factors in the border region which establish these progenitor territories by both cross-inhibitory and cross-autoregulatory interactions. The gradual assembly of different cohorts of transcription factors that results from these interactions is one mechanism to provide the competence to respond to inductive signals in different ways, ultimately generating the neural crest
and cranial placodes
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Fig. 1. A graphical summary of the signals received by embryonic ectoderm during the establishment of the neural plate, neural crest, pre-placodal region and epidermis. The diagram is a consensus of data taken from zebrafish, Xenopus, chick and mouse studies and is not intended to be an accurate representation of any one species. (A) Pre-Gastrula. Wnt and BMP signals in the ectoderm initiate differentiation of non-neural ectoderm, while these signals are counteracted by FGFs and BMP and Wnt inhibitors from the organizer or hypoblast. (B) Early Gastrula. Wnt and FGF signals start to induce the first neural crest genes; BMP signaling is not required for this step and may be actively inhibited. (C) Late Gastrula. Pre-placodal genes begin to be induced by FGFs and by an attenuation of Wnt and BMP signals. Wnts and BMPs begin to be expressed at the edge of the neural plate and continue to induce neural crest tissue. (D) Early Neurula. The final resolution of the border region into four distinct regions.
Fig. 2. A graphical summary of the expression pattern of different transcription factors in the establishment of the neural plate border. Panels (A)–(D) show the transition from the establishment of neural and non-neural ectoderm domains prior to gastrulation (A), to the early neurula stage where distinct neural, epidermal, placodal and neural crest territories can be observed. Shading of colored blocks implies that the edges of a particular gene expression domain is not sharply defined. (A) Pre-Gastrula, (B) Early Gastrula, (C) Late Gastrula and (D) Early Neurula.