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The role of the neural crest (NC) in the construction of the vertebrate head was demonstrated when cell tracing techniques became available to follow the cells exiting from the cephalic neural folds in embryos of various vertebrate species. Experiments carried out in the avian embryo, using the quail/chick chimera system, were critical in showing that the entire facial skeleton and most of the skull (except for he occipital region) were derived from the NC domain of the posterior diencephalon, mesencephalon and rhombomeres 1 and 2 (r1, r2). This region of the NC was designated FSNC (for Facial Skeletogenic NC). One characteristic of this part of the head including the neural anlage is that it remains free of expression of the homeotic genes of the Hox-clusters. In an attempt to see whether this rostral Hox-negative domain of the NC has a specific role in the development of the skeleton, we have surgically removed it in chick embryos at 5-6 somite stages (5-6 ss). The operated embryos showed a complete absence of facial and skull cartilages and bones showing that the Hox expressing domain of the NC caudally located to the excision did not regenerate to replace the anterior NC. In addition to the deficit in skeletal structures, the operated embryos exhibited severe brain defects resulting in anencephaly. Experiments described here have shown that the neural crest cells regulate the amount of Fgf8 produced by the two brain organizers, the Anterior Neural Ridge (ANR) and the isthmus. This regulation is exerted via the secretion of anti-BMP signaling molecules (e.g. Gremlin and Noggin), which decrease BMP production hence enhancing the amount of Fgf8 synthesized in the ANR (also called "Prosencephalic organizer") and the isthmus. In addition to its role in building up the face and skull, the NC is therefore an important signaling center for brain development., (Copyright © 2012 Elsevier Inc. All rights reserved.)

The combinatorial expression of Hox genes is an evolutionarily ancient program underlying body axis patterning in all Bilateria. In the head, the neural crest (NC)--a vertebrate innovation that contributes to evolutionarily novel skeletal and neural features--develops as a structure free of Hox-gene expression. The activation of Hoxa2 in the Hox-free facial NC (FNC) leads to severe craniofacial and brain defects. Here, we show that this condition unveils the requirement of three Six genes, Six1, Six2, and Six4, for brain development and morphogenesis of the maxillo-mandibular and nasofrontal skeleton. Inactivation of each of these Six genes in FNC generates diverse brain defects, ranging from plexus agenesis to mild or severe holoprosencephaly, and entails facial hypoplasia or truncation of the craniofacial skeleton. The triple silencing of these genes reveals their complementary role in face and brain morphogenesis. Furthermore, we show that the perturbation of the intrinsic genetic FNC program, by either Hoxa2 expression or Six gene inactivation, affects Bmp signaling through the downregulation of Bmp antagonists in the FNC cells. When upregulated in the FNC, Bmp antagonists suppress the adverse skeletal and cerebral effects of Hoxa2 expression. These results demonstrate that the combinatorial expression of Six1, Six2, and Six4 is required for the molecular programs governing craniofacial and cerebral development. These genes are crucial for the signaling system of FNC origin, which regulates normal growth and patterning of the cephalic neuroepithelium. Our results strongly suggest that several congenital craniofacial and cerebral malformations could be attributed to Six genes' misregulation. [ABSTRACT FROM AUTHOR]

Abstract: The neural crest (NC), a defining feature of vertebrate embryo, generates most of the skeletal tissues encasing the developing forebrain and provides the prosencephalon with functional vasculature and meninges. Recent findings show that early in development, the cephalic NC is also essential for the pre-otic neural tube closure and promotes the development of the prosencephalic alar plate by regulating the morphogenetic activities of forebrain organizers. [Copyright &y& Elsevier]

The neural crest (NC) yields pluripotent cells endowed with migratory properties. They give rise to neurons, glia, melanocytes and endocrine cells, and to diverse 'mesenchymal' derivatives. Experiments in avian embryos have revealed that the differentiation of the NC 'neural' precursors is strongly influenced by environmental cues. The reversibility of differentiated cells (such as melanocytes or glia) to a pluripotent precursor state can even be induced in vitro by a cytokine, endothelin 3. The fate of 'mesenchymal' NC precursors is strongly restricted by Hox gene expression. In this context, however, facial skeleton morphogenesis is under the control of a multistep crosstalk between the epithelia (endoderm and ectoderm) and NC cells. [ABSTRACT FROM AUTHOR]