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First published online June 6, 2008

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Shh noses into craniofacial development......and creates a ZPA in the jaw

Noses come in many shapes and sizes, and some human conditions are marked by characteristic nasal malformations. Facial morphogenesis depends on inductive interactions between cephalic neural crest cells (NCCs, which give rise to the nasal capsule and other head structures) and cephalic epithelia, but which molecules provide the instructive signals? Benouaiche and co-workers now report that sonic hedgehog (Shh) signalling from the foregut endoderm patterns the avian nasal capsule (see p. 2221). The surgical removal in ovo of the most rostral zone of the endoderm (EZ-I), they report, prevents the formation of mesethmoid cartilage (a ventral part of the nasal capsule that forms the upper beak), but this defect can be rescued by the implantation of Shh-loaded beads. Correspondingly, when the authors grafted an extra EZ-I into developing embryos, an ectopic mesethmoid formed, the development of which they inhibited by suppressing Shh signalling. These results support the notion that early endodermal regionalization drives normal facial morphogenesis and suggest that its disruption might result in craniofacial defects.

Another head structure that cephalic NCCs give rise to is the lower jaw, which forms when these cells migrate into the first branchial arch (BA1) of avian and mammalian embryos. Shh expression in the ventral foregut endoderm is crucial for the survival and development of these NCCs. Now, on p. 2311, Nicole Le Douarin and colleagues unexpectedly report that the transplantation of Shh-expressing quail cells into the presumptive territory of BA1 before NCCs migrate into this region induces mirror-image supernumerary jaws to form in the mandibular mesenchyme (the embryonic tissue that gives rise to the lower jaw). They show that the development of these extra jaws is preceded by the expression of Fgf8, Bmp4 and Shh in the caudal BA1 ectoderm in a spatial pattern similar to that normally seen in the oral epithelium. The activation of these genes leads to the formation of two extra lower-jaw-organizing centres with opposite rostrocaudal polarities. Thus, the researchers suggest, Shh-producing cells create a zone of polarizing activity (ZPA) in mandibular buds just as they do in developing limb buds.

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Related articles in Development:

Sonic hedgehog signalling from foregut endoderm patterns the avian nasal capsule

Laurence Benouaiche, Yorick Gitton, Christine Vincent, Gérard Couly, and Giovanni Levi
Development 2008 135: 2221-2225. [Abstract] [Full Text]  

Induction of mirror-image supernumerary jaws in chicken mandibular mesenchyme by Sonic Hedgehog-producing cells

José M. Brito, Marie-Aimée Teillet, and Nicole M. Le Douarin
Development 2008 135: 2311-2319. [Abstract] [Full Text]  

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in Development Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Search for Related Content Social Bookmarking                         What's this?