Molecular interactions coordinating the development of the forebrain and face

From an architectural point of view, the forebrain acts as a framework upon which the middle and upper face develops and grows. In addition to serving a structural role, we present evidence that the forebrain is a source of signals that shape the facial skeleton. In this study, we inhibited Sonic hedgehog (Shh) signaling from the neuroectoderm then examined the molecular changes and the skeletal alterations resulting from the treatment. One of the first changes we noted was that the dorsoventral polarity of the forebrain was disturbed, which manifested as a loss of Shh in the ventral telencephalon, a reduction in expression of the ventral markers Nkx2.1 and Dlx2, and a concomitant expansion of the dorsal marker Pax6. In addition to changes in the forebrain neuroectoderm, we observed altered gene expression patterns in the facial ectoderm. For example, Shh was not induced in the frontonasal ectoderm, and Ptc and Gli1 were reduced in both the ectoderm and adjacent mesenchyme. As a consequence, a signaling center in the frontonasal prominence was disrupted and the prominence failed to undergo proximodistal and mediolateral expansion. After 15 days of development, the upper beaks of the treated embryos were truncated, and the skeletal elements were located in more medial and proximal locations in relation to the skeletal elements of the lower jaw elements. These data indicate that a role of Shh in the forebrain is to regulate Shh expression in the face, and that together, these Shh domains mediate patterning within the frontonasal prominence and proximodistal outgrowth of the middle and upper face.