A zone of frontonasal ectoderm regulates patterning and growth in the face

Abstract

A fundamental set of patterning genes may define the global organization of the craniofacial region. One of our goals has been to identify these basic patterning genes and understand how they regulate outgrowth of the frontonasal process, which gives rise to the mid and upper face. We identified a molecular boundary in the frontonasal process ectoderm, defined by the juxtaposed domains of Fibroblast growth factor 8 and Sonic hedgehog, which presaged the initial site of frontonasal process outgrowth. Fate maps confirmed that this boundary region later demarcated the dorsoventral axis of the upper beak. Ectopic transplantation of the ectodermal boundary region activated a cascade of molecular events that reprogrammed the developmental fate of neural crest-derived mesenchyme, which resulted in duplications of upper and lower beak structures. We discuss these data in the context of boundary/morphogen models of patterning, and in view of the recent controversy regarding neural crest prepatterning versus neural crest plasticity.

Morphoregulation of avian beaks: Comparative mapping of growth zone activities and morphological evolution

“…They are distributed below the middle horizontal line of the two nasal pits at this stage. The upper border of this epidermis proliferation zone may correlate with the position of the frontonasal ectodermal zone (FEZ) restricted by FGF8 and Shh (Hu et al, 2003). In chickens, this epidermis proliferation zone is diffusely distributed (Fig.…”

“…Whereas elegant transplantation experiments showed that the shapes of duck or quail beaks are determined by the cephalic crest , the information residing in the crest mesenchyme still has to be executed. At stage 20, the interface of the FGF8 and Shh expression domains was shown to be the growth point of the FNM and was termed the frontonasal ectodermal zone (FEZ; Hu et al, 2003). Misexpression of Shh around stage 20 can duplicate the FNM and form duplicated beaks (Hu and Helms, 1999).…”

“…In the chicken, FGF8 was expressed at the upper part of the ectodermal frontonasal zone and may function to induce cartilage formation (Hu et al, 2003;. Its expression disappeared in this region after stage 20.…”

“…Chimerae between duck and quail cephalic crest cells suggested that the neural crest determines beak morphology . Transplantation studies showed that a frontonasal ectoderm, marked by fibroblast growth factor 8 (FGF8) and Sonic hedgehog (Shh) expression, could pattern the mesenchyme (Hu et al, 2003). Further studies showed that Shh and FGF8 act synergistically to drive cartilage growth .…”

Why the short face? Developmental disintegration of the neurocranium drives convergent evolution in neotropical electric fishes

“…One such mechanism is the modulation of a gradient of Shh and Fgf8 signaling molecules from the forebrain which can result in more or less brachycephalic phenotypes (Hu & Marcucio, 2009a; Hu et al., 2003; Marcucio et al., 2005). These signaling molecules act as an integrating force across the neurocranium between face and braincase regions.…”