Looking different: Understanding diversity in facial form

The face is perhaps the most distinguishing feature of the vertebrate body. Six billion human faces decorate the earth, each of them unique and exceptional in their own way. Likewise, facial variation is the cornerstone of species‐specific diversity within the animal kingdom. Yet despite this multiplicity in form, the underlying architecture of the vertebrate face is remarkably conserved. If early embryos of different species first resemble one another, then how is this facial diversity generated? Our primary goal is to elucidate the molecular origins of species‐specific craniofacial morphogenesis. We examined one facial primordia, the frontonasal prominence, of phylogenetically related (chick vs. quail vs. duck) and distant (mouse vs. chick) embryos and asked how such drastically different forms (e.g., beak, bill, or muzzle) could be generated from a once‐similar entity. We examined the morphological ontogeny and a number of molecular expression patterns in an attempt to shed light on when species‐specific variations occur and what molecules (BMPs, FGFs, etc.) are implicated in its differential growth. We hypothesize that subtle changes in the signaling of these morphogens can reproducibly alter the morphology of the frontonasal prominence. Taken together, these data facilitate our fledgling understanding of the process by which facial morphogenesis is regulated. © 2006 Wiley‐Liss, Inc.