4 Craniofacial Patterning

HISTORICAL BACKGROUND One of the most striking characteristics about the craniofacial bones is that, contrary to the rest of the vertebrate skeleton, they are not entirely of mesodermal origin. Embryological studies, which started at the end of the 19th century with the observations of Kastschenko (1888, for selacians) and Goronovitch (1892, 1893, for teleosts and birds), have established that mesenchymal cells can arise, not only from the mesodermal, but also from the ectodermal germ layer. During this period, Julia Platt was the first to propose in 1893 that ectoderm contributed not only to the mesenchyme, but also to the cartilage of the visceral arches and to the dentine of the teeth in the mud puppy, Necturus . This derivation of mesenchyme, bones and cartilages from the ectoderm, was shown to occur via a transient structure, the Neural Crest (NC), which was first described in the chick embryo by the German Histologist Wilhem His in 1868. These observations contradicted the germ layer theory first put forward by Christian Heinrich Pander (1817), who described the formation of three layers of cells from the chick blastoderm. Later, Karl von Baer (1828) extended Pander’s findings to all vertebrate embryos. In 1849, Thomas Huxley generalized the presence of germ layers to invertebrates and the terms ectoderm, mesoderm , and endoderm were first used to designate the vertebrate germ layers by Ernst Haeckel in 1874, in the context of the Gastrea concept. The observation that formation of germ layers precedes organ morphogenesis and cellular differentiation was followed by...