Craniofacial Development of Avian and Rodent Embryos

In this overview, we examine approaches to the embryological development of the craniofacial region of avian and rodent embryos. By craniofacial region we mean: externally, the face and head, excluding the caudal pharyngeal arches; internally, the brain, skull, jaws, and facial skeleton, teeth (in rodents), and associated soft tissues, including muscular, vascular, and connective tissues. The terms “avian” and “rodent” are often equated with the domestic fowl (Gallus domesticus ), mouse (Mus musculus ), and rat (Rattus sp.); most studies of craniofacial development have been performed on these three species, mostly on chick and mouse. Such a model organism approach is unfortunate. It is typical of the type approach of pre-Darwinian biology and negates the richness afforded by comparative and phylogenetic analyses (see refs. 1 and 2 ), but is perhaps unavoidable, given both the convenience of working with chick and mouse embryos and the breadth of knowledge that has accumulated on these two species. There is a rich literature on aspects of the development of different strains of mice but much of the literature on avian development assumes that all strains of chickens are alike. The major staging tables for the chick (3 ) does not identify the strain; that for the mouse (4 ) is based on hybrids between C57BL/6 females and CBA males. We have recently expanded staging tables for murine craniofacial development for several inbred strains (5 –8 ), but comparative analyses are few.