Highlights of Craniofacial Morphogenesis in Mammalian Embryos, As Revealed by Scanning Electron Microscopy

The craniofacial region has a complex developmental history. Some of the major morphogenetic events of this region are described for the mouse embryo, as revealed by the use of scanning electron microscopy. The three germ layers, the primitive building blocks of the embryo, are delineated during gastrulation. During neurulation, the outer ectodermal layer gives rise to the nervous system. Several other events occur concomitantly with neurulation. These include regional subdivision of the mesoderm, body folding, turning, and formation of the heart and gut. The craniofacial region, as well as a number of other regions or organ systems, undergoes major developmental events during the phase of embryogenesis called organogenesis. The visceral (branchial) apparatus develops around the primitive oral cavity and pharyngeal region of the foregut, and gives rise to many of the structures of the head and neck. The face develops from the first visceral arches and derivatives of the frontonasal prominence. The tongue also originates from tissue prominences derived from the visceral arches. The oral and nasal cavities are initially continuous broadly with one another. As a result of partitioning, paired nasal cavities develop and become largely separated from the underlying oral cavity. Scanning electron microscopy reveals much about the morphological events occurring during formation of the craniofacial region. However, emphasis in future studies should be placed on determining the underlying cellular and molecular mechanisms causing these events.