The development of the face, palate, and nose

Abstract

In Chapter 21, we described the development of the pharyngeal arches and their derivatives. Craniofacial abnormalities account for about one third of all live birth defects. These arise during the development of the pharyngeal arches described in Chapter 21 or during the events described in this chapter. The first pharyngeal arch, the mandibular arch, is one of the basic building blocks needed to form the face and associated structures. The other major building block is the frontonasal process that covers the developing forebrain. The development of the face begins after the first pharyngeal arch forms around four weeks post-fertilization. At this stage, the head consists of a large bulge over the developing forebrain, approximating to the forehead and the mandibular arch in the position of the lower jaw. A slit between the frontonasal process and mandibular arch is continuous with the foregut tube; this slit is the primitive oral cavity or stomodeum. This primitive mouth cavity has no side walls where the cheeks would be and more significantly, there is no nasal cavity. Development of the nasal and oral cavities internally and the face externally proceeds at the same time over the course of the next eight weeks of development. Essentially, the nasal cavity is formed, then divided into two and separated from the oral cavity by the palate. The sequence of events is: • Development of the nasal cavity and first part of the palate, beginning the separation the oral and nasal cavities; • Development of the maxillary arch from the mandibular arch to form the cheeks and important structures contributing to the palate; • Completion of the components required to form the palate and separate the nasal cavity into right and left cavities; • Fusion of the building blocks to complete the separation of the nasal cavity and the separation of the nasal cavity from the oral cavity. As you can see in Figure 32.1A, the stomodeum is roofed by the frontonasal process and its floor is the fused mandibular arches.