Embryology of the head and neck

Abstract

Embryology and development have been covered after the main anatomical descriptions in the previous sections, but it is going to precede them in this section. The reason for this departure is that the embryonic development of the head and neck explains much of the mature anatomy which can seem illogical without its developmental history. The development of the head, face, and neck is an area of embryology where significant strides in our understanding have been made in the last few years. The development of the head is intimately related to the development of the brain outlined in Chapter 19 and its effects on shaping the head will be described in Chapters 32 and 33. The major thrust of this chapter is the description of the formation of structures called the pharyngeal (or branchial) arches and the fate of the tissues that contribute to them. All four embryonic germ layers contribute to the pharyngeal arches and their derivatives, hence to further development of the head and neck. Figure 21.1 is a cross section through the neck region of a 3-week old embryo after neurulation and folding described in Chapter 8. It shows the structures and tissues that contribute to the formation of the head and neck: • The neural tube situated posteriorly and the ectomesenchymal neural crest cells that arise as the tube closes; • The paraxial mesoderm anterolateral to the neural tube; • The endodermal foregut tube anteriorly; • The investing layer of ectoderm. The development of all these tissues is intimately interrelated. The pharyngeal arches are very ancient structures in the evolutionary history of vertebrates. The arches and their individual components have undergone many modifications during their long history. In ancestral aquatic vertebrates, as in modern fishes, water was drawn in through the mouth and expelled through a series of gill slits (or branchiae, hence the term ‘branchial arch’) in the sides of the pharynx. Oxygen was extracted as the water was passed over a gill apparatus supported by a branchial arch skeleton moved by branchial muscles controlled by branchial nerves. Although ventilation and respiration is now a function of the lungs in land vertebrates, the pharyngeal arches persist during vertebrate development.