r Pharyngeal (branchial) arches, consisting of bars of mesenchymal tissue separated by pharyngeal pouches and clefts, give the head and neck their typical appearance in the fourth week (Fig. 15.3). Each arch contains its own artery (Fig. 15.4), cranial nerve (Fig. 15.7), muscle element, and cartilage bar or skeletal element (Figs. 15.8 and 15.9; Table 15.1, p. 366). En-doderm of the pharyngeal pouches gives rise to a number of endocrine glands and part of the middle ear. In subsequent order the pouches give rise to (a) the middle ear cavity and auditory tube (pouch 1), (b) the stroma of the palatine tonsil (pouch 2), (c) the inferior parathyroid glands and thymus (pouch 3), and (d) the superior parathyroid glands and ultimobranchial body (pouches 4 and 5) (Fig. 15.10).

Pharyngeal clefts give rise to only one structure, the external auditory meatus.

Molecular control of arch development resides in HOX genes, whose pha-ryngeal arch code is carried to the arches by neural crest cells migrating from hindbrain segments known as rhombomeres (Fig. 15.12). This code is then maintained by interactions between crest cells and arch mesoderm (Fig. 15.13).

The thyroid gland originates from an epithelial proliferation in the floor of the tongue and descends to its level in front of the tracheal rings in the course of development.

The paired maxillary and mandibular prominences and the frontonasal prominence are the first prominences of the facial region. Later, medial and lateral nasal prominences form around the nasal placodes on the front-onasal prominence. All of these structures are important, since they determine, through fusion and specialized growth, the size and integrity of the mandible, upper lip, palate, and nose (Table 15.2, p. 389). Formation of the upper lip occurs by fusion of the two maxillary prominences with the two medial nasal prominences (Figs. 15.22 and 15.23). The intermaxillary segment is formed by merging of the two medial nasal prominences in the midline. This segment is composed of (a) the philtrum, (b) the upper jaw component, which carries the four incisor teeth, and (c) the palatal component, which forms the triangular primary palate. The nose is derived from (a) the frontonasal prominence, which forms the bridge, (b) the medial nasal prominences, which provide the crest and tip, and (c) the lateral nasal prominences, which form the alae (Fig. 15.23). Fusion of the palatal shelves, which form from the maxillary prominences, creates the hard (secondary) and soft palate. A series of cleft deformities may result from partial or incomplete fusion of these mesenchymal tissues, which may be caused by hereditary factors and drugs (diphenylhydantoin).

The adult form of the face is influenced by development of paranasal sinuses, nasal conchae, and teeth. Teeth develop from epithelial-mesenchymal interactions between oral epithelium and neural crest derived mesenchyme. Enamel is made by ameloblasts (Figs. 15.31 and 15.32). It lies on a thick layer of dentin produced by odontoblasts, a neural crest derivative. Cementum is formed by cementoblasts, another mesenchymal derivative found in the root of the tooth. The first teeth (deciduous teeth, or milk teeth) appear 6 to 24 months after birth, and the definitive or permanent teeth, which supplant the milk teeth, are formed during the third month of development (Fig. 15.33).

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