Head and Neck

I. PHARYNGEAL APPARATUS (Figure 114) consists of pharyngeal arches, pouches, grooves, and membranes, all of which contribute greatly to the formation of the head and neck region. The Hox complex and retinoic acid appear to be important in the early stages of head and neck development. A lack or excess of retinoic acid causes striking facial anomalies.

A. Pharyngeal arches (1, 2, 3, 4, 6)* contain mesoderm (somitomeric) and neural crest cells. In general, the mesoderm differentiates into muscles and arteries (i.e., aortic arches 1-6; see Chapter 6 III A), whereas neural crest cells differentiate into bones. In addition, each arch has a cranial nerve associated with it. Table 11-1 summarizes the adult derivatives of the pharyngeal arches.

B. Pharyngeal pouches (1, 2, 3, 4) are evaginations of endoderm that lines the foregut (Table 11-2).

C. Pharyngeal grooves (1, 2, 3, 4) are invaginations of surface ectoderm. Pharyngeal groove 1 gives rise to the epithelial lining of the external auditory meatus, whereas all other grooves are obliterated (see Table 11-2).

D. Pharyngeal membranes (1, 2, 3, 4) are located at the junction of each pharyngeal groove and pouch. Pharyngeal membrane 1 gives rise to the tympanic membrane, whereas all other membranes are obliterated (see Table 11-2).

II. THYROID GLAND (Figure 11-1). This gland develops from the thyroid diverticulum, which forms in the floor of the foregut. The thyroid diverticulum migrates caudally down the midline to its adult anatomic position but remains connected to the foregut via the thyroglossal duct, which is later obliterated. The former site of the thyroglossal duct is indicated in the adult by the foramen cecum.

III. TONGUE (Figure 11-2)

A. Anterior two thirds (oral part) of the tongue

1. The oral part of the tongue forms from the median tongue bud and two distal tongue buds associated with pharyngeal arch 1. The distal tongue buds overgrow the median tongue bud and fuse in the midline, forming the median sulcus.

2. The oral part of the tongue is characterized by filiform papillae (no taste buds), fungiform papillae (taste buds present), and circumvallatc papillae (taste buds present).

*Pharyngeal arch 5 degenerates in humans. 60

Foramen Cecum Tongue

Figure U-l. (A) Lateral view of an embryo in week 4 of development, showing the pharyngeal arches. Note that pharyngeal arch 1 consists of a maxillary and mandibular prominence, which may cause some confusion in numbering of the arches. (B) Migration of the superior (SP) and inferior (IP) parathyroid glands, thymus (T), ultimobranchial body (UB), and thyroid gland (T). Note that the parathyroid tissue derived from pouch 3 is carried further caudally by the descent of the thymus than parathyroid tissue from pouch 4- The foramen cecum evaginates to form the thyroid diverticulum, which migrates caudally along the midline (dotted arrow). In addition, pharyngeal pouch 1, pharyngeal membrane 1, and pharyngeal groove 1 are shown to give rise to structures of the adult ear. 2 = pharyngeal pouch 2; 3 = pharyngeal pouch 3; 4 = pharyngeal pouch 4-

3- General sensation from the mucosa is carried by the lingual branch of the trigeminal nerve (CN V).

4. Taste sensation from the mucosa is carried by the chorda tympani branch of the facial nerve (CN VII).

B. Posterior third (pharyngeal part) of the tongue

1. The posterior third of the tongue forms from the copula and hypobranchial eminence, which is associated with pharyngeal arches 2, 3, and 4- The hypobranchial eminence overgrows the copula, thus eliminating any contribution of pharyngeal arch 2 in the formation of the definitive adult tongue. The line of fusion between the oral and pharyngeal parts of the tongue is indicated by the terminal sulcus.

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