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* The primordial pharynx is bounded laterally by pharyngeal arches. Each arch consists of a core of mesenchyme covered externally by ectoderm and internally by endoderm. The original mesenchyme of each arch is derived from mesoderm; later, neural crest cells migrate into the arches and are the major source of their connective tissue components, including cartilage, bone, and ligaments in the oral and facial regions. Each pharyngeal arch contains an artery, a cartilage rod, a nerve, and a muscular component.
* Externally the pharyngeal arches are separated by pharyngeal grooves. Internally the arches are separated by evaginations of the pharynx-pharyngeal pouches. Where the ectoderm of a groove contacts the endoderm of a pouch, pharyngeal membranes are formed. The adult derivatives of the various pharyngeal arch components are summarized in Table 9-1 and the derivatives of the pouches are illustrated in Figure 9-8.
* The pharyngeal grooves disappear except for the first pair, which persists as the external acoustic meatus. The pharyngeal membranes also disappear, except for the first pair, which becomes the tympanic membranes. The first pharyngeal pouch gives rise to the tympanic cavity, mastoid antrum, and pharyngotympanic tube. The second pharyngeal pouch is associated with the development of the palatine tonsil. The thymus is derived from the third pair of pharyngeal pouches, and the parathyroid glands are formed from the third and fourth pairs of pharyngeal pouches.
* The thyroid gland develops from a downgrowth from the floor of the primordial pharynx in the region where the tongue develops. The parafollicular cells (C cells) in the thyroid gland are derived from the ultimopharyngeal bodies, which are derived mainly from the fourth pair of pharyngeal pouches.
* Branchial cysts, sinuses, and fistulas may develop from parts of the second pharyngeal groove, the cervical sinus, or the second pharyngeal pouch that fail to obliterate.
* An ectopic thyroid gland results when the thyroid gland fails to descend completely from its site of origin in the tongue. The thyroglossal duct may persist or remnants of it may give rise to thyroglossal duct cysts and ectopic thyroid tissue masses. Infected cysts may perforate the skin and form thyroglossal duct sinuses that open anteriorly in the median plane of the neck.
* Cleft lip is a common congenital anomaly. Although frequently associated with cleft palate, cleft lip and cleft palate are etiologically distinct anomalies that involve different developmental processes occurring at different times. Cleft lip results from failure of mesenchymal masses in the medial nasal and maxillary prominences to merge, whereas cleft palate results from failure of mesenchymal masses in the palatal processes to meet and fuse. Most cases of cleft lip, with or without cleft palate, are caused by a combination of genetic and environmental factors (multifactorial inheritance).

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