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EMBRYOLOGY

Oral Cavity and Oropharynx

The oral cavity begins as a depression, the stomodeum, that invaginates until it meets the primitive foregut, creating the buccopharyngeal membrane. This membrane degenerates at 4 weeks of gestation, providing continuity between the ectodermally derived oral cavity and the endodermally derived oropharynx. The 5 branchial arches are mesodermal condensations on the lateral cervical area of the embryo and are separated by branchial clefts externally and branchial pouches internally. The brachial clefts are ectodermally lined, whereas the pouches are endodermally derived. Derivatives of the branchial system are shown in Table 366-1.

TABLE 366-1DERIVATIVES OF BRANCHIAL SYSTEM

The hard palate is divided into a primary and a secondary palate. The primary palate contains the anterior alveolus and the 4 upper incisors, and is derived from the medial nasal prominences. The secondary palate (the area posterior to the incisive canals) is formed by the medial growth of the lateral palatine processes of the maxilla. The primary palate is completely developed by the seventh week of gestation, and the secondary palate completes its fusion between weeks 10 and 12 of gestation.

The anterior two-thirds of the tongue are derived from ectoderm, whereas the posterior one-third is derived from endoderm of the primitive foregut. The prominces that form the future tongue begin to condense during the fourth week of gestation and are complete by the seventh week. The fungiform and filiform papillae develop by the 11th week, and the circumvallate papillae develop between weeks 8 and 20. The floor of the mouth is a first-arch derivative. The salivary glands are of ectodermal origin and are derived from the first pouch, developing between weeks 5 and 8.

The laryngeal cartilages are formed by the fourth and sixth branchial arches. During the fourth week of life, the tracheobronchial diverticulum develops ventral to the primitive pharynx. The arytenoid swellings will appear on the cranial end. The tracheoesophageal septum will develop to separate the trachea from the esophagus. The trachea and lung buds will elongate to form the ...

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