Chapter 371

### Embryology

The oral cavity begins as a depression that invades into the developing embryo. It invaginates until the ectoderm of the stomodeum contacts the endoderm of 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 five 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 cleft is ectodermally lined, whereas the pouch is endodermally lined. The first arch develops into the mandible, the portions of the ossicles, and the muscles associated with these structures. The second arch contributes to portions of the ossicles, the styloid process, the portions of the hyoid bone, the facial muscles, the posterior belly of the digastric muscle, and the buccinator muscles. The third arch differentiates into portions of the hyoid bone and pharyngeal muscles. The fourth arch develops into the anterior/superior portions of the larynx. Finally, the fifth arch contributes to the posterior, larynx, cricoid, and intrinsic muscles of the larynx and the inferior pharyngeal constrictor muscle.

The pharyngeal pouches produce a variety of structures. The first pouch becomes the middle-ear cavity. The first branchial cleft creates the external ear canal. The tonsils are formed from contributions from both the first and second pouches invading into the surrounding mesoderm. Between the third and fifth months of gestation, lymphatic tissue then invades these primitive structures. The third pouch gives rise to the thymus gland and the inferior parathyroid glands. The fourth pouch develops into the thyroid gland and the superior parathyroids.

The hard palate is divided into a primary and secondary palate. The primary palate contains the anterior alveolus and the four upper incisors and is derived from the medial nasal swelling. 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. Clefts of the soft palate are generally associated with clefts of the secondary hard palate. Complete clefts involve the primary, secondary, and soft palate structures.

The anterior two thirds of the tongue are derived from ectoderm, whereas the posterior one third is derived from endoderm of the primitive foregut. Swellings 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.

### Anatomy

#### Oral Cavity

The oral ...

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