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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.
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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.
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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.
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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.
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