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Related to Branchial arches: Branchial cyst, branchial fistula
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However, in general, the branchial apparatus structures develop between the 4th and 6th week of gestation and consist of 6 pairs of mesodermal branchial arches separated by 5 paired endodermal pharyngeal pouches internally and 5 paired ectodermal branchial clefts externally.
At the fifth week of fetal development, the first and second branchial arches of the pharyngeal wall undergo a downward proliferation and meet the epicardial ridge, thereby closing the communication of the second, third, and fourth clefts with the exterior surface of the neck.
According to the most recent hypothesis, it is caused by an impaired midline fusion of the first branchial arches.