Embryology

The human external auditory canal is derived from the first branchial groove. A primitive meatus is initially formed in the fifth week of gestation from an invagination of ectoderm medially toward the endoderm of the first pharyngeal pouch. This primitive meatus becomes the cartilaginous ear canal. The surface ectoderm also gives rise to a core of epithelial cells called the meatal plate which extends medially toward the tympanic cavity (pharyngeal pouch). The most medial ectodermal cells of the meatal plate form the superficial layer of the tympanic membrane. A layer of mesenchymal growth between the meatal plate and pharyngeal endoderm forms the middle fibrous layer of the tympanic membrane. Pharyngeal endoderm contributes to the medial mucosal layer.

In the third month of gestation, the medial canal is formed by ossification of the tympanic bone around the core of epithelial cells. Malformation of this bone medially results in atretic bone at the tympanic membrane and atresia of the bony ear canal. Similarly, if the core of epithelial cells does not grow toward the pharyngeal groove, tympanic bone is allowed to ossify producing atretic bone. The epithelial core canalizes late in developmentā€”the seventh month. Failure of these cells to absorb can result in a normal tympanic membrane and bony canal, with an atretic or stenotic cartilaginous ear canal. In 313 patients with major congenital ear malformations, Jahrsdoerfer,1 found atresia far more common than stenosis, in a ratio of 7:1. Pneumatization of the mastoid bone is also a late embryologic event and continues into postnatal life. As discussed later, size of the mastoid air cell system and tympanic cleft are crucial in evaluating candidacy for atresia surgery.

The pharyngeal pouch persists medially as the tubotympa-num and widens laterally to form the tympanic cavity. Interestingly, the eustachian tube generally develops normally, and patients with aural atresia have no increased incidence of eustachian tube dysfunction.6 The ossicles develop from the first and second branchial arches. The head of the malleus and body of incus are formed from Meckel's cartilage of the first branchial arch, while the manubrium of the malleus, long process of incus, and stapes superstructure are derived from Reichert's cartilage of the second branchial arch. The stapes footplate is a product of Reichert's cartilage and otic capsule bone; it develops normally, and is rarely fixed in aural atresia. Ballachandra7 gives an excellent review of developmental anatomy of the outer and middle ears.

The derivation of inner ear structures including the membranous labyrinth is from the ectodermal otocyst and is completely separate from the embryology of the middle and outer ears. As a result, vestibular function and sensorineural hearing in these patients is usually normal. Inner ear abnormalities, both anatomic and functional, can exist in patients with aural atresia, however, and may be a relative contraindication to atresiaplasty.

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