An epidermoid formation in the developing middle ear: possible source of cholesteatoma.

Thirty-seven of 68 middle ears from fetuses between 10 and 33 weeks gestation show an epidermoid formation (EF) on histological study. This is always found in exactly the same anatomical location: in the anterosuperior middle ear mucosa near the origin of the tympanic membrane and adjacent to the posterior edge of the anterior limb of the bony tympanic ring. The EF is not found after the 33rd week. It is likely that the EF is a vestigial relic of the epibranchial organ that is present in some vertebrates. Epidermoid tissue of primary and even some secondary cholesteatomas may have an origin in the EF.