A reversion to fetal type epidermis as a cause of common external ear disease

Objective: To study the histology of epidermal hyperplasia of the tympanic membrane and deep ear canal and to correlate the histopathological and optical features with clinical conditions. Design: Multimodality examination of deep meatal skin, keratosis, keratosis obturans, external canal cholesteatoma, retraction pockets, acquired cholesteatoma. Materials and Methods: Migratory properties were initially studied in 26 ears; then by using ink dot measurements (with appropriate imaging and distortino correction technology) in a further 14 ears. Optical analysis in the form of Elastic Scattering Spectroscopy was performed of areas of the tympanic membrane in 12 ears. The origin of auditory epithelial migration was studied in 179 fetal temporal bones. Whilst a study of 438 adult temporal bones from the Temporal Bone Laboratory, revealed that in 308 bones, there were representative areas of the external ear canal and tympanic membrane which were available for examination under the microscope. Archival registered material was studied. Low power pictures and inset higher power photomicrographs were taken of one or two relevant areas of the epidermis. Results: Migration occurred in 2 definite pathways. Despite having a similar histological appearance elastic scattering spectroscopy showed that the epidermis in specific regions of the tympanic membrane are different. In some mature ears, a marked change in the normal flat thin epidermis to one resembling that of the fetus is seen in the presence of local inflammation. This fetal type epidermis is present in varying lengths on the tympanic membrane and deep ear canal and displays hyperplastic bands growing down into sub-epidermal tissue. These sometimes penetrated the collagen layer of the tympanic membrane to enter the middle ear. Keratosis was frequent on the surfaces of the tympanic membrane and deep ear canal which may also be found in the invading fetal type epidermis, where it produced small keratin cysts. Conclusions: Different regions of the tympanic membrane were found to have distinct routes of migration, development and optical properties. Severe chronic otitis media, with inflammitory exudates, glandular metaplasia or tympanosclerosis, may be associated with fetal type epidermis. This type of epidermal hyperplasia appeared to be pathogenic and was observed in several structurally distinct entities such as keratosis, keratosis obturans, retraction pockets, squamous cell carcinoma and cholesteatoma of the deep external canal. In addition, invading fetal type epidermis in the middle ear, in some cases, is indistinguishable from acquired cholesteatoma. Thus the mature epidermis of the deep ear canal and the tympanic membrane may revert to an appearance consistent with fetal type epidermis in the presence of chronic local inflammation. This reverse differentiation highlights a primordial property of auditory epithelium which was not previously recognised. Furthermore, replacement by fetal type epidermis appears to be pathogenic and may be the cause of some specific conditions or their complications.