Immunohistochemical characterization of the epidermoid formation in the middle ear.

Objective: To study the incidence, size, and origin of epidermoid formations after accurately characterizing them by cytokeratin immunohistochemical analysis.

Study Design: A strategy of screening sections for possible epidermoid formations in the entire eardrum area in paraffin-embedded, serially sectioned developing temporal bones was used. Unstained adjacent sections were subjected to immunohistochemical staining, to provide accurately identified epidermoid formations for a study of their appearance and size in relation to age.

Methods: The immunohistochemical expression patterns for epidermoid cytokeratins of several antibodies at different gestational ages were determined. Then, epithelial structures suspected as epidermoid formations were characterized as epidermoid with age-appropriate antibodies in 36 paraffin-embedded temporal bones from 19 cases with an age range of 6 gestational weeks to 15 months postpartum using a novel method of antigen retrieval by heating sections in a 70 degrees C water bath.

Results: Each of 22 temporal bones ranging in age from 16 weeks gestation to 8 months postpartum contained one or more, often numerous, epidermoid formations. Sizes of epidermoid formations increased significantly with increasing age. The formations were found anywhere in the annular lateral wall region of middle ear, but the majority were in the anterosuperior region. A further study of the interface between annular external canal epidermis and middle ear epithelium in a larger group suggested that epidermoid formations arise at approximately 16 gestational weeks from the external canal epidermis.

Conclusion: The findings indicate that epidermoid tissue in the middle ear normally originates from external canal epidermis at approximately the 16th gestational week and grows before disappearing.