Epithelial Migration
Fortunately, the ordered growth of the ear canal epithelium facilitates a clearing mechanism termed epithelial migration. Simply stated, the epithelium in the ear canal grows outward from the tympanic membrane toward the opening of the external ear canal.
As the surface epithelial cells move, they carry along any debris on top of them. This physiologic epithelial movement process may be demonstrated by placing India ink on the eardrum and observing its dispersal along the ear canal over several weeks (Figure 11-1). The rate of epithelial movement is slow, and in older animals and people the rate becomes even slower. When the rate slows to the point of allowing accumulation of debris, the term failure of epithelial migration applies (Figure 11-2). When this condition causes accumulation of substances within the ear canal, ear flushes and ceruminolytic agents play important roles in its management.A simple squamous germinal epithelium lines the lateral surface of the tympanic membrane in the external ear canal, especially in the area of the handle of the malleus (the pars tensa). It has been hypothesized that these germinal cells differentiate into epithelial cells called basal keratinocytes, with very loose attachments to the basement membrane.
Basal keratinocytes migrate radially on the surface of the eardrum, becoming continuous with the epithelium of the external ear canal. Evidence of this phenomenon has been gained from studying the resultant migration of squamous epithelium into the middle ear cavity following myringotomy performed at the periphery of the eardrum.
Migration of epithelium from the (pars tensa of the) tympanic membrane toward the annular region of the eardrum to the epithelium of the horizontal canal provides a clearance mechanism for the migratory keratinocytes that result from normal stratified squamous epithelial physiology in the external ear canal. Cerumen, which covers the keratinocytes, is cleared from the ear canal along with the migrating epithelium.
Besides playing a role in protecting the ear canal as a mechanical barrier to environmental substances, the keratinocytes have been shown to possess immune functions. Interleukin-1 (IL-1) is stored in keratinocytes. When these cells are damaged, IL-1 is released, stimulating other cells to release more IL-1, and a cascade of immunologic events results in the migration of granulocytes, monocytes, and macrophages into the site of damage. When an area of the ear canal has been denuded by trauma or ulcer formation, loss of this protective immune mechanism allows unchecked bacterial colonization, favoring the development of otitis externa.