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.

Figure 11-1 For legend, see p. 224.

A through E, Movement of India ink across the eardrum demonstrates epithelial migration. India ink was placed on the eardrum at the junction with the horizontal canal. Photos are taken at 2-week intervals. The entire series covered 2 months.

Figure 11-2 Accumulation of ceruminous exudates in the ear. Epithelial migration is not able to remove the waxy matter.

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.

Failure of Epithelial Migration

Damage to the germinal epithelium of the eardrum from infection or ear mite infestation results in damage to the keratinocytes on the surface of the tympanic membrane (Figure 11-3). During the healing process, fibrosis may cause permanent changes to the tympanic membrane. Normal epithelial migration may be disrupted. Failure of epithelial migration may result in the accumulation of flakes of skin in the ear canal. More often, however, wax and keratin accumulate at the base of the eardrum and form either soft wax plugs (Figure 11-4) or large, hard concretions called ceruminoliths (Figure 11-5). To find them when examining the ear canals with an otoscope, the veterinarian should follow the bend in the ear cartilage and look in the horizontal canal toward the eardrum. Ceruminoliths are found attached to the eardrum lying along the floor of the horizontal ear canal.

Figure 11-3 The patient, a 3-year-old cat, was presented with severe ear mite infestation. The ear canal was gently flushed with a dilute povidone-iodine solution to remove the ear mite debris. After cleaning, the eardrum can be seen to have a tear in it. It is here that epithelial cells that would migrate to the ear canal will stop their journey. This will result in waxy accumulations on the eardrum in the future.

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