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Auditory epidermal cell migration. V. Transmission electron-microscopic study of tympanic membrane and external auditory canal in the mouse.

The morphology of the tympanic membrane (TM) and external auditory canal (EAC) in mice was examined by transmission electron microscopy, in seven different regions: the handle of the malleus (HM) region, the intermediate and annular regions of the pars tensa (PT), the region of the pars flaccida (PF), the annular and osseous regions of the EAC and the cartilaginous region of the EAC. Each region displayed its own unique findings. In most regions of the TM and EAC-except the intermediate region of the PT-basal layer cells formed cytoplasmic processes (CPs) on their basal surface and adhered to the basement membrane (BM) by means of hemidesmosomes (HDs). This observation may suggest the presence of stem cells. In contrast, basal cells in the intermediate region of the PT did not extend any CPs from the basal surface but attached to the BM by a few characteristic spots of HD-like density but lacking the characteristics of HDs. It is concluded that stem cels are not present there and that basal cells connect loosely with the BM, thus providing the necessary morphological conditions to migrate laterally toward the annular region. The outermost layer of the epidermis in the annular region of the EAC showed numerous cytoplasmic projections (so-called 'finger-like projections'), representing keratinization and desquamation of epidermal cells (keratinocytes). These projections are believed to be the terminal differentiation of epidermal cells originating from the general centre in the HM region of the PT. The epidermis of the PF is an extension of the upper wall of the EAC, but the two regions differ morphologically.