Fine structures of the normal mucosa in developing rat middle ear.

Forty-two fetal and 90 developing young rats were studied electron microscopically to obtain a normal cellular distribution pattern of middle ear mucosa. The results showed that while ciliated columnar or cuboidal cells interspersed with secretory cells were predominantly distributed around the tympanic orifice and transitional zone, cuboidal or squamous nonciliated cells without secretory granules were numerous in the remaining part of the bulla. Development of ciliated cells started at the 17th or 18 th fetal day,paralleling that of secretory cells. The number of ciliated and secretory cells increased rapidly after birth. Glands were not found in the osseous tube, tympanic orifice, or other portions of the middle ear cavity in fetal, newborn, or suckling rats, while they are abundant in the cartilaginous portion of the tube. The findings of this study indicate that both secretory and ciliated cells are normal components of middle ear mucosa although these cells are restricted to the tympanic orifice and transitional zone, and that the mucociliary defense system starts to develop during the fetal stage.