Surfactant-producing epithelium in the dorsal part of the cartilaginous eustachian tube of mice. Light, transmission, and scanning electron microscopic observations.

The nonciliated area in cartilaginous roof tubothelium of 14 conventional mice was examined histologically and classified as a modified transitional respiratory epithelium. On the free cell surface numerous short microvilli were found. On the lower roof surface in the midcartilaginous portion and especially near the pharyngeal orifice, a convoluted pattern of ridges and pits was observed. These structural peculiarities are interpreted as adaptive features, ensuring survival of the cells in a dynamic tubal environment. On the basis of systematic observations of the multilamellar bodies in the cytoplasm resembling the phospholipid lamellar bodies of pulmonary surfactant, which are discharged in the tubal lumen and to be found in the pits, it is suggested that the nonciliated cells are "specialized" surfactant-producing tubocytes. The synthesis of surfactant-precursors starts in the basal layer. Different phases of the secretory process were observed in the neighboring cells. This finding is related to the cell-cooperation constantly releasing the secretory product, and natural cell-turnover. Unlike the previously reported surfactant-producing cells in the lower tubothelium of other species, roof tubothelial cells of mice are morphologically similar to type II pneumocytes.