Ultrastructural evidence concerning the mode of secretion of electron-dense granules by Clara cells.

An ultrastructural study of the secretory activity of non-ciliated bronchiolar epithelial (Clara) cells has been made. Membrane-bound, electron-dense granules were observed in the apical cytoplasm of Clara cells, immediately beneath the terminal web, and within terminal web regions, with their limiting membranes in close association with the plasma membrane of the cell. Granules were also observed in cytoplasmic pockets with varying amounts of their surfaces exposed to the bronchiolar, lumen, or within cytoplasmic projections into the lumen. Granules that were enveloped in a thin, uniform coat of cytoplasm and granules with no associated cytoplasm were both found free in the bronchiolar lumen. The study is consistent with the conclusion that the granules are secretory in nature, being extruded by a merocrine and possibly an apocrine mechanism.