Presence of abundant filaments in apical caps of the nonciliated bronchiolar epithelial (Clara) cells.

The nonionic detergent Triton X-100 has often been used for the extraction of cytoplasmic materials. We used the detergent in a vascular perfusion medium when preparing rat lung in order to observe the cytoskeleton of the nonciliated bronchiolar epithelial (Clara) cells. To eliminate some cytoplasmic materials selectively and to maintain good fine cell structure simultaneously, the lungs were perfused sequentially with the detergent (0.2% Triton X-100) alone for 2 min, with a mixture of low-concentration (0.1%) glutaraldehyde and detergent (0.2% Triton X-100) for 15 min, and finally with 2.5% glutaraldehyde for 5-10 min. After fixation, the nonciliated bronchiolar epithelial (Clara) cells were observed by scanning and transmission electron microscopy. At the apical region of the cells, there were central cytoplasmic protuberances (apical caps) filled with microfilaments. These filaments were bound at one end to the cytoplasmic side of the cell membrane and ran into the interior of the cytoplasm at the other end. As a control, the Clara cells were observed by transmission electron microscopy after perfusion with 2.5% glutaraldehyde solution. The luminal surfaces of the cells were covered with short, thick microvilli. The apical caps also had microvillus-like protrusions. These results suggest that the apical cap is not an apocrine droplet but rather is a stable structure involved in the function of the Clara cells.