Anatomy of Clara cell secretion: surface changes observed by scanning electron microscopy.

The microanatomical alterations of the surface of lung Clara cells were studied during secretion. Stimulation of Clara cells was induced in rats by chronic inflammation caused by a single intratracheal instillation of bleomycin performed 2.5 months before the animals were killed. Bleomycin treatment resulted in marked stimulation of secretion by the bronchiolar Clara cells, 31% of the Clara cells from the treated rats showing signs of active secretion whereas only 6.3% of Clara cells in control rats presented similar features. High-resolution views of lung airways were obtained by scanning electron microscopy of critical point dried tissue samples. The surface of Clara cells underwent several modifications associated with the secretory events. These alterations followed 3 major phases: (1) formation of a smooth apical dome made up of a large volume of cytoplasm; (2) progressive narrowing of this dome-like body at its base with the formation of a cap-like structure; (3) in toto release of the cytoplasmic cap-like body. In favourable views, thin pedicles linking the cap-like bodies to the remaining cytoplasm of the Clara cell were detected. In other instances, release of the cap-like body occurred without the previous formation of stalks. Secretion of intracellular granules was observed in some cells before severance of the cap-like body. It is concluded that: (1) the cap-like bodies are not artifactual features of Clara cells; (2) Clara cell secretion is both apocrine and merocrine, the former predominating; (3) chronic inflammation is associated with an increased formation and release of the secretory cap-like bodies by Clara cells.