ANATOMY OF CLARA CELL SECRETION - SURFACE CHANGES OBSERVED BY SCANNING ELECTRON-MICROSCOPY

The microanatomical alterations of the surface of lung Clara cells wer e studied during secretion. Stimulation of Clara cells was induced in rats by chronic inflammation caused by a single intratracheal instilla tion of bleomycin performed 2.5 months before the animals were killed. Bleomycin treatment resulted in marked stimulation of secretion by th e bronchiolar Clara cells, 31% of the Clara cells from the treated rat s showing signs of active secretion whereas only 6.3% of Clara cells i n control rats presented similar features. High-resolution views of lu ng airways were obtained by scanning electron microscopy of critical p oint dried tissue samples. The surface of Clara cells underwent severa l modifications associated with the secretory events. These alteration s followed 3 major phases: (1) formation of a smooth apical dome made up of a large volume of cytoplasm; (2) progressive narrowing of this d ome-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 vi ews, thin pedicles linking the cap-like bodies to the remaining cytopl asm of the Clara cell were detected. In other instances, release of th e cap-like body occurred without the previous formation of stalks. Sec retion of intracellular granules was observed in some cells before sev erance of the cap-like body. It is concluded that (1) the cap-like bod ies are not artifactual features of Clara cells; (2) Clara cell secret ion is both apocrine and merocrine, the former predominating; (3) chro nic inflammation is associated with an increased formation and release of the secretory cap-like bodies by Clara cells.