Mnd. Peao et al., ANATOMY OF CLARA CELL SECRETION - SURFACE CHANGES OBSERVED BY SCANNING ELECTRON-MICROSCOPY, Journal of Anatomy, 182, 1993, pp. 377-388
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.