The Clara cell, the primary nonciliated cell type in the epithelial lining of distal, non-cartilaginous, conducting pulmonary airways, has at least 3 roles in normal lung function: (1) the source of secretory material for the acellular lining of bronchioles, (2) the site for metabolism of xenobiotic compounds via the cytochrome P-450 monooxygenase system, and (3) the progenitor for both itself and ciliated cells. Clara cell ultrastructure reflects the first 2 of these functions. The apex contains numerous electron-dense ovoid secretory granules and large amounts of agranular endoplasmic reticulum (AER). Granular endoplasmic reticulum (GER) is restricted to the perinuclear cytoplasm. This characterizes this cell type in these species: mouse, hamster, rat, guinea pig, rabbit, sheep, pig, horse, and llama. The Clara cell in humans and 3 species of macaque monkeys has little AER, but has abundant GER throughout the cell and numerous secretory granules. In dog, cat, steer, and ferret, the Clara cell lacks AER, has few granules and little GER, but possesses large amounts of cytoplasmic glycogen. In the 3 species studied, rat, rabbit, and pig, the cell does not reach the adult state for at least 4 wk after birth. In the fetal and neonatal lung, it resembles the cell of dogs, cats, cows, and ferrets, with large amounts of glycogen and few organelles. The transition occurs rapidly over the 4-wk postnatal period. In the adult, Clara cell distribution within the conducting airways varies from species to species. In mouse, hamster, and rabbit, it is the primary nonciliated secretory cell of the trachea and bronchi as well as bronchioles. The cytochrome P-450 monooxygenase system has been localized in the Clara cells of mouse, rat, hamster, rabbit, and pig. The secretory granules probably contain a protein, but it may be a carbohydrate or phospholipid in some species. In summary, the Clara cell is not the same cell in all species. There is considerable variability in ultrastructure, distribution, age of maturity, potential for carcinogen metabolism, and granule content between species.
|Original language||English (US)|
|Journal||American Review of Respiratory Disease|
|Issue number||2 II Suppl.|
|State||Published - Jan 1 1983|
ASJC Scopus subject areas
- Pulmonary and Respiratory Medicine