Contribution of ciliated cells to Clara cell regeneration following complete loss of bronchiolar Clara cells in bromobenzene-injected mice.

Abstract

The paper investigates, by electron microscopy, the mechanism of regeneration of Clara cells killed selectively and completely by bromobenzene (BB) in mouse terminal bronchioles. Male specific pathogen free mice received an intraperitoneal injection of 0.76g/kg BB. Within 24hr, Clara cells showed vacuolization of the massive smooth endoplasmic reticulum (SER) followed by severe cell damage, and all Clara cells then exfoliated into the airway. Only ciliated cells remained intact; 99.9% of the lining cells were ciliated cells, which flattened and covered all of the terminal bronchioles. Many ciliated cells then lost the cilia and basal bodies; concurrently, nonciliated cells occurred although cell proliferation was negligible. Between 3 and 5 days, these nonciliated cells divided and increased in number, but some of them still maintained a few basal bodies, strongly suggesting that they are “post-ciliated” cells. At the same time, some cells showed ciliogenesis to replenish the decreased ciliated cell population, and at day 5 some formed a cluster of SER. After eight days, granules were formed in the SER-containing cells, and resulted in Clara cell development. Cycloheximide did not affect the reservation of ciliated cells but spoiled the reappearance of Clara cells during the bronchiolar regeneration after BB injection. At day 2 when Clara cells were thoroughly destroyed, [3H]-thymidine was incorporated into some ciliated cells, which transformed into nonciliated cells at days 3 and 4. Therefore, the present results may prove that a bronchiolar regeneration mechanism involves dedifferentiation of ciliated cells to form nonciliated cells, from which columnar epithelium regenerates.