Quantitative three-dimensional reconstruction and carbohydrate cytochemistry of rat nonciliated bronchiolar (Clara)...

We have employed three-dimensional reconstructions and cytochemistry to analyze nonciliated bronchiolar epithelial (Clara) cells from rat lungs. We identified a previously unreported ovoid granule type, which was characterized by its relative electron-lucency and eccentric electron-dense core. These electron-lucent granules were less common than electron-dense granules and were polarized toward the Golgi. They occupied 0.51% of the cell volume. Quantitative analysis of cell volume fractions showed the more numerous electron-dense granules to be polarized toward the air border; they occupied 2.5% of the cell volume. Periodic acid-thiocarbohydrazide-silver protein staining for hexose sugars indicated no differences between the granule types with regard to their carbohydrate content. Multivesicular bodies occupied 0.42% of total cell volume and were distinctly polarized toward the surface of the Golgi. Rod-shaped granules were present in 4 of the 6 reconstructed cells, ranging in number from 0 to 9 per cell. Nine apparent secretory events were seen in 6 cells and all appeared as exocytosis. We conclude that rat Clara cells contain at least 3 morphologically distinct granule types located in different volumes of the cell, and that exocytosis is the mode of secretion. The functional relationship between the ovoid granule types could include precursor-product lineage or different secretory products. We suggest that multivesicular bodies and light granules are stages in dark granule genesis.