Ultrastructure of the Nonciliated Bronchiolar Epithelial (Clara) Cell of Mammalian Lung: I. A Comparison of Rabbit, Guinea Pig, Rat, Hamster, and Mouse

Abstract

Two morphologic characteristics have been used to define the nonciliated bronchiolar epithelial cell: (1) abundance of agranular endoplasmic reticulum (AER) and (2) numerous membrane-bound ovoid granules. To quantitatively and qualitatively assess the ultrastructural homogeneity of this lung cell type among laboratory mammals used in lung research, we examined tissue from adult male rabbits, guinea pigs, rats, hamsters and mice. Following fixation by airway infusion at constant pressure (30 cm H₂O), lungs were processed by a selective embedding technique and bronchioles of known anatomic location were examined by electron microscopy. Nonciliated bronchiolar epithelium of all five species contained ovoid granules and abundant AER. Granules were most abundant in the rat (11.1 ± 8.8 per cell) and least in the hamster (4.4 ± 5.2 per cell). Granules were largest in hamster (0.72 ± 0.25 μm) versus 0.44 μm or less in the other species. Granules were of uniform electron density except in the guinea pig. AER was found throughout the cell in hamster and restricted to apical regions in others. Large spherical mitochondria with few cristae were present only in mice, rabbit, and guinea pig. We concluded that there was considerable interspecies variation in abundance, size, and morphology of the granules, in abundance and distribution of AER; and in mitochondrial morphology.