Basal cells of the human bronchiole.

Basal cells of human bronchioles were studied by light and electron microscopic histochemistry, and compared with basal cells of major bronchi. Bronchiolar basal cells were found isolated or in isolated groups of a few linked cells. On the other hand, the basal cells of the major bronchi form virtually continuous layers covering the basement membrane. Bronchiolar basal cells were different in ultrastructure and alkaline phosphatase (ALP-ase) activity from basal cells of major bronchi. Bronchiolar basal cells showed intense ALP-ase activity on the plasma membrane, but basal cells of the major bronchi showed no activity. Basal cells of bronchioles were flattened, triangular or trapezoid in shape, and showed different organelle features from those of basal cells of major bronchi. ALP-ase positive bronchiolar flattened basal cells with pinocytoic vesicles were not myoepithelial cells, since actin was not demonstrated by immunohistochemistry. The role of basal cells in the renewal of epithelium appeared to be that of stem cells in both major bronchi and bronchioles, since basal cells of bronchioles appeared to differentiate directly to Clara cells, and to become ciliated cells via intermediate cells.