Respiratory bronchioles appear during the canalicular stage, marking the initiation of the formation of a gas exchange unit. Stem bronchi subdivide to tertiary bronchial buds, which in turn divide to yield terminal bronchial buds, the last portion of the nonrespiratory conducting airway. These then subdivide into respiratory bronchioles. The bronchioles are completely encircled by parabronchial smooth muscle and do not have cartilage.
The epithelial composition of the bronchioles, particularly the ratio of Clara:ciliated cells, varies along the proximal–distal axis; the smaller airways have a higher proportion of Clara cells than ciliated cells
Multiple AncestorsSingle AncestorNo DescendantsDevelops fromPart of Parent