Bronchioles

Bronchioles are air passages of 1mm or less in diameter. Terminal bronchioles are lined with simple columnar/cuboidal ciliated epithelium with ciliated and non-ciliated Clara cells (which have secretory vesicles). They then branch further to give rise to respiratory bronchioles. The epithelium of bronchioles as they branch changes from pseudostratified ciliated columnar to simple ciliated columnar to simple cuboidal with ciliated cells and non-ciliated cells. The goblet cells are present in the larger diameter bronchioles, not the terminal or respiratory (except in smokers and others exposed to irritants in the air).

• There is a transitional zone at the start of the respiratory bronchioles, which are involved in air conduction and gas exchange. The initial segments are lined with simple cuboidal epithelium with ciliated cells and Clara cells. Distally, the Clara cells predominate. The occasional granule cell or brush cell may be present. There are scattered thin walled outpocketings, the alveoli extending from the lumen. The walls have thick and thin regions, with collagenous and elastic fibers intermixed with the smooth muscle.