Number of cailages in bronchiole

The lung substance is made up of numerous alveoli. Alveoli are thin-walled spaces lined by simple squamous epithelium. The structure of large bronchus is similar to that of the trachea. Smooth muscle, cailage and glands are present in its wall; and it is lined by pseudostratified ciliated columnar epithelium with goblet cells. Small bronchioles are lined by a simple columnar epithelium, and have a wall of smooth muscle. There is no cailage in their walls The cailages in the walls of the bronchi become irregular in shape, and are progressively smaller. Cailage is absent in the walls of bronchioles: this is the criterion that distinguishes a bronchiole from a bronchus. REF : Inderbir Singh's Textbook of Human HistologY, seventh edition, pg.no.,45, 233.