Bronchioles are air-conducting passageways that are typically less than 1 mm in diameter. The smallest bronchioles, called terminal bronchioles, are the most distal airways that are still a part of the conducting portion of the respiratory system. The terminal bronchioles then lead to the first portion of the airways that have a respiratory function, called respiratory bronchioles. These bronchioles then lead to the alveolar ducts, which are transitional airways that gradually become increasingly involved in gas exchange and lead to the alveolar sacs and their individual alveoli.
If we compare images from a bronchus and bronchiole at low magnification, we can see that the larger bronchus has large supporting cartilage plates that aren’t seen in the smaller bronchiole. The bronchi also contain sero-mucous glands that are not present in bronchioles, but the bronchioles will still have a surrounding layer of smooth muscle present. The larger proximal bronchioles, similar to the one in this image, are called the primary bronchioles. If we zoom in to 40x, we can see that the primary bronchioles have ciliated, pseudostratified columnar epithelium with goblet cells. When using a Hematoxylin and Eosin stain, the goblet cells will appear lighter than the surrounding epithelial cells and as their name suggests, these cells are often shaped similarly to the top portion of a goblet. Moving distally through the airways, the bronchioles' diameter gradually decreases and the terminal bronchioles will have a diameter less than 0.5 mm. The epithelium also transitions from ciliated pseudostratified columnar epithelium to an epithelium that consists mostly of ciliated simple columnar and cuboidal cells with exocrine club or Clara cells instead of goblet cells. The Clara cells can be identified by their tall columnar and non-ciliated appearance, as well as their dome-shaped apical ends that contain secretory granules.