Figure 17-10.
Terminal bronchiole and Clara cells. The last parts of the air conducting system before the sites of gas exchange appear are called the terminal bronchioles, which generally have diameters of one to two mm. (a): Cross-section shows that a terminal bronchiole has only one or two layers of smooth muscle cells. The epithelium contains ciliated cuboidal cells and many low columnar nonciliated cells. X300. PT. (b): The nonciliated Clara cells with bulging domes of apical cytoplasm contain granules, as seen better in a plastic section. Named for Dr. Max Clara, the histologist who first described them in 1937, these cells have several important functions. They secrete components of surfactant which reduces surface tension and helps prevent collapse of the bronchioles. In addition, Clara cells produce enzymes that help break down mucus locally. The P450 enzyme system of their smooth ER detoxifies potentially harmful compounds in air. In other defensive functions, Clara cells also produce the secretory component for the transfer of IgA into the bronchiolar lumen; lysozyme and other enzymes active against bacteria and viruses; and several cytokines that regulate local inflammatory responses. Mitotically active cells are also present and include the stem cells for the bronchiolar epithelium. X500. PT.