Bronchiolar Structure

Bronchioles are air-conducting ducts that measure 1 111111 or less in diameter. The larger bronchioles represent branches of the segmental bronchi. These ducts branch repeatedly, giving rise to the smaller terminal bronchioles that also branch. The terminal bronchioles finally give rise to the respiratory bronchioles.

Cartilage plates and glands are not present in bronchioles

The larger-diameter bronchioles initially have a ciliated, pseudostratified columnar epithelium that gradually transforms into a simple ciliated columnar epithelium as the duct narrows. Goblet cells are still présent in the largest bronchioles but are not present in the terminal bronchioles that follow. An exception is in smokers and others exposed to irritants in the air. There are no subepithelial glands in bronchioles. Cartilage plates, characteristic of bronchi, are absent in bronchioles. Instead, small elements of cartilage may be present, particularly at branching points. A relatively thick layer of smooth muscle is present in the wall of all bronchioles.

Small bronchioles have a simple cuboidal epithelium. The smallest conducting bronchioles, the terminal bronchioles, are lined with a simple cuboidal epithelium in which Clara cells are interspersed among the ciliated cells (see Fig. 18.11). Clara cells increase in number as the ciliated cells decrease along the length of the bronchiole. Occasional brush cells and small granule cells are also present. A small amount of connective tissue underlies the epithelium, and a circumferential layer of smooth muscle underlies the connective tissue in the conducting portions.

Clara cells are nonciliated cells that have a characteristic rounded or dome-shaped apical surface projection. They display TEM characteristics of protein-secreting cells (Fig. 18.12). They have a well-developed basal rER, a lateral or supranuclear Golgi apparatus, secretory granules that stain for protein, and numerous cisternae of sER in the apical

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