Tracheal Epithelium

Tracheal epithelium is similar to respiratory epithelium in other parts of the conducting airway

Ciliated columnar cells, mucous (goblet) cells, and basal cells are the principal cell types in the tracheal epithelium (Figs. 18.6 and 18.7). Brush cells are also present but in small numbers, as are small granule cells.

• Ciliated cells, the most numerous of the tracheal cell types, extend through the full thickness of the epithelium. Cilia appear in histologic sections as short, hairlike profiles projecting from the apical surface. Each cell has approximately 250 cilia. Immediately below the cilia is a dark line formed by the aggregated ciliary basal bodies (Fig. 18.8). The cilia provide a coordinated sweeping motion of the mucous coat from the farthest reaches of the air passages toward the pharynx. In effect, the ciliated cells function as a "mucociliary escalator" that serves as an important protective mechanism for removing small inhaled particles from the lungs.

• Mucous cells are similar in appearance to intestinal goblet cells and are, thus, often referred to by the same name. They are interspersed among the ciliated cells and also extend through the full thickness of the epithelium (see Fig. 18.8). They are readily seen in the light microscope after they have accumulated mucinogen granules in their cytoplasm. Although the mucinogen is typically washed out in hematoxylin and eosin (H&E) preparations, the identity of the cell is made apparent by the remaining clear area in the cytoplasm and the lack of cilia at the apical surface. In contrast to ciliated cells, the

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