In human respiratory mucosa, ciliated pseudostratified in certain regions to a stratified squamous form. The alcolumnar epithelium may change to stratified squamous ep-tered epithelium is more resistant to physical stress and inithelium. This change from columnar to squamous epithelium sult, but it is less effective functionally. In smokers, a similar epithelium is referred as columnar-to-squamous metapla-epithelial change occurs. Initially, the cilia on ciliated cells sia or simply the squamous metaplasia. Epithelial alter-lose their synchronous beating pattern as a result of noxations of this kind are reversible and are characterized by ious elements in smoke. As a result, removal of mucus is change from one type of fully differentiated adult cell to a dif-impaired. To compensate, the individual begins to cough, ferent type of adult cell. A given mature cell does not change thereby facilitating the expulsion of accumulated mucus in to another type of mature cell; rather, basal cell proliferation the airway, particularly in the trachea. With time, the numgives rise to the new differentiated cell type. These cellular ber of ciliated cells decreases because of chronic cough-changes are considered to be controlled and adaptive. ing. This reduction in ciliated cells further impairs the
Squamous metaplasia is a normal occurrence on the normal epithelium and results in its replacement with stratrounded, more exposed portions of the turbinates, on the ified squamous epithelium at affected sites in the airway. If vocal folds, and in certain other regions. factors (i.e., tobacco smoking) that predispose to squa-
Changes in the character of the respiratory epithelium mous metaplasia are not eliminated, the metaplastic epmay, however, occur in other ciliated epithelial sites when ithelium may undergo malignant transformation. Therefore, the pattern of airflow is altered or when forceful airflow oc-one of the two most common forms of cancer in the respicurs, as in chronic coughing. Typically, in chronic bronchi-ratory tract, the squamous cell carcinoma, has its origin tis and bronchiectasis, the respiratory epithelium changes from the squamous metaplastic cells.
expiration. Fibroelastic tissue and smooth muscle, the trachealis muscle, bridge the gap between the free ends of the C-shaped cartilages at the posterior border of the trachea, adjacent to the esophagus.
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.
19.7 and 19.8). 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, hair-like profiles projecting from the apical surface (Plate 71, page 692). Each cell has approximately 250 cilia. Immediately below the cilia is a dark line formed by the aggregated ciliary basal bodies (Fig. 19.9). 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.