The epithelium of the airways is a continuous sheet of cells lining the lumenal surface of the airways. It separates the internal environment of the body (i.e., subepithelial structures) from the external environment (i.e., airway lumen). The lumenal surface of the epithelium is, therefore, exposed to inhaled substances, such as gases, particulates, or aerosols. Connecting adjacent epithelial cells are specialized tight junctional processes [5,6] that limit the penetration of inhaled substances by the intercellular route of administration. Under normal or physiological conditions, larger molecules must past through the epithelial cell. Therefore, the epithelium serves the important function of limiting access of inhaled substances to the internal environment of the body. Under pathophysiological conditions, the epithelium may be damaged, leading to enhanced penetration of substances present in the airway lumen [7].

The airway epithelium comprises a variety of cell types (Table 1), the distribution of which confers different functions on the airway region. Extending from the trachea to the terminal bronchus, the lumenal surface of the airways are lined by ciliated cells. Mucus, a viscous fluid containing mucin glycoproteins and proteoglycans, floats on a watery layer of periciliary fluid (or sol) and covers the lumenal surface of the epithelium. The secretions fulfill four important functions. Firstly, they protect the epithelium from becoming dehydrated. Secondly, the water in the mucus promotes saturation of inhaled air. Thirdly, the mucus contains antibacterial proteins and peptides, such as defensins and lysozyme, that serve to repress microbial colonization of the airways [8,9]. Fourthly, the mucus is involved in airway protection from inhaled xenobiotics or chemicals. Coordinated beating of the epithelial cilia propels the blanket of mucus toward the upper airways and pharynx, where the mucus may be either swallowed or ejected. The rate of mucus propulsion varies according to the airway region such that movement in the smaller airways is slower than in the larger airways, a situation that arises from the proportionately larger number of ciliated cells in the larger airways and the higher ciliary beat frequency in the larger airways [10]. Syllogistically, this process is advantageous, given that many small airways

Table 1 Cells of the Airway Epithelium


Putative function

Ciliated columnar

Mucus movement

Mucous (goblet)

Mucus secretion


Periciliary fluid; mucus secretion

Clara (nonciliated

Xenobiotic metabolism; surfactant production


Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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