The conducting airways of humans are lined by a superficial layer of epithelial cells which comprise an important primary line of defense to the entire respiratory tract. This superficial cellular layer consists primarily of mucus-producing (goblet) cells and ciliated cells. These cells function in a coordinated fashion to entrap inhaled biological and inert particulates and remove them from the airways. While this "mucociliary escalator" functions with great efficiency in the face of potentially injurious stimuli, it is a delicately balanced system relying on maintenance of appropriate complements of ciliated and mucus-producing cells and the normal functioning of those cells to accomplish effective clearance. Perturbations in epithelial cell type distribution and function can lead to adverse health effects.
Watch a video clip illustrating normal ciliary activity in nasal epithelium obtained from a normal healthy human subject.
Mucus Cells And Other Non-Ciliated Epithelial Cells
Mucus and other non-ciliated cells represent approximately 20% of the epithelial cells lining the luminal borders of the large airways. Mucus cells often are distended with secretory product and exhibit a characteristic "goblet" shape. Other non-ciliated cells with fewer or no granules also may be present along the luminal border. These may represent mucus cells which have emptied their contents onto the luminal surface or cells which have not yet differentiated. The entire epithelial layer sits on a basement lamina comprised of collagen and connective tissue. All the cells of the epithelial layer are anchored to this "basement membrane." Smaller, more polygonal basal epithelial cells reside along the basement lamina. There is good evidence based on their ability to incorporate nucleic acid precursors that these cells are capable of undergoing mitosis and contributing to the growth and maintenance of the epithelial layer and in regeneration and reorganization of the epithelial layer subsequent to injury.
Ciliated cells represent approximately 80% of the epithelial cells residing on luminal borders of the large airways. While they are the most prevalent epithelial cell type lining the airways, many studies suggest that they also are among the most vulnerable to injury by infection, irritant, and pollutant exposure. The identifying characteristic of ciliated cells, the cilia which cover the luminal border, are highly organized appendages of the cell. A cross-section through the ciliary axoneme reveals an array of nine peripheral microtubules and two central microtubules. Present on each microtubular pair are two dynein arms, high molecular weight ATPases which effect sliding of the microtubular pairs relative to one another. Accessory structural elements such as radial spokes and interdoublet linkers also present in the axoneme are thought to provide shear resistance to microtubular sliding and thus impose a beat or waving motility.
Transmission electron micrograph of a cross-section of an airway cilium from a healthy human subject.