The airway epithelium, extending from the nasal cavity through the trachea and bronchial passages to the alveoli, is a complex tissue that performs a multitude of vital functions. This epithelium is generally considered to be composed of two contiguous divisions: the proximal conducting airway regions that serve as a conduit for inhaled air and the distal gas exchanging regions of the peripheral lung. This simple view does not adequately reflect the range of specialised activities necessary for normal lung functions.1 These include not only the key function of gas exchange between the respiratory and circulatory systems but also the synthesis and secretion of components of the extracellular lining of the lungs, uptake and metabolism of secreted products and xenobiotics, fluid and electrolyte balance, and protection and repair of the epithelium following injury. Accomplishing these various tasks requires an array of differentiated epithelial cells capable of sustaining these functions in distinct portions of the lungs.1 The surface epithelium of the cartilaginous conducting airways is a pseudostratified columnar epithelium composed of ciliated, non-ciliated, goblet, neuroendocrine and basal cell types. Interspersed between the cartilaginous rings of the larger airways are the submucosal glands (SMGs). Airway SMGs are microscopically similar to other minor mucosal glands that underlie the epithelium of the nasal passages and oral cavity and it is now clear that they are a major source of the secretions that coat the airway epithelium. As the epithelium extends further down into the lungs the structure becomes a less complex cuboidal epithelial layer containing a mixture of ciliated and secretory cells until the epithelium in the alveolus is a mix of type I and type II pneumocytes intimately associated with the capillary bed.

There have been significant advances in the understanding of the molecular drivers of cellular commitment and differentiation of individual …