Summary

T he respiratory epithelium is the first cell layer to come intocontact with the approximately 7,000 liters/air humans atrest breath in every day. Thus, epithelial layer defense andrepair processes must be robust enough to withstand the onslaughtof infectious, chemical, and other insults likely to comeits way. The significance of pulmonary epithelial injury and repairto pulmonary homeostasis and diseases takes many forms.Perhaps of most importance is repopulation of the denuded airwayor alveolar epithelial basement membrane – a key process inlung tissue repair. In asthmatic conditions, airway epithelial cellresponses to injury, be it infectious, immunologic, chemical, orphysical injury, can induce the release of soluble mediators thatalter airway responsiveness to a contractile stimulus or inducesubepithelial fibrosis (1). Similarly, in conditions with alveolarinflammation/fibrosis (i.e., acute lung injury) type II alveolarepithelial cells must clear alveolar fluid, reform a tight barrier,repopulate the denuded alveolar basement membrane, and participatein provisional alveolar matrix remodeling, includingfibrinolysis (2). Evidence for a role of epithelial-mesenchymalcell interactions and/or epithelial-mesenchymal transition infibroproliferative diseases of the lung is rapidly mounting (2, 3).In summary, migration of epithelial cells to cover wounded areasof airway or alveolar wall, as well epithelial cell physiologicaland biological responses that depend on tight epithelial cell-cellcontacts are of paramount importance to lung homeostasis anddisease.