Plasticity in the airway epithelium.

Normal cell turnover as well as the response to injury require cell proliferation and differentiation. The airway epithelium maintains these processes throughout adult life. Controlled homeostatically, cell proliferation and differentiation usually restore, as an end point, the pseudostratified architecture of the normal mucociliary epithelium. After injury, however, cell proliferation and differentiation sometimes establish, as an end point, regions of metaplastic cells. In this brief review, we have tried to summarize research findings that 1) describe the development of metaplastic lesions in morphological terms, 2) identify cells the proliferation of which forms the basis of these lesions, and 3) identify molecular changes within these cells that control development of the metaplastic phenotype.