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Illustration of putative stem/progenitor cells which have been demonstrated within the four trophic regions of the human lung, ie, trachea, bronchi, bronchioles and alveoli. Reprinted with permission of the American Thoracic Society. Copyright © 2008, American Thoracic Society. X, Engelhardt JF. The glandular stem/progenitor cell niche in airway development and repair. Proc Am Thorac Soc. 2008;5(6):682–688.
