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The lung is a multi-functional organ that executes gas exchange and innate defense functions. The epithelium of respiratory tree consists of four regions from proximal to distal: trachea, bronchi, bronchiole, and alveolar regions. Multiple stem or progenitor cells are required for maintaining lung functions during normal condition and repair. The maintenance and repair of epithelium at different regions mainly depend on its resident stem or progenitor cells 1 , 2 , 3 .
Liu et al. (Tue,) studied this question.