Figure 1: Illustration of potential stem cell niches in the adult lung. Scheme represents the regionally spatial location and distribution and differentiation of potential lung epithelial stem/progenitor cells along the airway. Distinct region-specific putative stem cell niches exist along the proximal-distal axis of the airway; they are SMG ducts in proximal trachea, basal cells of intercartilaginous zones in cartilaginous airway, NEBs in bronchioles domain, BADJ, and alveolar spaces. The potential progenitor/stem cells reside in their respective local niches, in which they maintain their stem/progenitor properties and are able to differentiate into various lung cell types. SMG: submucosal gland, NEB: neuroepithelial body, and BADJ: bronchoalveolar-duct junction.