Schematic diagram of the microenvironmental niches that may contain lung stem/progenitor cells. In the trachea and main bronchi, undifferentiated basal cells (stained for transcription factor p63) can function as classical stem cells. Duct cells from submucosal glands located in the cartilaginous airways are also a potential niche. In the more distal lung, Clara cells and 'variant' Clara cells (stained red) are found in the bronchioles and bronchiolar-alveolar junctions, respectively, while the alveoli contain type II cells (stained green) that can regenerate type 1 pneumocytes. bv, blood vessel; D, dorsal; H and E, hematoxylin and eosin; V, ventral. Reprinted with permission from The Company of Biologists Ltd. .