Fig. 7. A model showing putative stem cells in the distal lung. Left: depiction of the epithelial cell layer, with cells resting on a basal lamina (black line) and facing the lumen to the right. Stem cells are located in specific regions (red stars): Clara^V cells near neuroendocrine bodies (NEBs) and putative bronchioalveolar stem cells (BASCs) at the bronchioalveolar duct junction. Key indicates the different cell types possibly involved in steady-state turnover and injury response. The Clara^V and putative BASCs (which may be the same cells or members of the same stem cell population) differ from classical stem cells in that they co-express proteins also expressed by fully differentiated cell types. The existence of putative transit amplifying (TA) cells (hexagons), intermediate between stem cells and fully differentiated cells (squares) is hypothetical. Alternatively, Clara cells may be the TA population. A gradation of potential for self-renewal and contribution to different lineages might exist throughout the epithelium, with Clara^V and BASCs having the highest potential and fully differentiated cells the lowest.