Fig. 5. Potential repair mechanisms in the distal bronchi, bronchioles and alveoli. Type II alveolar cells are in yellow, type I alveolar cells are in gray, smooth muscle cells are in red. The broken red line indicates that blood vessels are associated with the bronchiolar epithelium but not obviously enriched in particular regions. In the alveoli, blood vessels are tightly apposed to type I cells. Putative regenerative cell populations are as follows (1-6). (1) Variant Clara (Clara^V) cells adjacent to neuroendocrine bodies (NEBs) and at bronchioalveolar junction (blue/pink or blue/yellow), which are resistant to naphthalene and proliferate soon after injury by this agent. If all Clara cells are destroyed, there is no repair and the mice die. (2) Putative bronchioalveolar stem cells (BASCs) (blue/yellow), which self-renew in culture after flow sorting and give rise to multiple lineages in culture. These may be the same as Clara^V cells. (3) Ciliated cells, which can proliferate and transdifferentiate into Clara cells after naphthalene injury but other evidence argues against this. (4) Clara cells, which proliferate after NO₂ injury and give rise to ciliated cells but it is not known whether all Clara cells have this potential. (5) Type II cells, which give rise to type I cells after bleomycin injury to type I cells. (6) Neuroendocrine cells, where present, can self-renew and proliferate but do not give rise to other lineages.