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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): ClaraV 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
ClaraV 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 ClaraV and BASCs having the
highest potential and fully differentiated cells the lowest.