ReviewTarget cell types with stem/progenitor function to isolate for in vitro reconstruction of human bronchiolar epithelia

Recent advancement in research on stem/progenitor cells of respiratory organs is breathtaking, benefiting from the rapid development of technology to create transgenic mice. There is now a great deal of knowledge capable of direct translation from mice to humans. Nevertheless, one has to be careful, since there may be unexpected pitfalls. First of all, there are differences anatomically, histologically and ultrastructurally in the airway epithelia of the two species. In parallel with these structural differences, regionally specific cell types behave and function, particularly in regenerative instances, differently between the two species, at least to some extent. From the viewpoint of important human respiratory diseases, one of the most susceptible regions of the respiratory tract is the bronchiole. In our approach to develop in vitro systems utilizing human bronchiolar epithelial cells, we are currently leaning on the data obtained from mouse studies in spite of the above-mentioned species differences. With the help of such in vitro systems we should be able to investigate the damaging effects and mechanisms of environmental pollutants in the human respiratory epithelium and consequently achieve results useful for quantitative analyses of the impact on human respiratory health. While pursuing this goal, the mouse data have suggested that it should be worthwhile to pay close attention to the stem/progenitor cells contained in the human bronchiolar epithelia and eventually make use of them. The mouse data have further shown that these stem/progenitor cells possess a very close association with the immature and variant club cells and the neuroendocrine cells, and our own unpublished preliminary data with human cells are, apparently, at least partly consistent with what the mouse data are telling us.