Study design of BrdU pulse chase of control and bleomycin-induced lung fibrosis model in mice. C57Bl/6 mice had intratracheal instillation of 0.075 U/ml bleomycin in 40 ml volume under brief isofluorane anesthesia and animals were maintained under SPF conditions in the University of Washington animal facilities, and sacrificed periodically to assess the above. Slow cycling cells are assumed to be stem cells and pulsing of control vs. bleomycin-treated wildtype C57Bl/6 mice over two, four and six days i.p. at 12-hr intervals and pulse chase over 10 weeks, was expected to yield BrdU+ cells and BrdU- cells. While negative cells are assumed to be mature regularly cycling cells, BrdU+ cells after 10 weeks of chase are most likely label retaining cells (slow cycling stem cells) that started cycling late and hence retain the label for longer. BrdU is a DNA analogue. Abbreviations: i.p. intra-peritoneal; i.t. intra-tracheal; BALf, bronchoalveolar lavage fluid; BM, bone marrow; PB, peripheral blood. BrdU, bromodeoxyuridine; FACS, fluorescin-activated cell sorter.