Flow cytometric study of the type II pneumocyte cell cycle in vivo and in vitro

The type II pneumocyte cell cycle was studied in vivo and in vitro through bivariate flow cytometric analysis of DNA content vs. incorporated 5-bromo-2-deoxyuridine (BrdUrd). The cell cycle phase durations Ts (7.8 h) and TG2/M (1.1 h), measured in vivo, agreed well with earlier estimates obtained by thymidine labeling. Left unilateral pneumonectomy increased the BrdUrd labeling index of type II cells in the remaining lung from an initial value of 1.9 +/- 0.3% to 4.8 +/- 1.0%, but had no effect on Ts or TG2/M in vivo. In both normal and pneumonectomized animals, BrdUrd-positive cells in vivo rapidly completed mitosis but did not enter a second S-phase. These results demonstrate that proliferating type II cells do not form a continuously cycling population in the normal or regenerating adult lung. When cell cycle parameters were measured in vitro immediately after type II cell isolation, Ts increased 2-fold and TG2/M rose up to 10-fold above the value obtained in vivo. After 2 d of primary culture under customary plating conditions, Ts had returned to the same value as that in vivo, but TG2/M remained elevated. Little variability was observed in the duration of S-phase within each treatment group. In contrast, type II cells exhibited considerable heterogeneity in the rate of G2/M-phase traverse, especially in vitro. These data suggest that the inability of adult rat type II pneumocytes to proliferate in primary culture is related to delayed G2/M-phase transit and imply the existence of pneumocyte subpopulations which differ in susceptibility to growth arrest.