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 thr ough bivariate flow cytometric analysis of DNA content vs. incorporate d 5-bromo-2-deoxyuridine (BrdUrd). The cell cycle phase durations T-s (7.8 h) and T-G2/M (1.1 h), measured in vivo, agreed well with earlier estimates obtained by thymidine labeling. Left unilateral pneumonecto my increased the BrdUrd labeling index of type II cells in the remaini ng lung from an initial value of 1.9 +/- 0.3% to 4.8 +/- 1.0%, but had no effect on T-s or T-G2/M in vivo. In both normal and pneumonectomiz ed animals, BrdUrd-positive cells in vivo rapidly completed mitosis bu t did not enter a second S-phase. These results demonstrate that proli ferating type II cells do not form a continuously cycling population i n the normal or regenerating adult lung. When cell cycle parameters we re measured in vitro immediately after type II cell isolation, T-s inc reased a-fold and T-G2/M rose up to 10-fold above the value obtained i n vivo. After 2 d of primary culture under customary plating condition s, T-s had returned to the same value as that in vivo, but T-GB/M rema ined elevated. Little variability was observed in the duration of S-ph ase within each treatment group. In contrast, type II cells exhibited considerable heterogeneity in the rate of G2/M-phase traverse, especia lly 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 subpopulatio ns which differ in susceptibility to growth arrest. (C) 1994 Wiley-Lis s, Inc.