The postnatal age of rat lung fibroblasts influences G1/S phase transition in vitro

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Abstract
In the neonatal rat lung, alveolar development occurs from postnatal Days 4–13, during which time there is a fourfold increase in interstitial fibroblasts. Factors influencing emergence of new septa and cell proliferation associated with septal elongation have yet to be identified, in part because of difficulties inherent in studying this process in vivo. Using flow cytometric analysis of the DNA content of freshly isolated lung fibroblasts, we found that proliferation, as indicated by the percentage of cells in S plus G2/M phases, peaked on postnatal Day 4 (PP3H-thymidine at a faster rate than did fibroblasts obtained from pups at other postnatal ages (PP=0.0001). Cells from 5-, 9-, and 13-d-old pups reentered G0/G1 by 21 h after release from serum starvation, in contrast to fibroblasts from 2- and 23-d-old rats which did not. Throughout the 15-h period after release from serum starvation, levels of cyclin E, which peaks at the G1/S border, were highest in the 5-d-old cells (PM hydroxyurea which inhibits DNA synthesis completely abolished age-related differences in cell cycle transit time, implying that age-dependent differences in lung fibroblast proliferation rates are the result of events occurring before S-phase entry.