Summary 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 G
2/M phases, peaked on postnatal Day 4 (
P<0.04). By Days 9–10 the proliferation rate was lower than on Days 3, 4, 5, or 6 (
P<0.005). We then evaluated rates of in vitro proliferation as a function of postnatal age in first passage fibroblasts and
found that the proliferative phenotype expressed in vivo persists in vitro. Fibroblasts from 4–5-d-old pups increased in number
and incorporated
3H-thymidine at a faster rate than did fibroblasts obtained from pups at other postnatal ages (
P<0.0001). Age-dependent differences in cell cycle transit time were compared in fibroblasts synchronized by serum starvation
and analyzed by flow cytometry at 2-h intervals from 13–21 h after release from serum starvation. A greater percentage of
cells from 5-d-old pups entered S phase during this period than was seen for cells obtained from 2-, 9-, 13-, or 23-d-old
rat pups (
P=0.0001). Cells from 5-, 9-, and 13-d-old pups reentered G
0/G
1 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 G
1/S border, were highest in the 5-d-old cells (
P<0.025). Synchronization with 2.5 m
M 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.
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