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It was shown by Hayflick and Moorhead (2) that human embryonic lung fibroblasts, after a period of rapid proliferation (phase II) in vitro, enter a period of slow growth (phase III) culminating in death of the population. We have previously described some changes in the division cycle that take place in phase III cultures (4, 5) . These changes were manifested by a decreased number of cells entering deoxyribonucleic acid (DNA) synthesis during a 24 hr period, a decreased number of cells dividing between subcultivation and resting stage, and a longer generation time mainly due to a prolonged G2 period . Cell lines originating from human adult fibroblasts also go through an initial period of more rapid proliferation followed by a period of slower growth. The cell-doubling potential, however, is decreased in adult human cells compared with cells of embryonic origin (1) . We have now found that a line of human adult fibroblasts in phase II shows the same changes in the division cycle as those that take place in late-passage embryonic cells.

ANALOGY IN GROWTH BETWEEN LATE PASSAGE HUMAN EMBRYONIC AND EARLY PASSAGE HUMAN ADULT FIBROBLASTS