| Abstract: | Sixty to eighty per cent of the cells in a culture of human diploid fibroblasts may be stimulated from the state of density dependent inhibition of replication to active DNA synthesis and division. The maximum response is effected by 50% serum within the pH range 7.2–8.0. The proportion of cells responding depends on the concentration of serum protein in the medium which may be effectively substituted by crystalling serum albumin. There is a differential sensitivity to the stimulus of cells in the densely packed centers of whorls and in the less dense areas between the whorls. The cell response is parasynchronous and the median durations of the various phases of the cell cycle are: G1I 6 β ?æ® ¿ ∞ 8 hours, G2 = 6 hours and doubling time = 30 hours. The stimulatory effect of fresh medium is lost during contact with dense cultures so that it has only 50% of its initial capacity after 14 hours. It can be restored by dialysis against serum-free medium. The stimulus must be applied for at least ten hours to be effective in inducing DNA synthesis. During the latter half of ten hour induction period subsequent DNA synthesis becomes exquisitely sensitive to actinomycin D. After this time an increasing number of cells become irreversibly committed to replicate. The data are interpreted to indicate that during contact with serum proteins (including albumin) changes in the cell surface, if continued long enough, trigger a mechanism which involves the synthesis of a unique RNA species during the fifth to tenth hours. After this RNA has been synthesized the cells are then committed to DNA synthesis. |
