The re-entry of long term density growth inhibited cells into the cell cycle

The pre-replective period of newly subcultivated human glial cells, following periods of G1 block as a result of density-dependent growth inhibition (DDGl), for 1 week, 3 months and 1 year were studied and compared with sister cultures blocked in G1 by serum starvation. The effects of the trypsination procedure on this prereplective period was also documented. Sister cultures of the 3 month growth inhibited cultures have previously been found to have 30% of their cytoplasm filled with residual bodies. Trypsination caused an almost 10 hour delay in the labeling-index-peak (LIP) of the serum starved, G1-blocked cells. The LIP of the 1 week growth inhibited cultures (DDGL), showed no significant delay when compared with that of the serum-starved and trypsinised cultures. The LIP of the 3 month growth-inhibited cultures occurred only slightly delayed and that of the 1 year growth inhibited cells (DDGL) showed no further delay. Thus the period of growth inhibition did not significantly delay the timing of the LIP while it did decrease the proportion of the cells which entered the S-phase (LIP tops of 30%, 22% and 6% for 1 week, 3 months and 1 year respectively). Daughter cultures did not enter phase III any earlier than controls.