Abstract
DNA synthesis and cell division are markedly reduced in confluent monoβlayers of WIβ38 diploid fibroblasts, but resume again if the depleted medium is replaced by fresh medium containing 10% fetal calf serum. If the cells are kept quiescent for prolonged periods of time after confluence (1 or 2 weeks), the fraction of cells that can be stimulated to proliferate by fresh serum decreases and the length of the prereplicative phase increases. The template activity of isolated nuclei decreases with increasing time of quiescence, and parallel changes occur in chromatin as evidenced by circular dichroism spectra and capacity to bind the intercalating dye, ethidium bromide.
When WIβ38 cells are stimulated to proliferate after prolonged quiescence, the increase in template activity of nuclei is delayed by several hours in comparison to cells stimulated after short periods of quiescence. Two distinct steps, both requiring serum, can be identified in the prereplicative phase of cells stimulated to proliferate after prolonged quiescence.
We interpret the results as indicating that, during prolonged quiescence, WIβ38 fibroblasts go into a deeper G~0~ state from which they can be rescued only after prolonged stimulation. In this respect, prolonged quiescence may bear some resemblance to the process of aging.