Abstract
Chick embryo cells which have been kept overnight at pH 6.8 in the absence of serum multiply very slowly. Only a small fraction of cells is in the S period at any given time, and the rate of uptake of 2‐deoxy‐D‐glucose is very low. Upon raising the pH to 7.4 and adding serum (“turn‐on”) the uptake of 2‐deoxy‐D‐glucose increases immediately; the rate of DNA synthesis increases after a lag of about 4 hours, and represents an increase in the fraction of cells synthesizing DNA. The uptake of 2‐deoxy‐D‐glucose is rapidly returned to its original low rate at any time by again lowering the pH and removing serum (“turn‐off”). The synthesis of DNA in the culture remains constant or continues to rise at a markedly reduced rate following the same treatment. Lowering pH or removing serum independently of each other is less efficient at inhibiting the increase in DNA synthesis than the combined treatment but each accomplishes a similar result. Cultures which have been “turnedoff” during the early stages of the rapid increase in DNA synthesis, resume their prior rate of increase immediately if “turned‐on” again within 2.5 hours. If the cultures have been “turned‐off” for 5.5 hours before restoring the “turn‐on,” there is a 2 hour delay before they resume an increased rate of DNA synthesis. The results indicate that chick embryo cells do not become committed to the initiation of DNA synthesis until shortly before, or at the time of the onset of the S period.
Up to 96% of the cells in post‐confluent cultures growing in conventional medium become labeled upon continuous, prolonged exposure to ^3^H‐thymidine. Seventy‐eight percent of the cells in serum‐deprived cultures growing at a very low rate become labeled. These and other considerations suggest that the inhibition of cell multiplication by high population density or serum deprivation is caused by a lengthening of the time cells remain in the prereplicative G~1~ period rather than by shifting cells into a qualitatively distinct G~0~ period. There may, however, be a period common to all cells regardless of growth rate, in which cells are not progressing toward the S period. The length of this variable period would then determine the growth rate of a population of cells.