Stimulation by serum of multiplication of stationary chicken cells

Abstract

Abstract When serum was added to cultures of stationary chicken fibroblasts, there was an increase in the percentage of cells synthesizing DNA after a delay of about five hours. If the serum was removed before DNA synthesis began, some cells were still found to start DNA synthesis and to enter mitosis. The delay between the time of addition of serum and the start of DNA synthesis was not affected by the type of serum, the concentration of serum, nor the means of preparing the stationary cells. However, all of these factors affected the proportion of cells stimulated by serum. If two pulses of serum were given immediately following each other, their effects were synergistic. If there were twelve hours between them, there was no effect of the first pulse. These results were summarized in a new model of the cell cycle which subdivided the G1 phase into Gla, b, and c. The effects of serum were seen in a balanced salt solution, and were sensitive to several metabolic inhibitors.