Initiation of DNA synthesis in Ehrlich ascites tumor cells in their plateau phase of growth.

The E2 hypotetraploid Ehrlich ascites tumor reached its peak size at 1 X IO9 cells on the 14th day after i.p. inoculation of 0.30 X 10* cells. During this period the % nonviable Ehrlich ascites tumor cells did not change. Autoradiographic studies during the first 14 days of Ehrlich ascites tumor growth dem onstrated no decrease in the % tumor cells incorporating tritiated uridine, proline, phenylalanine, or leucine. The % Ehrlich ascites tumor cells in DNA synthesis (thymidine-3H incorporation), however, decreased with increasing tumor age. Aspiration of most of the tumorous ascitic fluid from a 14day-old Ehrlich ascites tumor host resulted in recurrent growth of the remaining Ehrlich ascites tumor. Recurrent growth of the Ehrlich ascites tumor was characterized by a significant increase in the thymidine index (% Ehrlich ascites tumor cells incorporating thymdine-3H in 20 min). The peak thymidine index of recurrent growth, 51.8%, occurred between the 14th and 24th (19th) hr after aspiration of most of the 14-day-old tumorous ascitic fluid. During recurrent growth of the Ehrlich ascites tumor a lag period of at least 4 hr occurred between the time of ascites removal and the increase in the thymidine index. Thymidine index determinations made after transplan tation of saline-washed 14-day-old Ehrlich ascites tumor cells to new hosts also demonstrated (a) a 4 hr lag period before the thymidine index began to increase and (6) a peak thymi dine index, 56.1%, 19 hr after inoculation. The data are dis cussed in relation to the events in the cell cycle.