A Characterization of the Boundary Between the Cycling and Resting States in Ascites Tumor Cells

Growth deceleration of an Ehrlich ascites tumor with increasing mass is associated with a prolongation of the cell cycle and a decline in the growth fraction. These effects are reversed upon transfer of cells from an older tumor into a new host. Studies were made to locate the stages at which a cell cycle could be suspended or resumed. Transplantation caused a prompt rise in both mitotic and flash H 3 TdR labeling indices. When all the cells in cycle including mitoses were prelabeled with H 3 TdR in older tumors, the fraction of labeled mitoses did not decline for a considerable period after transplantation into new hosts. This suggests that the early rise in mitoses is not due to a flow of resting (G o ) cells from a G 2 store (G 2 ‐G o transition). It appears rather to be a reflection of a lag of the mitotic process relative to other stages during the initial readjustment of the cycle. A prompt rise in flash H 3 TdR indices in the transplants suggested cell entry into S from either a suspended G I (G 1 ‐G o transition) or a suspended S (S‐G o transition). These possibilities were examined by relating micro‐spectrophotometric estimates of DNA to the cell cycle stage as revealed by H 3 TdR autoradiography. Since G o cells had DNA values corresponding to G I , it was concluded that decycling or recycling could occur only after mitosis and before DNA synthesis.