Microphotometric and autoradiographic studies on the cell cycle and cell size during growth and decline of Chinese hamster cell cultures

Abstract

Summary A combination of (1) microphotometric determinations of the DNA and protein content and the dry mass of single cells and (2) autoradiography was used to follow the changes in cell cycle parameters and cell growth in cultures of Chinese hamster cells as they passed from the initial stages of growth, through the exponential phase, into decline. Cell counts and total protein determinations were used to follow the overall culture growth. In the lag phase and early part of the exponential growth phase, cell size and the relative duration of the combined periods S through M increased. In the latter half of the exponential phase, cell size at mitosis and the duration of S through M decreased, while the length of the cell cycle and the size of the cells at the initiation of DNA synthesis remained essentially constant. As the cultures went into the decline phase resulting from cell death, the cell cycle increased greatly, primarily because of an increase in G1. At thesame time, cell size at mitosis and at the initiation of DNA synthesis decreased, because the cells continued to go through the cycle with very little net increase in protein content. There was evidence that at least 40% of the cells remained in the cycle well into the decline period, and no evidence that any cells left thecycle. A description of the events can be given in terms of the rates of three processes: protein synthesis, initiation of DNA synthesis, and initiation of mitosis. There are indications that the rate of initiation of mitosis is less closely associated with the rate of protein synthesis than is the rate of initiation of DNA synthesis.