Schwann cells adjust their size slowly when shifted from serum-free (SF) medium to serum-containing (SC) medium. The cells were plated at 100,000 cells per well and were passaged when they reached a density of about 300,000 cells per well. (a,b) The mean volume of cells proliferating in either SC or SF medium was measured in a Coulter Counter at the time of passage. The raw data for each condition are shown in (a), and the mean ± standard deviation of the mode cell volume at passage is shown in (b). (c,d) The cell-cycle time of Schwann cells proliferating either in SC medium or in SC medium after a shift from SF medium was measured by determining the rate at which cell number increased. The raw data for each condition are shown in (c), and the mean ± standard deviation of four population-doubling times is shown in (d). (e) The size of cells proliferating in SC medium, in SF medium, or in SC medium after a shift from SF medium ('switched' cells) was measured every day in a Coulter Counter. Because the cells in SC medium and the switched cells had similar cycle times see (d) they were passaged about every 3 days in both cases, when they reached around 300,000 cells per well; the cells in SF medium cycled more slowly and were thus passaged less often. These experiments were performed three times with similar results.