Schwann cell growth remains linear for 9 days but increases with increasing concentrations of serum. In (a) the cells were cultured in 1% FCS, forskolin, and aphidicolin, while in (b) they were cultured in forskolin and aphidicolin and various concentrations of FCS. Cell volume was measured in a Coulter Counter at the time points indicated. Each point represents the mean ± standard deviation of at least three cultures. The experiments were performed at least three times with similar results. (c) The cells were cultured as in (a), but each point represents the mean ± standard deviation of cell volumes from one plate of cells.