Subdivision of the G1 phase of human glia cells in culture

Human glia cells blocked post-mitotically by serum deprivation require about 8–12 h of continuous stimulation by growth factors to become committed to DNA synthesis. DNA synthesis begins about 5 h after growth factor withdrawal. The length of time until the S phase began and the length of the apparent commitment period, i.e. the time when cells progressed towards the G1/S transition point even in the absence of growth factors were independent of the nature of the growth factors studied (calf serum, platelet-rich human serum, epidermal growth factor). Epidermal growth factor and calf serum were mutually interchangeable during the pre-commitment period. Increasing cell density reduced the number of cells which entered DNA synthesis, but had no effect on the length of the apparent commitment period or the latent time until DNA synthesis commenced. The requirement for a long exposure to a growth factor may be an important safeguard in normal cells against “accidental” entry into the cell cycle, since malignant glia cells do not show the same requirement.