Fig. 1. Cell cycle control by growth factors and adhesion in N2A and CHO cells entering G1 phase from mitosis. (A) Experimental set-up. Mitotic cells were collected from randomly proliferating cell cultures by shake-off and released in the presence or absence of inhibitors as indicated. Cells were harvested at several time points after synchronization to assess cell cycle progression. Phase-contrast images depict a randomly cycling culture of CHO cells (R), a synchronous population obtained by mitotic shake-off (M) and a population 2 hours after synchronization (G1). Bar, 20 µm. (B) Kinetics of cell cycle progression in adherent N2A and CHO populations traversing G1 phase from mitosis in the presence of growth factors. Upper panel, representations of expression patterns of G1-S cyclins over time. Lower panel, kinetics of S-phase entry as measured by thymidine incorporation. Representative experiments are shown; data are expressed as means ±s.e.m. (n=3). (C) Summary of the main cell cycle controls in M-G1-S phase regulation by growth factors and the ECM in N2A and CHO cells synchronized by mitotic shake-off. Adherent cells in the presence of growth factors progress into S phase, whereas post-mitotic growth factor depletion or incubation in suspension induce an arrest in G1. Adh, adherent; gf, growth factors; P-Rb, phosphorylated retinoblastoma protein; sus, suspension.