Growth arrest, confluency, and ciliogenesis. (A) RPE1 cells were plated on polystyrene-coated glass coverslips at various densities and serum starved. At different time points, cells were fixed and labeled for acetylated tubulin (green) to reveal primary cilia and DNA (blue). The graph shows the quantification of the proportion of ciliated cells in these conditions (the number of quantified cells is given for every time point: 150 cells/mm², n = 20, 50, 58, and 34; 400 cells/mm², n = 118, 183, 101, and 171; and 1,000 cells/mm², n = 174, 312, 108, and 166). Bar, 10 µm. (B) Working model: cells can adopt two distinct states in G0 with or without growing a primary cilium.