Senescent cells fail to induce primary cilium elongation. (a and b) Wild-type and IFT88^ORPK chondrocytes (which are deficient in primary cilia; Wann et al., 2012) were grown until confluent and subjected to serum starvation overnight. Cells were either lysed and subjected to Western blotting (a) or fixed and immunostained with antibodies against acetylated tubulin and phospho-S6 (b). (c–f) Confluent control and senescent (20 Gy irradiation; Sen (IR)) fibroblasts were serum-starved overnight (–FCS), fixed, and immunostained for acetylated tubulin and phospho-S6 (c). Percentage of cells with primary cilium (d), percentage of cells with elongated primary cilium (e), and mean cilium length (f) were quantified. Error bars represent SEM; all experiments, n = 3 (for immunofluorescence, at least five fields of view imaged per experimental repeat). Student’s t test performed between groups: **, P < 0.01; ***, P < 0.001; NS, not significant. Bars: (main) 10 µm; (insets) 5 µm.