Figure 1. Linkage of the centrosome–cilium cycle to the cell cycle.

Primary cilia assemble specifically when cells exit the cell cycle and become quiescent or differentiate. Cells are also competent to form cilia in G1. Phases of the cell cycle are indicated, and blue and yellow arrows indicate cilium assembly and disassembly, respectively. Only the mother centriole (light blue) can initiate ciliogenesis. The daughter centrioles are shown in dark blue. During the process of ciliogenesis, an axoneme is assembled. This microtubular structure (indicated with parallel green rods) is disassembled as cells progress towards S phase, concomitant with remodelling of the distal end of the basal body (aqua ring). During S phase, centrosomes commence duplication, at which point cilia have largely disassembled. After mitosis, centrosomes are again competent to assemble primary cilia, either in G0 or in early G1 phase.