Figure 1.

Routes of primary ciliogenesis. (A) The intracellular route. Ciliogenesis begins with the formation of a large ciliary vesicle at the distal end of the appendages of the mother centriole by fusion of smaller vesicles. The axoneme starts forming intracellularly and, as it grows, deforms the ciliary vesicle and establishes an inner membrane (shaft) and an outer membrane (sheath). The incipient cilium is finally exocytosed and the cilium becomes exposed in the plasma membrane. The sheath gives rise to the ciliary pocket, and the shaft forms the ciliary membrane. (B) Model of the alternative route. (a) In polarized epithelial cells, the intercellular bridge containing ciliary proteins forms at the apical cell surface during cytokinesis. (b) When abscission occurs, the MBR is inherited by one of the daughter cells, acquires a RAMP and localizes at the periphery of the apical surface. (c) The MBR/RAMP moves over the apical surface toward the centrosome, which is docked at the center of the apical membrane. (d) When the MBR is proximal to the centrosome, the RAMP splits into two patches, one of which remains at the MBR and the other, known as the CAMP, occupies the centrosome zone. (e) The ciliary membrane stems from the CAMP. The entire process of primary cilium formation takes place in the plasma membrane. The events occurring at the apical surface in (a–e) are shown in more detail in the corresponding enlargements in (a'-e').