Figure 1.

Schematic representation of the primary cilium structure. The primary cilium protrudes from the apical surface of cells, with a central shaft composed of nine microtubule doublets (axoneme) that extend from the basal body, a modified centrosomal mother centriole. The transition zone (TZ) is characterised by Y-shaped links connecting the axonemal microtubules to the ciliary membrane contributing to compartmentalisation of the organelle. Proteins and other cargos are transported from the basal body to the tip of axoneme by anterograde intraflagellar transport (IFT) through IFT-B complex and kinesin-motor protein; whereas dynein-motor protein associated with IFT-A complex contributes to retrograde IFT (from the tip to the basal body). The basal body associates with the BBSome, a heptameric complex involved in ciliogenesis and ciliary trafficking. The ciliary membrane contains specialised lipids, proteins and receptors (e.g., Patched 1 (PTCH1) receptor and the associated Smoothened (SMO) co-receptor, that bind different hedgehog, HH, ligands), through which the cilium coordinates different pathways (e.g., Hedgehog signalling). See text for further details.