Vesicular transport to the primary cilium: basic principles
The primary cilium is composed of a microtubule core enclosed within a membrane sheath, the ciliary membrane, that is continuous with the plasma membrane. Despite this continuity, ciliary and plasma membranes contain distinct complements of lipids and proteins, thus making the cilium a bona fide compartment exposed to the extracellular milieu. Consequently, the cilium has been compared to a "cellular antenna" that capture and transduces developmental signal to the inside of the cell.