Fig. 1.

General structure of the primary cilium. The mother centriole contains nine triplet microtubules (MT) and is anchored to the periciliary membrane by transition fibers. Nine doublet microtubules protrude from the triplet microtubules and form the axoneme. The proximal end of the cilium is termed the transition zone and is the site for gated entry into the ciliary compartment. Here, Y-shaped structures (the Y-links) connect the doublet microtubules to the ciliary membrane. At the distal end of the cilium in some cells and species, the doublet microtubules convert to singlet microtubules