Figure 1. Structural basis of canonical ciliary subcompartments. (A) Left; representation of a canonical C. elegans amphid channel sensory neuronal cilium. Ciliary microtubules (MT) extend from a fully degenerated basal body at the ciliary base, with microtubules flaring proximally into the periciliary membrane compartment (PCMC). The PCMC is a swelling of the distal dendrite tip, bounded at its proximal side by a belt-like adherens junction between the sensory neuron and the enveloping sheath support cell (not shown). An ~0.8 μm long ciliary transition zone emerges from the ciliary base, consisting of nine closely tethered doublet microtubules (via an internal apical ring (AR); see cross section), with each doublet connected to the ciliary membrane via Y-links. Also present are inner singlet microtubules (gray) extending to varying degrees along the axoneme. The TZ is followed by an ~4 μm long middle segment consisting of 9 doublet microtubules, after which the B-tubule of each doublet terminates to establish the nine singlet A-tubule arrangement of the ~3 μm long distal segment. Right; representation of a canonical human primary cilium, showing the basal body (and associated distal (DA) and subdistal (sDA) appendages), transition zone and main axonemal compartments. Example also shows the emergence of the cilium from a ciliary pocket, which is an invagination of the periciliary membrane observed for some ciliary subtypes. (B) Schematic of the amphid sensillum—the largest sensory organ in C. elegans—in the nematode nose region. Ten ciliary axonemes occupy the environmentally exposed channel (only 3 shown for illustration purposes), created by ciliary axonemes punching through the surrounding sheath (glial) cell. Belt-like adherens junctions (AJ) at the base of each periciliary membrane compartment (PCMC) seal the channel.