NeuroFIG 31: CEP, CEM, and OL cilia ultrastructure, all transverse sections.

A. Section through the DR lip of an adult male. The distal segments of CEP and OL cilia contain supernumerary microtubules and an amorphous dark material associated with them (TAM) that is common to many mechanocilia (Perkins et al., 1986). The CEP cilium widens as it enters the cuticle, and this distal segment is filled with numerous MTs organized around electron-dense TAM cores. In OLL cilium, TAM forms a large central aggregate around which a single layer of many MTs is seen (inset). In both types of cilia, the outermost MTs are connected to the membrane through fine attachments. The OLQ cilium contains less robust TAM at the periphery. In all three types of cilia, the distal MTs are all supernumerary singlets. They do not derive from the nine doublets nor the inner singlets of the apical ring of the transition zone, and they only exist in the distal segments. The CEM cilium is exposed to the outside through a small bump and hole in the cuticle at this level.
B. Section slightly more posterior to A from the same animal. CEM and CEP cilia lie within the same channel. The chemosensory CEM cilium is ultrastructurally different from the mechanosensory CEP; it is narrow and has no TAM or supernumerary MTs, but contains 12–13 singlet MTs.
C. Section from a similar level as in B from a hermaphrodite animal. There is no CEM cilium. The A and B subfibers of the MT doublets of the OLQ cilia have filled centers and appear dark. Four of the original nine doublets continue to the distal end and are connected to one another by side bridges to form a square and to the center by fine radial arms (inset).
D. Section through the nubbin (N) of the CEP cilium. The OLQ nubbin is more anterior to this section (not shown). (Inset) An OLL cilium from approximately the same level of section. At this level no TAM is left and only the singlet MTs are seen in the short cilium.
E. Section through the transition zone of the OLQ cilium. It contains nine doublet MTs that are connected to the membrane through Y-shaped attachments. Only four of the doublets continue in the middle and distal segments. (M) Matrix.
F. Section through the transition zone of the CEP cilium. Only five of these doublets continue to the middle segments. A striated rootlet (SR) is seen in the OLQ dendrite. (M) Matrix.
G. Section through the transitional fibers of the CEP cilium.
H. Section proximal to the transition zone of the CEP cilium. SR continues in the OLQ dendrite, and adherens junctions are seen between the dendrite and OLQsh. The CEP dendrite contains a few vesicles and several intermediate filaments. Lamellae (arrowheads) are seen within both sheath cells. (aj) Adherens junction. Bar, 0.5 μm; applies to all panels. (Image sources: [Hall] 493-2/9103 [A] and 9105 [B]; [MRC] N2nose 3730-16 to 3731-17 [C–H].)

Click on picture for full resolution image.