NeuroFIG 34: Ultrastructure of an inner labial sensillum.

A. Schematic of longitudinal section through an inner labial sensillum with six transverse views; anterior is at the top. The six inner labial sensilla have identical structures except for the endings of the FLP neurons that are seen associated only with the lateral labial sensilla; the BAG neuron endings terminate close to the ventral labial sensilla. Each sensillum contains one sheath cell, one socket cell, one IL1 cilium, and one IL2 cilium. The IL2 cilium protrudes outside through a hole in the cuticle and is suggested to be chemosensory. The IL1 cilium terminates in the cuticle approximately 0.5 μm below this hole and is mechanosensory. The IL1dendrite contains a striated rootlet. There are adherens junctions (aj) between the socket cell and hypodermis, sheath and socket cell, and neuron and sheath cell. An ECM material is seen within the cuticle and socket channel. A specialized subcuticle surrounds the ending and protrudes from the lip cuticle.
B. TEM, transverse section. This section is through the disc of dark material attached to the IL1 ciliary membrane at its tip. This disc is suggested to help sense the deflections of the inner labial papilla when it is compressed by contact with external objects (Ward et al., 1975; Perkins et al., 1986). Some of the MTs that originate from the transition zone reach and fuse with the disc. The IL2 transition zone, which is not as well defined as the IL1 transition zone, is about 2.5 μm from its tip (not shown). Five to seven doublet MTs originate from this zone. These doublets become singlets as they extend into the distal region. (Image source: [Hall] 493-2B/9118.)
C. Transverse section through the transition zone of IL1 cilium. Nine MT doublets are seen in a ring structure with the tip of the striated rootlet (SR) becoming visible in the center. (Image source: [MRC] N2nose 3730-16.)
D. Transverse section through the transitional fibers (arrowheads) in IL1 cilium. IL2 cilium is in the same channel surrounded by the sheath cell with matrix-filled, large vesicles (V). (Image source: [MRC] N2nose 3730-18.)
E. Transverse section through IL2-sheath junction (arrows). IL1 dendrite contains an SR. (Image source: [MRC] N2nose 3730-20.)
F. Transverse section through the transition zone of BAGR cilium and transitional fibers of FLPR cilium. (Ph) Pharynx lumen. Bar, 0.5 μm. (Image source: [MRC] N2nose 3731-4.)

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