NeuroFIG 30: Structure of the cephalic and outer labial sensilla.
(Left) Schematic view of longitudinal section through the CEP and OL sensilla with additional transverse schematics shown to the right. Each of the four cephalic and six outer labial sensilla are similar in their morphologies, with one ciliary neuron ending surrounded by one sheath cell and one socket cell. In each sensillum, the cilium passes through a narrow channel and terminates embedded within the cuticle. OLL neurons are shorter and smaller and end about 0.5 μm more posteriorly than OLQ cilia. In addition, it has no ciliary rootlet (Ward et al., 1975). CEP, OLQ, and OLL cilia are also connected to the cuticle by a small nubbin. The nubbin is located at the base of the distal region in CEP and OLQ cilia. In contrast, the nubbin is at the tip of the OLL cilia and distal to the supernumerary microtubules and the tubule-associated material (TAM) (inset, bottom left). In males, the cilium of a second neuron, CEM, extends through the CEP channel and cuticle to the outside. There are adherens junctions between socket cell and sheath cell, neuron and sheath cell, and hypodermis and socket cell. CEP and OLQ sensilla are positioned next to each other within the two ventral and two dorsal labia. (Inset, top left) Longitudinal TEM section through CEP and OLQ sensilla. Visible are the dark TAM within the CEP cilium as well as a nubbin anchoring the base of the distal segment of CEP cilium to the cuticle. (Image source: Hall archive.) (Right) Transverse TEMs through CEPVL and OLQVL sensilla at three different levels, from distal tip (top panel) to more proximal (bottom panel). (Top panel) Termini of cilia within the cuticle. (Middle panel) Socket cells form the distal segment of the sensillar channels. (aj) Adherens junctions. (Bottom panel) Proximal part of the channel is created by the sheath cell. Both the CEP and OL sheath cells have membraneous lamellae stacked within the cytoplasm near the sheath channels (OLQshVL lamellae are not seen in this section). Individual lamellae connect to the channel lumen. A striated rootlet (SR) is visible in the OLQVL cilium. Bar, 1 μm. (Image source: E. Hartwieg and H.R. Horvitz 1339/96 [top], 1339/96 [middle], and 1345/96 [bottom].)
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