NeuroFIG 24: Anatomy of amphid sensillum.

Shown are four representative examples of channel and wing cilia.
A. Structure of the amphid opening, longitudinally. Amphid channel (Ch) is lined by the cuticle in the distal (socket) part and an electron-dense lining supported by a scaffold of cytoskeletal filaments (Fs) in the anterior sheath part. The socket cell is connected to the hypodermis and the sheath cell by adherens junctions (aj). Adherens junctions are also seen between the dendrites and sheath cell (neuron-sheath junction) proximally to the level where the dendrites enter the channel. A large Golgi apparatus located at the base of the sheath-cell process (left) gives rise to matrix-filled vesicles bound toward the channel. Mitochondria (not shown) are also present in this region. (Modified, with permission, from Perkins et al., 1986. ©Elsevier.)
B. Illustration of invagination of the sheath-cell cytoplasm by amphid dendrites (long gray arrows), longitudinally. The process of the AFD neuron stays embedded within the sheath cell, whereas all others enter the matrix-filled amphid channel. The cilia of the wing cells penetrate back into the sheath cell, whereas the eight channel cilia are exposed to the outside through the cuticle pore.
C. Illustration of longitudinal microtubule (MT) structure of amphid cilia. Each cilium is about 7.5 μm long in adult C. elegans and is composed of three segments: the proximal, middle, and distal segment. MTs project distally from the proximal segment (the transition zone). The nine doublet(d) MTs and inner singlets (s) continue through the middle zone. The distal zone contains some A fibers (not shown) and singlet MTs. The fan-like AWC cilium is an exception because the A fibers do not extend distally, and both the singlet and doublet MTs terminate at approximately the same level (Evans et al., 2006).
D. TEM of amphid channel cilia. Transverse section through middle segments of cilia. The AWB and AWC cilia interrupt the electron-dense lining of the channel and the filamentous scaffold as they invaginate back into the sheath cell (Amsh). The doublet and singlet MTs in AWB and AWC fan out into these flattened processes. AWA cilium branches into several small fingers, each of which contains one or more of the original nine doublet MTs. Numerous AFD fingers (also called villi) are seen within the sheath cell around the single AFD cilium. Bar, 0.5 μm. (Image source: N2nose [MRC] 3731-19.)

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