The amphids are a pair of laterally located sensilla in the head which are open to outside at the base of the lips (See WA editors' note below)(See a 3-D reconstruction of amphid and labial nerves by R. Newbury & Moerman lab. Cell labels are shown in GliaFIG12B-D. 3-Dmovie was created from confocal images of a strain expressing the GFP marker linked to the promoter for Y105E8A.5 using Zeiss LSM 5 Pascal software v. 3.2). They are the principal chemosensory organs of nematodes. Each amphid is made up of 12 sensory neurons (ADF, ADL, AFD, ASE, ASG, ASH, ASI, ASJ, ASK, AWA, AWB, AWC) with ciliated dendrites as well as one sheath and one socket cell in C. elegans. The sensory dendrites of 11 neurons, except that of AFD, completely penetrate the large tubular ending created by the sheath cell through 11 membrane-lined holes in a sieve-like fashion. The cilia of 8 of these neurons, except those of AWA, AWB and AWC, extend into the doughnut-like pore created by the socket cell and are exposed to the external medium. The cilia of AWA, AWB and AWC in turn invaginate into the sheath cell. The dendrite of AFD is embedded within the sheath cell and terminates in a rudimentary cilium with many villi. The ending of the socket cell does not form a tube like the sheath cell but rather wraps around the distal portion of the cilia-filled channel and seals onto itself with an adherens junction. The sheath cell is connected to the amphid sensory cilia and the socket cell by adherens junctions and the socket cell, in turn, is connected to the hypodermis by adherens junctions creating a restricted pocket that is exposed to the outside.
WA editors' note: Reviewing EM sections from 3 different animals, we noted that amphid openings on the nose can be quite variable in terms of anteroposterior location. One animal has them located more anteriorly than the other and a third one shows difference between R and L sides. It must be emphasized that currently anterior Slidable Worm sections show 2 different animals which have amphid openings at different levels; the animal that #4 EM is selected from has amphid openings more posteriorly placed compared to the animal that #2 EM section was selected from. As a result, amphid socket cells are already terminated in #4 section and is, therefore, not labeled, whereas they are seen in #2 section.
For reasons of simplicity, out of 12 amphid neurons, only ADFL is depicted in this image.
AMPHID SOCKET CELLS
AMPHID SHEATH CELLS
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