Besides glial cells, the cells in C. elegans that are located
between the junction of hypodermal/epithelial and other tissues are:
a- Arcade cells of the nose
b- Excretory duct: Two interfacial epithelial cells, the excretory duct cell and the excretory pore cell, connect the excretory system to the ventral opening at the excretory pore. Both the excretory duct cell and the excretory pore cell show typical hypodermal specializations while forming an extensive narrow channel. For a full description, see Excretory system.
c- Pharyngeal epithelium: Two rings (ant and post) of the pharyngeal epithelium are made of three cell groups; e1, e2 and e3. For a full description, see Alimentary system Part I
d- Marginal cells of pharynx: See Alimentary system Part I
e- Rectal epithelium: See Alimentary system Part III
f- Vulval epithelium: See Reproductive system
g- Cloacal epithelium (in male): See Male Reproductive system.
The nine arcade cells (see ArcFIG1-3)
were first carefully described in C. elegans by Wright
and Thomson (1981):
1. Anterior Arcade cells
arc ant DL
arc ant DR
arc ant V
2. Posterior Arcade cells. See a 3-D reconstruction of posterior arcade cells by R. Viverios & Moerman lab. Cell labels are shown in ArcFIG3b (3-D movie was created from confocal images of a strain expressing the GFP marker linked to the promoter for C08C3.2 using Zeiss LSM 5 Pascal software v. 3.2).
arc post D
arc post DL
arc post DR
arc post V
arc post VL
arc post VR
These interfacial epithelial cells must firmly hold the outer body wall and the lips to the inner cylinder of the pharynx in a manner that keeps these organs from breaking apart, while still giving each organ freedom of movement during feeding. The anterior hypodermis consists of three specialized rings of cells, (hyp1, hyp2, hyp3, or cheilostom see Hypodermis), which underlie the lips and anterior edge of the bodywall, while the anterior end (mesostom) of the pharynx consists of an interfacial cell group known as the pharyngeal epithelium (ArcFIG9 and 10). Spanning the gap between hyp1 and the pharyngeal epithelium are the arcade cells (prostom), a highly specialized ring of syncytial interfacial epithelia that line the extreme inside edge of the buccal opening (ArcFIG4). While the arcade tissue bridges the gap between hypodermal tissue and pharyngeal epithelium, the anterior arcade cells also secrete a narrow ring of bridging cuticle that seals the lip cuticle to the buccal cuticle via a local, notched expansion joint" (ArcFIG9 and 10). Wright and Thomson (1981) refer to this notch as the cheilostom groove. C. elegans is categorized as an astomatous species based upon this distinction between buccal and body cuticles (Wright, 1976; Wright and Thomson, 1981). These specialized cuticular zones within the buccal passage have also been termed the rhabdia (Bird and Bird, 1991 after Goodey, 1963 and Jones 1965), and thus,cheilorhabdion, prorhabdion, and mesorhabdion refer to the cuticular coverings of the lips, arcade and buccal regions respectively. During molting, the lip cuticle and buccal cuticles are formed at separate times and are probably shed independently (Wright and Thomson, 1981). It appears that the arcade cuticle is shed with the buccal cuticle.
The nine arcade cells form two tight rings of syncytial tissue surrounding the buccal cavity, the anterior arcade and the posterior arcade (Wright and Thomson, 1981). The arcade cell bodies lie between the buccal opening and the anterior bulb of pharynx, and each cell body extends a separate process towards the more anterior, ring-like syncytial region (ArcFIG 1-3, 4-8). In some cases cell fusion occurs between these processes, before reaching the syncytial ring. Thus, the arcade cells adopt a general shape not unlike the socket or sheath cells. The anterior arcade ring consists of the fusion of processes from three cells extending three separate processes, while the posterior arcade ring is formed by the fusion of processes extended from six cells. The region between the arcade cells and the hypodermal layers often includes several large extracellular spaces which may reflect some looseness in tissue adherence here, perhaps allowing for maximum flexibility when pharyngeal pumping puts stress on the connection between pharynx and lips. Overall, the arcade function is primarily one of supplying tissue continuity and flexibility to a region of tissue under maximum stress as well as bridging the gap between two dissimilar organs, the pharynx and lips.The arcade cell bodies and their processes lie within the outer body cylinder, in close apposition to hypodermis, and facing a narrow gap that separates them from the pharynx.
The arcade cell cytoplasm generally appears quite clear compared to other epithelia (electron lucent), except for the portion closest to the buccal cavity, which often shows increased density (probably due to local actin networks). The arcade cells contain many free ribosomes and mitochondria, but very little ER. Just prior to the molts the arcade cells become filled with dense core vesicles that cluster near the cuticle (Wright and Thomson, 1981; Hall, unpublished). This suggests that the arcade cells build new cuticle by vesicle secretion. The two arcade rings are firmly sealed to each other and to neighboring tissues via adherens junctions (aj's). The aj's between the posterior arcade and the most anterior ring of pharyngeal epithelium are especially robust and form one continuous belt desmosome along the entire border (black arrow in ArcFIG10). The aj's linking the two rings of arcade cells are less robust and probably discontinuous. Similarly, the aj's linking the anterior arcade to the ring of hyp 1 syncytium are less robust. There is preliminary evidence that the same cell appositions also support some gap junctions (gj's). While not yet confirmed by examination at higher magnification, it appears that gj's link the two arcade rings to each other, and that gj's possibly link the arcades to hyp 1 and to the most anterior pharyngeal epithelial cells.
One of the most distinctive elements of the anterior arcade is the extremely thin cylinder of tissue at its anterior margin (marked "anterior arcade cylinder" in ArcFIG10). Here the arcade cell syncytium forms two distinct apical zones on opposite sides of the cylinder, each secreting a different cuticle, the prorhabdion cuticle on the inward face, and the bridging cuticle (part of the cheilorhabdion) on the outward face. In electron micrographs, this arcade tissue stains very densely, suggesting that the two apical faces are tightly bound by cytoskeletal elements, perhaps securing the two distinct cuticles together locally. Further posterior, the outward face of the arcade (facing away from the buccal lumen) creates a lateral membrane zone where it faces the posterior arcade, but at no place does the anterior arcade appear to form a true basal face or touch basal lamina. Instead, the remaining outward side (presumed lateral and basal pole) of the anterior arcade remains in close contact with hyp 1 tissue. The posterior arcade only briefly touches the bridging cuticle, and does form a normal apical face towards the prorhabdion cuticle. It also has lateral plasma membrane domains facing the anterior arcade and pharyngeal epithelium. The posterior arcade makes only a glancing contact with the heavy basal lamina of the pharyngeal epithelium. Neither arcade ring seems to make a basal lamina of its own, except where the arcade processes proceed towards their cell bodies, in which case their basal lamina is thin, matching that of the rest of the body hypodermis. Thus the entire syncytial region of the arcade principally consists of apical and lateral zones, and generally lacks any specializations typical of the basal pole.
Listed below are some epithelial cells in C. elegans that
are not interfacial and may have scaffolding, hypodermal or neuroendocrine functions:
a- XXX cells of the nose
b- Tail spike scaffold cells
XXXL/R act as head hypodermal cells embryonically (White J., 1988). In the embryo, they are located in symmetrical positions close to early hyp4 cells (See TailspFIG1below). Later in embryogenesis, the XXX cell bodies migrate posteriorly towards their final positions, close to anterior pharyngeal bulb and lose their hypodermal character ( XxxFIG1, XxxFIG6-8) . In larvae, these cells seem to be carrying out a neuroendocrine function (A. Antebi, pers. comm.). In the adult, each cell extends a short, thin, flattened, posterior process that faces the pseudocoleom outside the pharynx rather similar to the processes of GLR cells (See XxxFIG1, XxxFIG4-5). Each cell also extends a short, tubular anterior process which lies adjacent to the ventral labial process bundles (XxxFIG3, 7-9 ). XXX cell bodies are compact (neuron- or glial-like) and lie between the lateral hypodermis and pseudocoelom (XxxFIG2 and 4). These cells have no intercellular junctions and make no obvious synapses, but they remain in close contact with labial neurons and hypodermis. They have abundant ribosomes, some smooth ER and a few vesicles.
Postembryonically, XXX cells seem to play a key role in dauer formation by regulating the metabolism and/or function of a steroid hormone that is involved in the dauer pathway. Ablation of XXX cells in L1 larvae leads to 30% of animals to become dauer-like (Ohkura K. et al, 2003). XXX cells express daf-9 which encodes a cytochrome p450 of the CYP2 family (Gerisch B. et al, 2001; Jia K. et al, 2002). Members of this family function in steroid metabolism. XXX cells also express npc-1 which is a C. elegans homolog of the Niemann-Pick type C disease gene (Sym M. et al, 2000). This gene encodes for a protein which plays a role in intracellular sterol transport. SDF-9, a protein tyrosine phosphatase which is suggested to increase the activity or help the function of DAF-9 is also expressed in XXX cells. DAF-9 itself functions upstream of DAF-12 which mediates the choice of dauer stage versus reproductive growth and is structurally most similar to nuclear hormone receptors that bind sterol derivatives (Antebi A. et al, 2000).
There are two short-lived hypodermal cells, ABplppppppa and ABprppppppa,
in the tail tip of the early embryo that fuse to form a syncytium by the tadpole
stage embryo. Their cell bodies lie in the anal ridge, near hyp10 and hyp9 (TailspFIG1
and TailspFIG2). The syncytial cell is small,
has somewhat darker cytoplasm than nearby hypodermis,and extends a single posterior
process to the extreme tail tip. The tail spike cells undergo programmed cell
death during late embryogenesis, and leave behind a narrow spike of cuticle
et al., 1983). This thin cuticle specialization itself is often called the
The posterior process of the fused tail spike cell is filled with microtubules (See TailspFIG3) (called filaments by Sulston et al., 1983), which give this process the appearance of a primitive cilium. The process is wrapped by hyp10 over most of its length (See TailspFIG3 and TailspFIG4), while only the very tip of the process is exposed to the cuticle. Specialized junctions (probably gap junctions, but possibly also some adherens junctions) are seen between the tail spike cell and hyp10 when the embryo reaches two-fold stage. The posterior cilium may, therefore, provide a scaffold over which hyp10 forms a broader, more stable posterior extension to give shape to the tail spike. When the spike cells die, hyp10 still has a posterior process in place to directly maintain and support the cuticular spike.
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