Interfacial epithelial cells are located at the interface between the hypodermis and another type of tissue, at the points where these tissues attach to the hypodermis or open to outside through holes in the hypodermis. They are often intermediate in function and morphology between hypodermis and these adjacent tissues. They arrange in toroidal conformations that surround the junction site. All have epithelial characteristics and some secrete cuticle, forming smooth transitions between cuticles with different compositions (White, 1988). Interfacial epithelial cells include arcade cells and the pharyngeal epithelium of the buccal cavity, the vulval epithelium at the vulval opening, the rectal epithelium at the rectal canal and anus (proctodeum), socket and sheath cells (glia) at sensory openings, and the duct cell and pore cell forming the excretory pore. Detailed discussions of these cells, except for the arcades, are provided in chapters regarding the alimentary system (pharyngeal epithelium and rectal epithelium), the reproductive system (vulval epithelium), the nervous system (glia) and the excretory system (the duct and pore cells). In males, rectal epithelial cells generate the cloacal-gonadal connection (see Proctodeum), instead of rectal canal and anus (see also Male Anatomy).

The anterior hypodermis consists of three specialized rings of cells (hyp1, hyp2, hyp3; cheilostom) (see HypFIG 10) that outline the anteriormost edge of the bodywall at the mouth and lips. The anterior end of the pharynx consists of an interfacial cell group known as the pharyngeal epithelium. Spanning the gap between hyp1 and the pharyngeal epithelium are the arcade cells (InterFIG 1) (Mango, 2007). Together, arcade cells and the pharyngeal epithelium form the buccal cavity and link the digestive tract to the outside and to body hypodermis.

There are nine arcade cells that organize into two separate epithelial syncytia: the anterior arcade and the posterior arcade (InterFIG 2, InterMOVIE 1). Each of these syncytia has a cytoplasmic ring that surrounds the anterior of the buccal cavity (InterFIG 3). Their cell bodies, which are located in the body wall more posteriorly and close to the anterior bulb of pharynx, are connected to these rings by thin cytoplasmic processes (InterFIG 2 and InterFIG 4). Similar to other precursor cells of the buccal cavity, the arcade cells move inwards from their ventral position during embryogenesis, generating these processes (Wright and Thomson, 1981). In some cases cell fusion also occurs between these processes before reaching the syncytial ring. The anterior arcade ring is generated by the fusion of processes from three cells (arc ant DL, arc ant DR and arc ant V), whereas the posterior arcade ring is formed by the fusion of processes extended from six cells (arc post D, arc post DL, arc post DR, arc post V, arc post VL and arc post VR) (Wright and Thomson, 1981).

The two arcade rings are firmly sealed to each other and to neighboring tissues via adherens junctions. The adherens junctions between the posterior arcade and the most anterior ring of pharyngeal epithelium are very robust and form one continuous belt desmosome along the entire border. The adherens junctions linking the two rings of arcade cells and the anterior arcade to the ring of hyp 1 syncytium are less robust. Preliminary evidence suggests that there are also gap junctions between these cells. Although not yet confirmed by examination at higher magnification, it appears that gap junctions link the two arcade rings to each other, and also possibly link the arcades to hyp 1 and to the anteriormost pharyngeal epithelial cells (D.H. Hall, unpublished).

The arcade cell cytoplasm generally appears quite clear (electron lucent) compared to other epithelia, except for the portion closest to the buccal cavity, which often shows increased density at the plasma membrane. The arcade cells contain many free ribosomes and mitochondria, but very little endoplasmic reticulum (ER). Just prior to the molts, the arcade cells become filled with dense core vesicles that cluster near the cuticle, which suggests that the arcade cells build new cuticle by vesicle secretion (Wright and Thomson, 1981; D.H. Hall, unpublished; see Cuticle).

3 Buccal Cavity

The nematode digestive system consists of three parts: the stomodeum, intestine and proctodeum (rectum and anus). Embryologically, the stomodeum and proctodeum have a mixed lineage derived from cells of both ectodermal and mesodermal origin, whereas intestine (gut) is wholly endodermal in origin. Stomodeum includes the mouth and the lips, the buccal cavity (stoma), and the pharynx (esophagus) (see Alimentary System for detailed descriptions of the pharynx, intestine, rectum and proctodeum).

Rhabditids that feed on bacteria, such as C. elegans, have a relatively narrow and cylindrical, smoothly lined buccal cavity. In earlier literature this buccal cavity had been described as divided into five regions: cheilostom, prostom, mesostom, metastom and telostom, from anterior to posterior (Bird and Bird, 1991). More recently, however, throughout the class Secernentea, including Rhabditida, the cuticle-lined lumen of the buccal capsule has been subdivided from anterior to posterior into three chambers: the cheilostom, gymnostom, and stegostom (InterFIG 1B) (De Ley et al., 1995). Cheilostom is surrounded by hypodermis, gymnostom (former prostom) is surrounded by the pair of arcade syncytia, and stegostom (former mesostom, metastom and telostom) is surrounded by a longitudinal series of four sets of radial pharyngeal cells (e1, e3, pm1, pm2; see Alimentary System - Pharnyx) (Dolinski et al., 1998).

In rhabditid nematodes, the specialized cuticular zones that line the mouth and the buccal passage have the suffix "rhabdia" (Bird and Bird, 1991). Hence, cheilorhabdion and prorhabdion refer to the cuticular coverings of the lips, and arcade regions, respectively, whereas mesorhabdion, metarhabdion and telorhabdion refer to the cuticular lining of the posterior buccal regions. Where the arcade tissue bridges the gap between hypodermal tissue and pharyngeal epithelium, the arcade cells also secrete a narrow ring of cuticle that seals the body cuticle of the lips to the buccal cuticle via a local, notched joint known as the cheilostom groove (InterFIG 1) (Wright, 1976, Wright and Thomson, 1981). Although, the cuticles of the body and buccal capsule are distinct in C. elegans, the cuticles of the different regions of the capsule are similar. Since body cuticle does not enter the buccal cavity, C. elegans is categorized as an “astomatous” species (Wright, 1976; Wright and Thomson, 1981). During molts, the cuticular linings of the buccal capsule and the pharynx are also shed. The body cuticle undergoes apolysis and a new cuticle is laid down before the apolysis of the buccal capsule cuticle and pharyngeal cuticle. Unlike body cuticle, pharyngeal cuticle and buccal capsule cuticle are broken down following apolysis. (Wright and Thomson, 1981; see Cuticle).

The morphogenesis of the pharynx starts at the late comma stage, after gastrulation is complete and approximately 330 minutes after first cell division. At this stage, 78 of the 80 pharyngeal cells have been born and the pharyngeal primordium has formed a cystic ball inside the embryo (InterFIG 5). This compact tissue borders the nascent intestine at the posterior and is connected to it by adherens junctions, but is not yet connected to the buccal cavity. The hypodermal and nine arcade cells fill the 11- to 12-μm space between the pharyngeal cells and the anterior of the embryo. Over the next 60 minutes, the pharyngeal cells go through a process called “pharyngeal extension,” when they convert from a cyst to a short, linear tube that links the digestive tract to the buccal cavity (InterFIG 5 and InterFIG 6) (Portereiko and Mango, 2001; Mango, 2007). Pharyngeal extension takes place through coordinate formation of new epithelia without any cell migrations and has three stages: reorientation of pharyngeal epithelial cells, epithelization of the arcade cells, which are originally mesenchymal, and contraction of the buccal cavity and the pharyngeal epithelial cells. During the first stage, pharyngeal epithelial cells rotate and reorient their apicobasal polarity along the dorsoventral axis from the rostrocaudal axis. This rearrangement aligns the pharyngeal epithelial cells of the anterior edge of the pharyngeal primordium with the arcade cells. During the second stage, the arcade cells form adherens junctions with the pharyngeal epithelium and the hypodermis, generating a continuous epithelium between the hypodermis and the anterior pharynx. The arcade cell epithelium is the last epithelium to form in the embryo, and its formation takes place in less than 10 minutes (Portereiko et al., 2004). In the last stage, contraction of the apical surface of this continuous epithelium pulls the cells together, moving the hypodermis backward while moving the pharynx forward (Portereiko and Mango, 2001). During later embryogenesis, the linear pharyngeal tube that forms at the end of pharyngeal extension develops a lumen and undergoes an extensive morphogenetic program to produce the characteristic two-lobed structure of the fully differentiated pharynx (see Alimentary System - Pharynx).

5 List of Interfacial Epithelial Cells

6 References

Albert, P.S. and Riddle, D.L. 1988. Mutants of Caenorhabditis elegans that form dauer-like larvae. Dev. Biol. 126: 270-293. Abstract

Bird, A.F. and Bird, J. 1991. Digestive System in The Structure of Nematodes. pp 183-229. S. Diego, CA, Academic Press.

De Ley, P., Van De Velde, M.C., Mountport, D., Baujard, P. and Coomans, A. 1995. Ultrastructure of the stoma in Cephalobidae, Paragrolaimidae and Rhabditidae with proposal for a revised stoma terminology in Rhabditida (Nematoda). Nematologica 41:153-182. Abstract

Dolinski, C., Borgonie, G., Schnabel, R. and Baldwin, J.G. 1998. Buccal capsule development as a consideration for phylogenetic analysis of Rhabditida (Nemata). Dev. Genes Evol. 208: 495-503. Abstract

Mango, S.E. 2007. The C. elegans pharynx: A model for organogenesis. In Wormbook (ed. The C. elegans Research Community), doi/10.1895/wormbook.1.129.1. Article

McKay, S.J., Johnsen, R., Khattra, J., Asano, J., Baillie, D.L., Chan, S., Dube, N., Fang, L., Goszczynsk,i B., Ha, K., Halfnight, E., Hollebakken, R., Huang, P., Hung, K., Jensen, V., Jones, S.J.M., Kai, H., Li, D., Mah,, A., Marr, M., McGhee, J., Newbury, R., Pouzyrev, A., Riddle, D.R., Sonnhammer, E., Tian, H., Tu, D., Tyson, J., Warner, A., Wong, K., Zhao,  Z. and Moerman, D.G. 2004. Gene expression profiling of cells, tissues, and developmental stages of the nematode C. elegans. Cold Spring Harbor Symp. Quant. Biol. 68: 159-169. Abstract

Portereiko, M.F. and Mango S.E. 2001. Early morphogenesis of the Caenorhabditis elegans pharynx. Dev. Biol. 233: 482-494. Article

Portereiko, M.F., Saam, J. and Mango S.E. 2004. ZEN-4/MKLP1 is required to polarize the foregut epithelium. Curr. Biol. 14: 932-941. Abstract

White J. 1988. The Anatomy. In The nematode C. elegans (ed. W. B. Wood). Chapter 4. pp 81-122. Cold Spring Harbor Laboratory Press, New York. Abstract

Wright, K.A. 1976. In "The Organization of Nematodes" (N. A. Croll, ed.) pp71-105. Academic Press, London.

Wright, K.A. and Thomson, J.N. 1981. The buccal capsule of C. elegans (Nematoda: Rhabditoidea): An ultrastructural study. Canad. J. Zool. 59: 1952-1961. Article