ALIMENTARY SYSTEM - (Part Ia ) The Pharynx

General info - The Pharynx - Emb. Development - Buccal Cavity & Epithelium - Muscle Cells - Marginal Cells - Gland Cells - Feeding behavior - Pharyngeal-Intestinal Valve - Pharyngeal Cuticle - Cell List - Back to Contents

General information

The alimentary system is one of the most complex portions of the nematode anatomy, made up of a large variety of tissues and cell types. Direct intercellular connections between the alimentary tissues and the rest of the body are minimal. Topologically, this system forms a separate epithelial tube running inside the cylindrical bodywall, separated from it by the pseudocoelomic space, and often parallel to the tube-like gonad. At the anteriormost end the pharyngeal epithelium connects to the arcade cells of the lips (See Specialized epithelial cells part III). The posteriormost tissue, the rectal epithelium and anus, touch the tail hypodermis and the dorsorectal ganglion. One pair of somatic neurons (RIPL/RIPR) penetrate the pharyngeal basal lamina to make gap junctions with a pair of pharyngeal neurons (I1L/R). Otherwise the only connection to span the two systems (somatic vs. enteric) are the four specialized enteric muscle cells which act on the intestine and rectal valve while being driven by somatic motor neurons, AVL and DVB.
The alimentary system is divided into the foregut (stomodeum; buccal cavity and the pharynx), the midgut (intestine), and the hindgut (proctodeum; rectum and anus in hermaphrodites and cloaca in males) and contains a total of 127 cells. The stomodeum and proctodeum are lined with cuticle which is shed along with body cuticle during molts (Bird A. F. and Bird J., 1991. pp183-229). The stomodeum and proctodeum are also regions into which glands open. Pharyngeal glands open to the stomodeum and rectal glands open to the proctodeum. Ingested material flows through the digestive tract by the muscular pumping and peristalsis of the pharynx at the anterior end and the opening of anus at the posterior end by the enteric muscles. The intestine itself is devoid of any muscular structure in C. elegans, although the rear portion can be contracted by the stomatointestinal (SI) muscles. During defecation, the body wall muscles also contribute to the control of internal pressure and concentration of the gut contents before the expulsion of the waste material.
Developmentally the intestine is endodermal in origin whereas the stomodeum and proctodeum have a mixed lineage from ectodermal and mesodermal origins (Bird A. F. and Bird J., 1991. pp183-229).
The alimentary system is described here, organ by organ, in their order of appearance along the length of the body, from anterior to posterior. Note that the lips which include, hyp1, hyp2, hyp3, the lip sensilla, and the arcade cells are described in the hypodermis, and specialized partners within hypodermis part II and III sections of the handbook, respectively.

The Pharynx

The pharynx is a single, unified, epithelial organ with its own nervous system, muscles, gland cells and structural cells contained within a single specialized basal lamina. Most of its features are organized radially. There may be hormonal signaling between this organ and the rest of the body; otherwise the two systems appear to operate almost independently. The pharyngeal musculature is divided into 8 segments along its length, which show three-fold symmetry and radial orientation. The nervous system includes 20 neurons, some of which have polymodal functions. These neurons extend along 3 longitudinal pharyngeal nerve cords, two subventral and one dorsal, and form one nerve ring and a posterior commissure (AlimFIG16and AlimFIG17). The basal side of the pharyngeal epithelium lies towards the outside, facing the pseudocoelom, bound by the basal lamina. The apical side of the pharyngeal epithelium lies inward towards the lumen, where the pharynx is bound by cuticle and several specialized cuticular structures; the anterior “flaps”, the “sieve” and the “grinder”. Specialized gland cells secrete into the pharyngeal lumen in three locales (see AlimFIG14). The pharyngeal lumen is lined with cuticle, and some of these gland cells may participate in the digestion of the pharyngeal cuticle during molting (Albertson and Thomson, 1976) (Hall D.H. and Hedgecock E.M., 1991).
The pharynx has a distinctive shape with two separate bulbs and an extended anterior portion. The narrow gap between the two bulbs is the isthmus, and is the location of the nerve ring in the surrounding somatic tissue. The regions of the pharynx are as follows (See AlimFIG4):
PHARYNGEAL (BUCCAL) EPITHELIUM; surrounds BUCCAL CAVITY
CORPUS = PROCORPUS + METACORPUS (1st BULB)
ISTHMUS
TERMINAL BULB (2ND BULB); for cell identifications see L. Avery's Terminal Bulb Atlas
PHARYNGEAL-INTESTINAL VALVE (CARDIA)


Embryonic development of the pharynx

Unlike the intestinal cells, all of which arise from a single founder cell, E, the pharynx is made up of cells that arise from both AB and MS founder cells (See AlimFIG5 and AlimFIG6). In these lineages, no strict clonal and lineal derivation can be recognized and there is a heterogeneity of ancestry with respect to final cell fate. In the anterior divisions, no lineal boundaries exist between hypodermal, arcade and pharyngeal daughters. The pharyngeal components derived from MS and AB do not possess any functional boundaries either, such that in pm3, pm4 and pm5 muscle rings identical cells arise from these two lineages and some MS descendants fuse with those coming from the AB cell (See Alimentary tract in Sulston J. E. et al, 1983).

During gastrulation, at around 100-cell stage and after the movement of gut precursors (Ea, Ep) to inside of the embryo, MS descendants start to move inward through the blastopore (AlimFIG7). Slightly later, AB-derived precursors of the pharynx move inside of the embryo. When gastrulation is completed about 330 min after fertilization, pharyngeal and intestinal precursors are surrounded by hypodermal and neuronal precursors on the outside similar to the other internally migrated cells, and pharyngeal precursors are in the form of a compact ball-like primordium. These cells are attached to each other and the midgut by adherens junctions, however, they are not yet attached to the buccal cavity (Portereiko M. F. and Mango S., 2001). At this time 78 of the 80 pharyngeal cells are already born. Over the next hour, pharyngeal extension occurs such that pharyngeal precursors reposition themselves to form a central cylinder and link to the buccal cavity while the body myoblasts position themselves between this cylinder and the outer layer of cells. Pharyngeal extension is accomplished in roughly three steps; reorientation where pharyngeal epithelial cells readjust their apicobasal polarity, epithelization where a continuous epithelium is formed between arcade cells and the anterior epidermis, and contraction where pharynx moves anteriorly and the mouth moves posteriorly to link the digestive track to the outside of the animal (Portereiko M. F. and Mango S. E, 2001). Later, the pharyngeal tube morphs into a bilobed structure and develops a lumen. Lumen formation is thought to be carried out by retraction of the tips of the marginal cells from the midline of the pharyngeal primordium when the pharyngeal muscles differentiate and contract (AlimFig7)(Leung B, et al, 1999). Cell fusions in pharynx occur either before or soon after hatching (Sulston J. E. et al, 1983). C. elegans initiates pharyngeal pumping right before hatching.

Buccal Cavity and Buccal Epithelium

The anteriormost limit of the pharyngeal lumen is called the buccal cavity. This cavity is lined by a thin pharyngeal epithelial tube made from nine cells which form two successive rings of (non-syncytial) tissue with six-fold symmetry (see AlimFIG8 and Albertson D.G. and Thomson N. J., 1975). This epithelial tissue forms a rather rigid narrow cylinder that restricts entry of food into the pharynx. Its cylindrical shape is heavily reinforced by short radial bundles of intermediate filaments that are anchored to the apical and basal membranes of the epithelial cells by large hemidesmosomes. These well anchored filaments seem to limit the epithelial cylinder from stretching or collapsing, despite the vigorous movements of the nearby pharyngeal musculature. However, the buccal epithelium itself is perhaps immobile, and is not innervated. The cuticle here may also be reinforced by tonofilament bundles (aggregated intemediate filaments). At its posterior limit, the buccal cavity is bounded by three cuticular flaps which extend into the lumen from the border of the buccal epithelium and pm1 (See ArcFIG9). Opening or closure of these flaps may also regulate food entry into the pharynx.
Buccal cavity is separated into 5 sections; cheilostom, prostom, mesostom, metastom and telostom (See ArcFIG9) (Bird A. F. and Bird J., 1991. pp183-229). The cheilostom is lined by epidermal cuticle, the prostom by arcade cuticle. These two cuticles are separated in cross section by a narrow cheilostom groove (See Slidable Worm section #4). The mesostom is lined by cuticle derived from nonmuscular epithelial cells of the pharynx (e1-e3), whereas the metostom and telostom cuticles are secreted by 2 anteriormost pharyngeal muscle cells.

a': anterior arcade

a": posterior arcade

a: dorsal metastomal flap

b: subventral flap

e': pharyngeal epithelium (e1 and e2)

e": pharyngeal epithelium (e2 and e3)

The cell bodies of the buccal epithelial cells lie far posterior to the buccal cavity and extend thin processes anteriorly within the pharynx to reach their epithelial territories surrounding the cavity (See AlimFIG 8, AlimFIG 9 and Albertson D.G. and Thomson J. N, 1976). In the region where these cells surround the buccal cavity, the most anterior epithelial cells, e1, lie as a single dorsal and two subventral cells, and cover markedly thinner territories (as measured along the a/p axis) than the other six cells. The most posterior cells, e3, lie in the same pattern, and their epithelial territories lie directly behind those of e1 cells. The intervening e2 cells lie at the three apices of the triangular lumen, and their epithelial territories span the space of both the e1 and e3 cells, yielding a six-fold symmetry. At their lateral borders, all of these epithelial cells are firmly connected to their immediate neighbors by large adherens junctions (running continuously at the apical border). Where the epithelial cells border the posterior arcade cells, there are again very prominent adherens junctions.
The inside caliber of the buccal epithelial lumen increases markedly at each larval molt, and this increase in size of the buccal cavity is suggested to permit increased growth rate after each molt. (Knight C. G. et al, 2002). The buccal cavity is much narrower in the dauer larva, which stops actively feeding.

Muscle cells of the pharynx

The pharyngeal muscles are grouped into eight separate segments (pm1 through pm8) arranged to form 8 consecutive rings of radial musculature encircling the pharynx (AlimFIG9) (Avery L. and Thomas J. H., 1997). Unlike the bodywall muscles, no hypodermal layer separates the muscle anchorage from the cuticle lining the inside of the pharynx, on the lumenal side. Indeed most of the pharyngeal muscles appear to participate directly in secreting cuticle, and thus qualify as having myoepithelial properties (Albertson and Thomson, 1976, Hall, unpublished). Most of the pharyngeal muscle rings are made up of 3 cells giving the pharynx a three-fold symmetry. Each of these cells contains two nuclei as a result of fusion of two cells around the time of hatching (See Table 1 in Albertson and Thomson, 1976, also pm table below). The exceptions are pm1 and pm6-pm8; pm1 is a syncytial cell which contains 6 nuclei, pm6 and pm7 are made of 3 nonsyncytial cells each and and pm8 is a single cell (AlimFIG9, AlimFIG10 and AlimFIG11). In pm2-pm5 in adult pharynx, there are still small remnant adherens junctions at the sites of former cell fusions between pairs of muscle cells, marking the exact apical border where fusion has occurred (Hedgecock and Thomson, 1982) (see AlimFIG13). These remnant junctions express AJM-1 protein and can be stained by immunocytochemistry (Koppen et al., 2001; Hall, unpublished). Most pharyngeal muscles are separated from their neighbors within the segment by a marginal cell, each of which generally spans more than one successive muscle segment, so that within a segment the three syncytial muscle cells do not directly touch each other ( AlimFIG12 and AlimFIG13). However, muscle cells often touch muscle cells in neighboring rings directly via interlocking short fingers on the anterior and posterior margins and link to them by gap junctions. On their lateral membranes, muscle cells and marginal cells within a segment are also linked by gap junctions, as well as by adherens junctions at their apical margins.

In most pm segments, all muscle filaments are oriented radially to help opening and closing the pharyngeal lumen. However, in the terminal bulb, the pm7 muscles are oriented obliquely in regard to the anterior-posterior axis to pull on the grinder region.

On the basal side, each muscle cell wraps around a longitudinal pharyngeal "nerve cord" which encloses the cell bodies and processes of neurons, gland cells, and anterior epithelial cells (AlimFIG9 and AlimFIG12). Many intercellular junctions, including neuromuscular junctions onto the pharyngeal muscles, occur along these three sets of cell processes.
Specialized zones mark the apical regions of several of the pharyngeal muscles where they secrete cuticle. Here, the cytoplasm contains electron dense tubules and sometimes dense core vesicles just under the plasma membrane. The cytoplasm of pm6 is particularly specialized in the region underlying the grinder, which is a very elaborate cuticular structure secreted by the pm6 muscle cells (AlimFIG20). There are quite prominent networks of sarcoplasmic reticulum along the borders of each muscle sarcomere. These presumably sequester calcium needed for muscle contractility. These muscle cells also contain many mitochondria. These features are perhaps to be expected in a set of muscles that undergo such a high rate of contractions during normal function.
pm1 6 nuclei form one syncytial cell innervated by M1
pm2 6 nuclei form 3 syncytial pairs innervated by M1
pm3 6 nuclei form 3 syncytial pairs innervated by M1
pm4 6 nuclei form 3 syncytial pairs innervated by M2, M3, MC (L. Avery pers. comm.)
pm5 6 nuclei form 3 syncytial pairs innervated by M2, M3, M4
pm6 3 nuclei not syncytial innervated by M5, also possibly M4 (L. Avery, pers. comm.)
pm7 3 nuclei not syncytial innervated by M5
pm8 1 nucleus does not receive any direct innervation from any of the motor neurons, however, makes gap junctions to mc3 which are innervated by M5

Marginal cells of the pharynx

The pharynx is a mosaic of several non-equivalent cell types that construct a unified non-stratified epithelium only one cell deep, in a checkerboard pattern. This epithelium shows 3-fold symmetry, due to the placement of three large cells, the “marginal cells”, at the three corners of the pharyngeal lumen at any one cross section.There are a total of 7 marginal cells; 3 mc1 cells, 3 mc2 cells and a syncytial mc3 cell with 3 nuclei (See AlimFIG9, AlimFIG12 and AlimFIG15). As described above, within each muscle segment of the pharynx, the marginal cells separate each of the three syncytial pharyngeal muscle cells from their neighbors. From segment to segment, the marginal cells lie in rows at the corners of the lumen. They vary markedly in size, but each of the three cells within a segment are essentially equivalent.
The marginal cells supply reinforcing strength to this muscular organ. Their large size and block-like shape is fortified by the placement of large radial bundles of intermediate filaments running from apical to basal borders of each cell. These filaments are anchored to the plasma membrane by large hemi-adherens junctions. Within each segment, the marginal cells are linked to neighboring muscle cells on their lateral borders by large gap junctions as well as apical adherens junctions. Large interlocking finger-like extensions also hold marginal cells to muscles within each segment, and these may also add to the structural integrity of the whole organ. Due to this arrangement, marginal cells within a segment communicate with muscle cells of the same segment, but not with other marginal cells. In contrast, between segments, these cells form interlocking fingers and gap junctions to other marginal cells.

The marginal cells contain many mitochondria, which suggests that these cells must perform some active role beyond merely providing continuity and strength to the epithelium. Since marginal cells are coupled to pharyngeal muscles via gap junctions, they may have some motor function, i.e., they may be myoepithelial in nature. Alternatively, they may act as relay stations to synchronously transmit signals from motor neurons to surrounding pharyngeal muscles so that all pharyngeal muscles within a segment can contract and relax at the same time. It is noteworthy that when the pharyngeal muscles contract, the muscle cells become thinner and the lumen opens. Since the marginal cells are already relatively thin, this suggests that at full contraction, the pharyngeal lumen may open practically as wide as the inside corners of the marginal cells and forms an open triangular lumen, whereas when the muscles relax, the lumen is practically closed except for the three channels at the ends of the pharyngeal rays (See AlimFIG13).

mc1 2 subdorsal mc1's receive innervation from M2; ventral mc1 is not innervated.
mc2 (WA editors' note: the original finding that mc2's are innervated by MC neurons has recently been challenged on the grounds that no functional evidence has been found to support it. Functional evidence suggests that MC neurons innervate pm4 and not mc2-L. Avery pers. comm.)
mc3 innervated by M5

Gland cells of the pharynx

Two classes of gland cells, g1 (two cells, one of which is syncytial) and g2 (two cells), are found in the second bulb of the pharynx (AlimFIG14 & 14B. See a 3-D reconstruction of phrayngeal gland cells by R. Newbury & Moerman lab. Cell labels are shown in AlimFIG14B. 3-D movie was created from confocal images of a strain expressing the GFP marker linked to the promoter for B0280.7 using Zeiss LSM 5 Pascal software v. 3.2. Pharyngeal lumen is rendered artistically on this image). One pair of g1 cells (dorsal g1 and right ventral g1) are fused. The g1 cells extend three cuticle-lined ducts anteriorly within the narrow pharyngeal nerve cords. Two of these ducts pass through the isthmus before emptying into the pharyngeal lumen near the first bulb. The dorsal g1 duct travels much farther and empties near the anterior limit of the pharynx. Similarly, the g2 cells extend shorter ducts which empty into the lumen of the second bulb. The g1 cells contain a lamellar cytoplasm and few vesicles, while the g2 cells have a rather clear cytoplasm and more vesicles. These contents may vary from animal to animal, and vesicle sizes are quite large and variable. Gland cells receive motor innervation from M4 and M5 motor neurons which suggests that they may be stimulated to secrete digestive enzymes synchronously with pharyngeal pumping activity. Periodic episodes of secretion (vesicle motion) have been seen in g1 ducts by light microscopy (Hall D. H. and Hedgecock E. M., 1991; also unpublished observations of J. Sulston reported by Albertson and Thomson, 1976) which are apparently associated with molting.

Feeding behavior

C. elegans feeds in a filter-feeding fashion; particles (bacteria) are taken in as suspended in liquid, and then trapped in the pharynx while the liquid is spit out by the function of the corpus and anterior isthmus (Avery L., and Thomas J. H., 1997). The feeding behavior consists of two motions; pumping, which is accomplished by near simultaneous contraction of the muscles of the corpus, anterior isthmus and terminal bulb followed by a near-simultaneous relaxation, and isthmus peristalsis. Contraction of the corpus and anterior isthmus opens this section of the lumen, sucking particles and liquid into the lumen while contraction of the terminal bulb muscles breaks up already-trapped bacteria and passes the debris posteriorly towards the intestine. At this stage corpus and isthmus are separated from the terminal bulb by a closed isthmus. During the relaxation period that follows the lumen of the corpus closes allowing for the liquid to be somehow expelled through radial channels while bacteria are retained and the grinder returns to its resting position. Roughly one out of four pumps is followed by an isthmus peristalsis where the anteriorly trapped bacteria are carried backwards to the grinder by a peristaltic wave of contraction in the posterior isthmus. Although each muscle cell runs the entire length of the isthmus, contraction occurs as a wave that propagates from anterior to posterior instead of simultaneously along the length of it. This capacity of isthmus for asynchronous contraction is suggested to permit for the terminal bulb and corpus lumen to be at different pressures (Avery L., and Thomas J. H., 1997).

Pumping is suppressed in wild-type worms during the dauer larval stage and in response to a touch stimulus in adults. The latter response depends on RIP/I1connection and there is data to suggest that this circuit is also important for suppression of pumping in the dauer larva (Keane J. and Avery L., 2003). Inhibition of pumping during dauer stage which the animal enters in response to adverse environmental and nutritional conditions may function to prevent precious energy loss by pumping and ingestion of environmental toxins when no appropriate food is available.

As described above, neural control of feeding can be managed completely by the pharyngeal nervous system. There are three pharyngeal motor neuron types that are necessary and sufficient for normal feeding of the animal under laboratory conditions; M3, MC and M4.
M3's control the timing of the end of a pump, i.e. initiation of pharyngeal relaxation. They are inhibitory type motor neurons and because of their free endings in the metacorpus they are also suggested to be proprioceptive sensory neurons (Albertson and Thomson, 1976). Through a self-acting proprioceptive loop, M3's are postulated to fire in response to corpus muscle contraction and cause muscle relaxation. M3's are glutamatergic and the fast inhibitory glutamatergic transmission from M3's to the pharyngeal muscle is thought to be mediated through the (ionotropic) glutamate-gated chloride channel, a subunit of which is AVR-15 (Dent J. A. et al, 1997 and 2000).
MC motor neurons control the rate of excitation of pharyngeal muscle. They mediate rapid pharyngeal pumping and are necessary for normal stimulation of pumping in response to food (Avery L. and Horvitz H. R., 1989). MC's are cholinergic excitatory motor neurons (Raizen D. et al, 1995, Keane J. and Avery L., 2003). Mutants in which acetylcholine synthesis (e.g. cha-1) or packaging (e.g. unc-17) is disrupted show a pumping defect similar to that seen after MC ablation. Functional studies suggest that MC's synapse onto pm4 and the synaptic transmission from MC's to pm4 requires the nicotinic acetylcholine receptor (nAchR) subunit, EAT-2 . Mutations in eat-2 mimick MC cell loss and EAT-2 is expressed on pm4 (Keane J. and Avery L., 2003, Avery L. pers. comm.). MC's may also be mechanosensory since they have free endings at the boundary between the procorpus and metacorpus and may sense the presence of bacteria in the pharynx.
M4 neuron is required for posterior isthmus peristalsis. It synapses onto the posterior half of the isthmus muscles and M4 ablation leads to growth failure since the animals can not ingest any food (Avery L and Horvitz H. R., 1987). These animals continue to pump, however, and as a result anterior isthmus and corpus become stuffed with bacteria. M4 is cholinergic but may also be utilizing another neurotransmitter since cholinergic antagonists and mutations that affect nicotinic Ach receptors only partially block M4 function (L. Avery pers. comm.).

Pharyngeal-intestinal valve

A group of six equivalent cells forms a tightly constructed “valve” that links the posterior bulb of the pharynx to the anterior four cells of the intestine. These six cells comprise a small epithelial channel with a cuticular lining in continuity with the pharyngeal cuticle and link the lumen of the pharynx to the large lumen of the anterior intestine. They are basically organized into three sets of cells forming consecutive rings containing 1, 3 and 2 cells from anterior to posterior (AlimFIG1).The inside cuticle is reinforced by a series of closely spaced circumferential ridges, rather like the cuticle of the anterior buccal cavity, formed by the pharyngeal epithelial cells. The valve cells are not syncytial, but are firmly linked to their neighbors and to the pharynx and/or intestine by robust adherens junctions at their apical borders and by gap junctions [valve to valve cell; valve to intestine] (See AlimFIG19). Pharyngeal intestinal valve may be opened by the contraction of the pm8 muscle when the grinder is active (Avery L. and Thomas J .H., 1997).

There are no apparent muscular elements operating within the valve cells, nor are there any muscles attaching to this valve from the outside. Thus, the valve is probably a passively open and patent channel at all times, but rather narrow in caliber. The pm8 muscle of the pharynx is in appropriate position to act alone as a sphincter just rostral to the valve cells, however, pm8 shows no direct innervation. Hence, restrictions to regurgitations of ingested material may be due to operations of the pharyngeal grinder within the second bulb of the pharynx, immediately anterior to this valve. Squeezing motions of the terminal bulb would force food through the grinder and into the intestine via the pharyngeal-intestinal valve during the pumps when the pressure in terminal bulb is high. The intestinal lumen widens considerably just behind this region, which would result in a drop in internal pressure and may limit any reflux from the intestine back through the valve.

Each valve cell has a very electron dense cytoplasm and occupies a thin wedge-shaped domain surrounding about one half of the lumen of the valve (AlimFIG 18 and AlimFIG19). The nuclei of valve cells are flattened in shape, and the cytoplasm contains radial bands of intermediate filaments anchoring the apical cuticle to the basal lamina of the epithelium via hemidesmosomes, again in a very similar fashion to those seen in the buccal epithelium (Hall, unpublished).

Click here to see how vpi cells stack (created by Brian Henick and Adam Hartley). For vpi cell numbers refer to the images below. Note that WA color code was not used in these images.

Cuticle of the pharynx

There is a thin cuticle lining that extends to cover the interior surface of the pharyngeal passageway from the lips to the back of the pharynx, ending at the rear of the pharyngeal/intestinal valve. Unlike the body cuticle this cuticle shows no layers. However, it shows some reinforcement at points of stress. For instance, it becomes keratinized to stain more densely by TEM along much of its length. In other places the cuticle shows radial bands of tonofilaments, thought to consist of clusters of intermediate filaments, such as along the buccal cavity, under the pharyngeal epithelium (For more detailed description see Cuticle chapter).
The pharyngeal cuticle is apparently formed jointly on apical surfaces of many cells acting in concert, including the pharyngeal muscles, marginal cells, valve cells and pharyngeal epithelium. This tissue is shared by many cells much as the thickened pharyngeal basal lamina is formed jointly on basal surfaces of the same cell groups. Other specialized portions of the pharyngeal cuticle include the following:

Bridging Cuticle
There is a small discrete region of cuticle connecting the body wall cuticle covering the lips to the cuticle lining the buccal cavity (see ArcFIG10). This bridging cuticle lies on the outer face of the anterior arcade and may also be touched in a short region by the posterior arcade. The anterior arcade grows an extremely thin extension between the bridging cuticle and the buccal cavity cuticle. Although the two major cuticles (the lip cuticle and buccal cavity cuticle) barely touch one another at the anterior margin of the arcade, they still appear to be in contact there. The bridging cuticle seems to form a discrete ring in some specimens, but in other cases seems to exist as a direct extension of the lip cuticle; thus it is in firmer connection to the lip cuticle than to the buccal cuticle. This bridging structure may permit some flexibility in the relative position of the buccal lining and the lip cuticle during pharyngeal pumping, while at the same time sealing off any fluid exchange from the exterior into the body cavity between the two discrete cuticles. This arrangement may also permit the interlocking cuticles to act as a local expansion joint, eliminating any chance of tearing, during large rough motions of the pharynx.

Flaps
These structures close the opening of the buccal cavity to the pharyngeal lumen (AlimFIG 21). Three flaps extend inward to restrict flow at the rear of the buccal cavity. They extend from the pharyngeal muscle cells pm1 and pm2, and their cuticle is very electron dense, suggesting a sclerotic hardening to stiffen the flaps and the entryway to the true pharynx. See ArcFIG9). These may correspond to the onchia, or buccal teeth described in larger nematodes (See Chitwood and Chitwood, 1950).

Grinder
Grinder is a cuticle specialization made primarily by pm6 and pm7 muscles along their lumenal surfaces. Its three segments made by the three pairs of muscle cells rotate when the muscles contract (Avery L. and Thomas J. H., 1997). These interlocking "teeth" function in macerating the food, and may function as a valve to regulate one-way traffic of food into the intestine ((AlimFIG 20).
While the pm6 muscle filaments are mostly seen to orient in radial fashion to the grinder, some portions of the pm7 muscles are oriented obliquely to the anterior-posterior axis, and are anchored on the basal pole to the rear of the terminal bulb. These pull from the posterior side of the teeth and coordinated action of these muscles may then rotate the grinder segments and force the teeth to scrape past and engage one another. The food caught and ground up between them is passed back to the intestine through the pharyngeal-intestinal valve. Relaxation of the terminal bulb returns the grinder to its resting state.

Pharyngeal Sieve
Cuticular fingers extend from pharyngeal cuticle that probably act to trap bacteria in front of the grinder (AlimFIG 21). These long fingers of cuticle extend from the cell borders where pm4 muscles meet the neighboring mc1marginal cells and the fingers project over a region of about 20 microns in length within the narrow lumen ( See Fig 16 of Albertson and Thomson, 1976), near the transition of the metacorpus to the isthmus.

Pharyngeal Channels
These are three narrow grooves in outer corners of pharyngeal cuticle which seem to allow an escape route for liquid to be regurgitated out of the pharynx via the buccal cavity. This results in food particles being trapped in the sieve while fluid is expelled. These channels extend under the mc1 marginal cells and empty into the buccal cavity (See AlimFIG13 and Fig. 16 of Albertson and Thomson, 1976 for further description).

List of cells of the pharynx

i. Buccal epithelium of the pharynx (e)

1. First epithelial ring
e1D
e1VL
e1VR
2. Second epithelial ring
e2DL
e2DR
e2V
3. Third epithelial ring
e3D
e3VL
e3VR

ii. Pharyngeal muscles (pm); Note that in earlier publications these cells are labeled as "m"; m1, m2, m3 etc., here they are labeled "pm" for "pharyngeal muscle" (Avery L. and Thomas J. H., 1997)

1. First pharyngeal muscle ring; all fuse into one syncytium around hatching
pm1DL
pm1DR
pm1L
pm1R
pm1VL
pm1VR
2. Second pharyngeal muscle ring
pm2DL; fuses with DR around hatching
pm2DR; fuses with DL around hatching
pm2L; fuses with VL around hatching
pm2R; fuses with VR around hatching
pm2VL; fuses with L around hatching
pm2VR; fuses with R around hatching
3. Third pharyngeal muscle ring
pm3DL; fuses with DR around hatching
pm3DR; fuses with DL around hatching
pm3L; fuses with VL around hatching
pm3R; fuses with VR around hatching
pm3VL; fuses with L around hatching
pm3VR; fuses with R around hatching
4. Fourth pharyngeal muscle ring
pm4DL; fuses with DR around hatching
pm4DR; fuses with DL around hatching
pm4L; fuses with VL around hatching
pm4R; fuses with VR around hatching
pm4VL; fuses with L around hatching
pm4VR; fuses with R around hatching
5.Fifth pharyngeal muscle ring
pm5DL; fuses with DR around hatching
pm5DR; fuses with DL around hatching
pm5L; fuses with VL around hatching
pm5R; fuses with VR around hatching
pm5VL; fuses with L around hatching
pm5VR; fuses with R around hatching
6. Sixth pharyngeal muscle ring
pm6D
pm6VL
pm6VR
7. Seventh pharyngeal muscle ring
pm7D

pm7VL
pm7VR
8. Eighth pharyngeal muscle ring
pm8

iii. Marginal cells (mc)

1. First marginal cell ring
mc1DL
mc1DR
mc1V
2. Second marginal cell ring
mc2DL
mc2DR
mc2V
3. Third marginal cell ring (syncytial, cells fuse around hatching)
mc3DL
mc3DR
mc3V

iv. Pharyngeal-intestinal valve (vpi)

1. First pharyngeal valve ring
vpi1
2. Second pharyngeal valve ring
vpi2DL
vpi2DR
vpi2V
3. Third pharyngeal valve ring
vpi3D
vpi3V

v. Pharyngeal neurons

1.Motor neurons
M1
M2L
M2R
M3L
M3R
M4
M5
MCL
MCR
2. Other neurons
I1L
I1R
I2L
I2R
I3
I4
I5
I6
MI
NSML
NSMR

vi. Pharyngeal glands (g)

1. First pharyngeal gland ring
g1AL; ventral left g1 gland cell
g1AR; ventral right g1 gland cell, fuses with P around hatching
g1P; dorsal gland cell, fuses with AR around hatching
2. Second pharyngeal gland ring
g2L
g2R

 

Acknowledgements

We would like to thank Leon Avery (University of Texas Southwestern Medical Center, TX) for critically reviewing this chapter for us.

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