ALIMENTARY SYSTEM - (Part II ) The Intestine

General description - Int. Development (transcriptional mechanisms)- Int. Development (structural mechanisms)- Intestine structure & function- Cell List - Back to Contents

General description-Intestine

Intestine is made of 20 large epithelial cells which form a tube and are mostly situated as bilaterally symmetric pairs around the tubular lumen. Each of these cell pairs forms an intestinal ring ( II-IX int rings). The most anterior intestinal ring (int ring I), however, is made of four cells (AlimFIG1). Intestinal cells contain large nuclei with large nucleoli and numerous autofluorescent granules in their cytoplasm (See IntFIG7 below). Although the intestine initially fills the entire body cavity behind the pharynx, it eventually becomes deflected to permit the outgrowth of the gonad within the same cavity as the animal grows older (IntFIG1-4). The intestine is not rigidly attached to the bodywall but is firmly anchored to the pharyngeal and rectal valves at either end. More tenuous linkages between the basal laminae of the intestine and the bodywall form via lengthwise stripes of hemicentin (Vogel B. E. and Hedgecock E. M., 2001). The intestine is not directly innervated and has only one associated muscle (the stomato-intestinal muscle) at its posterior extreme (See Alimentary system Part III).

 

In worms that are cultivated at 20°C, the adult intestine shows a dextral handedness to its position along the length of the animal such that in the anterior portion of the animal it is localized to the left side and and in the posterior, to the right side (IntFIGs 5 and 6) (Wood W. B. et al, 1996). Furthermore, the anterior intestinal rings II, III, IV undergo a left handed twist of 180° along the longitudinal axis with respect to the neighboring rings in the region of the primordial gonad (AlimFIG1 and IntFIG10-see WA editors' note below, however). It has been suggested that the bilateral asymmetry of the position of the intestine (i.e. its handedness) is caused by this rotational twist (Sulston J. E. and Horvitz H. R., 1977).

Intestinal development - transcriptional mechanisms

The control of cell fates within C. elegans lineages is primarily determined by cell autonomous transcriptional decisions within the nucleus of each cell, but there are a few levels at which inductive signals from other cells can impact these decisions. In the case of the intestine, these transcriptional mechanisms have been explored in detail, and thus provide an excellent example of how this animal regulates cell fates. Unlike many lineages, the intestinal cells derive from a single progenitor cell, E, such that the clonal proliferation of E lineage constitute the whole intestine (IntFIG8)(Goldstein B., 1992). Some maternally contributed mRNAs play key roles in the transcriptional patterns in early steps of this lineage.

E is the posterior daughter of the mesendodermal precursor, EMS. The anterior daughter of EMS is the mesodermal precursor MS. EMS itself derives from the blast cell P1, which divides to generate EMS and P2 (IntFIG9). The descendents of P2 go on to produce many mesodermal and some ectodermal lineages, as well as the germline precursor P4. During regulation of early blastomere fates, a bZIP/homeodomain transcription factor SKN-1, whose mRNA is contributed maternally, is produced at asymmetrically high levels in P1 and its descendants. SKN-1 is required for proper E and MS development and high levels of ectopic SKN-1 expression throughout the embryo leads to conversion of non-EMS descendants into mesendoderm generating cells. (Maduro M. F. and Rothman J. H., 2002; Schnabel R. and Priess J. R., 1997).

In P2 and in the following germline lineage, SKN-1 function is suppressed by a maternally provided transcriptional repressor PIE-1. In pie-1 (-) embryos P2 adopts EMS-like fates. In EMS, SKN-1 activates expression of med genes which encode GATA-type transcription factors and this marks the switch from maternal to zygotic control in mesendoderm specification.

At 4-cell level a cell-cell inductive interaction between EMS and P2 engenders an endoderm-producing E cell. As a result of this interaction the posterior part of EMS that contacts P2 gives rise to E while the anterior part produces MS. In the absence of this cell-cell communication EMS divides symmetrically into two MS-fate harboring cells. This cell-fate decision pathway involves MOM proteins (MOM-1:porcupine; MOM-2: Wnt ligand, MOM-5: frizzled), APR-1 (adenomatous polyposis coli homolog), WRM-1 (beta-catenin) and LIT-1 (a Ser/Thr kinase) and ultimately leads to low levels of nuclear POP-1 in the posterior daughter of EMS. High levels of POP-1 in the anterior daughter, MS (which does not receive the Wnt/MAPK signal), represses endoderm specification in this cell (Lin R. L., et al 1995; Lin R. L. et al, 1998; Rocheleau C. E. et al, 1999).

Along with lowering the nuclear amounts of POP-1 in E, Wnt/MAPK signaling is suggested to convert POP-1 from a transcriptional repressor to a transcriptional activator such that it functions as a positive factor in endodermal fate specification (Maduro M. F. and Rothman J. H., 2002). MED-1,2 proteins are thought to activate transcription of the earliest known genes, end-1,3, expressed specifically in the E lineage. end genes themselves encode GATA-type transcription factors and their expression is temporally restricted to the early E lineage. It has been found that both end-1 and end-3 have been conserved in a close C. elegans relative, C. briggsae, since the two nematodes diverged evolutionarily. Downstream of END-1,3, intestinal differentiation and maintenance are carried out by activation of other GATA-type transcription factors such as ELT-2,7. Later, intestine specific genes such as acid-phosphatase encoding pho-1, cysteine protease encoding cpr-1 and metallothionein encoding genes mtl-1 and mtl-2 are transcriptionally activated to bring about a fully-functional intestine.

Intestinal development - structural mechanisms

The E blastomere is born on the surface of the embryo at about 35 min after fertilization at 20°C ambient temperature. From this point on, the specific stages of intestinal development are indicated by the number of E descendants present such as E2, E4, E8, E16 and E20 (Occasionally due to an extra cell division during development the mature intestine is seen to be made of 21 cells instead of the usual 20 (Sulston J. E. and Horvitz H. R., 1977)). The daughters of E, E.a and E.p, migrate into the interior of the embryo initiating gastrulation when the embryo is at the 26-cell stage (Bucher E. A. and Seydoux G., 1994). At E16 stage the intestinal primordium has a ventral tier of 6 cells and a dorsal tier of 10 cells. About 30 min into the E16 stage, cytoplasmic polarization of intestinal cells occurs such that the nuclei of cells move towards and cytoplasmic components move away from the midline (See IntFIG10, IntFIG11) (Leung B. et al, 1999). Shortly after cytoplasmic polarization, cell separation starts at the midline as small gaps, and these small gaps eventually become the lumen of the intestine. Similarly, soon after cytoplasmic polarization, irregularly electron dense vesicles appear in the cytoplasm and localize to the cytoplasmic side. These vesicles are thought to correspond to the birefringent, autofluorescent, intestine-specific gut granules which are suggested to be involved in catabolism. In E16-E20 primordium two ventral cell pairs intercalate between the dorsal cells resulting in a single layer of intestinal cells with bilateral symmetry. As the second cell intercalation occurs, neighboring int II, int III and int IV rings initiate a coordinated 90° rotation around the axis of the midline (IntFIG10). By hatching these intestinal rings make an additional 90° rotation which leads to the twisted appearance of the intestine in the newly hatched larva (Sulston J. E. and Horvitz H. R., 1977- see WA editors' note, however). These cell movements are suggested to cause a superhelical twist of the intestine displacing the anterior half to the left side of the larval body and the posterior half to the right side. The superhelical twist of the intestine, in turn, is suggested to lead to the asymmetrical growth of the gonad later in life (Hermann G. J. et al, 2000). The left-right rotational asymmetry of this twist is determined by the LIN-12/Notch pathway and involves LAG-2, APX-1 and LAG-1 proteins. Also a pathway involving POP-1 and LIT-1 limits this twist to the anterior half of the intestine in the A-P axis.Subsequently the intestinal primordium elongates (Hermann G. J. et al, 2000).

*WA editors' note: It should be noted that after examining a number of larval stage and adult animal EM's, we detected that the cells in each of int rings II-IV are still located dorsoventrally to each other (see IntFIG3 and IntFIG12, for example). This suggests that either the second 90° turn of these cells around the longitudinal axis of the intestine does not take place in some animals or these cells at one point revert back to their dorso-ventral position after making this turn before hatching as reported. Similarly, there is variation in the orientation of rings VI-IX in larval and adult animals. Rings VI-VIII tend to adopt L/R positions while ring IX tends to be a dorso-ventral pair. The forces or developmental processes which influence the positions of these cells in postembryonic life are still unknown.

During epithelial polarization which follows cell intercalation, punctate foci of adherens junction proteins organize into rectilinear junctions surrounding the lumen of the intestine. Through this process, each cell acquires distinctive apical and basal surfaces. During subsequent embryogenesis, the apical membranes of cells between the adherens junctions increase greatly in area as microvilli develop and correspondingly the apical surface of the intestine expands. Also subsequently, the cytoplasmic polarization disappears such that intestinal nuclei are found in the center of the cell cytoplasms (Leung B. et al, 1999).

IntFIG11. Development of the intestine from E8 to L2. Strain marker: pW02H5.8-NLS-GFP; pRF4 , strain source: Molin M., Blomberg A., Pilon M.. A: anterior, P: posterior.
DIC GFP DIC-GFP overlay
DIC GFP

Intestinal cells become binucleate and polyploid during postembryonic development (Hedgecock E. M. and White J. G., 1985). At the beginning of the lethargus of the first molt, most of the intestinal nuclei, except the anteriormost 6, divide without accompanied cell divisions giving rise to 20 intestinal cells with a total of 30-34 nuclei (IntFIG12). After this stage intestinal cells and nuclei continue to increase in size but not in number. Also, the intestinal nuclei go through repeated endoreduplications (chromosome duplication without karyokinesis) increasing the ploidy of each nucleus to 32C by the final molt. These endoreduplications are generally synchronized to each period of lethargus resulting in a 2-fold increase in chromosomal number at the end of each molt.

Intestine structure and function

Intestinal cells are large and cuboidal, with distinct apical, lateral and basal regions. Each intestinal cell forms part of the intestinal lumen at its apical pole, and a basal lamina at its basal pole, while its lateral membranes are sealed apically by large adherens junctions wherever they meet neighboring cells in the epithelium, and communicate to the same cells via gap junctions on their lateral membranes (IntFIG13). The lateral membranes often form a region of tightly folded plasma membranes that may represent another specialized intercellular junction of novel form.

Many microvilli extend into the lumen from the apical face, forming a brush border. The microvilli are rooted into a strong cytoskeletal network of intermediate filaments at their base, the terminal web. There is an extracellular electron-lucent coating (a glycocalyx) over each microvillus, and the cytoplasmic core of each microvillus has a bundle of actin filaments running lengthwise, rooted in the terminal web. The villi may be somewhat shorter in the first int ring than in subsequent cells along the body axis (Sulston J. E. and Horvitz H. R., 1977).

The intestinal cells are each very large, and contain very large nuclei with a prominent nucleolus, many mitochondria, extensive RER and many ribosomes, and a very prominent collection of membrane-bound vesicles and vacuoles (IntFIGs 12 and 14). The nature of these organelles changes gradually as the animal ages. The digestive and metabolic activities of the intestine are central to the growth and development of the animal, and the contents of these organelles must reflect these ongoing functions. They include yolk granules, recycling endosomes, autophagic vacuoles, and autofluorescent (gut) granules. By light microscopy, some of these gut granules become visible as birefringent objects in older adults, and are inferred to be secondary lysosomes involved in catabolism (Clokey G. V. and Jacobson L. A., 1986).

In C. elegans, intestine carries out multiple functions that are executed by distinct organs of higher eukaryotes. The primary function of intestinal cells seems to be digestive since they secrete digestive enzymes (e. g. cysteine protease, endodeoxyribonuclease) into the lumen and take up processed material and nutrients. Intestine also seems to be a large storage organ since it contains a large number of assorted storage granules (White J., 1988). These granules change in size, shape and number during various developmental stages of the animal. In hermaphrodites, it is also involved in synthesis and secretion of yolk material which is then transported to the oocytes through the body cavity (Kimble J. and Sharrock W. J., 1983). The intestinal contents may also play role in miscellaneous functions carried out by nonintestinal cells in higher animals. For instance, the glycosyltransferases may comprise more than 70 genes in the C. elegans genome, and at least some appear to be expressed in the digestive tract (C. elegans Consortium, 1998; Chen S., et al 1999). Along with muscle, intestine is thought to be the major organ where fatty acid metabolism takes place. Through the function of a glyoxylate cyclase (SRH-1) yolk fatty acid-derived acetylcoA is converted to succinate from which carbohydrates are synthesized (Liu F. et al, 1995).

Anatomical and gene expression data both suggest that these functions differ along the length of the organ. For instance, the collection of membrane-bound organelles and vacuoles is more diverse and much more extensive in int rings I and II than further posterior (Borgonie G. et al., 1995). Without histochemical staining, it is still difficult to assign functions to each type of endosome, but open vacuoles of the anterior organ were proposed to release digestive enzymes into the gut lumen. In support of this observation, cysteine protease (CPR-1) expression is restricted to the anterior portions (between int ring I-VI) of the intestine (Britton C. et al, 1998). In posterior portions of the intestine, yolk and lipid vacuoles predominate, and these cells may be more active in nutrient and energy storage. The posterior intestine also functions as the pacemaker of the defecation cycle. In C. elegans, defecation occurs in a rhythmic manner in tightly regulated cycles that are approximately 50 sec long and have three distinct muscle contraction steps (See Defecation behavior in Alimentary System Part III). The Inositol triphosphate (IP3) receptor-drived Ca++ oscillations in the posterior intestinal cells initiates the muscle contractions of the defecation cycle and the IP3 receptor is a central component of the timekeeping mechanism that regulates this behavioral rhythm (Dal Santo P. et al, 1999).

The intestine may change in shape and function rather dramatically in the dauer larva, which do not feed (Popham J. D. and Webster J. M., 1979). The lumen becomes shrunken and the size and number of microvilli are greatly reduced. When the animal emerges from the dauer state, these changes are reversed in the new L4 larva.

List of cells of the intestine

i. Intestinal cells (int)

1. First intestinal ring
int1DL
int1DR
int1VL
int1VR
2. Second intestinal ring
int2D
int2V
3. Third intestinal ring
int3D
int3D.a- postembryonic nuclear division
int3D.p- postembryonic nuclear division
int3V
int3V.a- postembryonic nuclear division
int3V.p- postembryonic nuclear division
3. Fourth intestinal ring
int4D
int4D.a- postembryonic nuclear division
int4D.p- postembryonic nuclear division
int4V
int4V.a- postembryonic nuclear division
int4V.p- postembryonic nuclear division
3. Fifth intestinal ring
int5L
int5L.a- postembryonic nuclear division
int5L.p- postembryonic nuclear division
int5R
int5R.a- postembryonic nuclear division
int5R.p- postembryonic nuclear division
3. Sixth intestinal ring
int6L
int6L.a- postembryonic nuclear division
int6L.p- postembryonic nuclear division
int6R
int6R.a- postembryonic nuclear division
int6R.p- postembryonic nuclear division
3. Seventh intestinal ring
int7L
int7L.a- postembryonic nuclear division
int7L.p- postembryonic nuclear division
int7R
int7R.a- postembryonic nuclear division
int7R.p- postembryonic nuclear division
3. Eighth intestinal ring
int8L
int8L.a- postembryonic nuclear division
int8L.p- postembryonic nuclear division
int8R
int8R.a- postembryonic nuclear division
int8R.p- postembryonic nuclear division
3. Ninth intestinal ring
int9L
int9L.a- postembryonic nuclear division
int9L.p- postembryonic nuclear division
int9R
int9R.a- postembryonic nuclear division
int9R.p- postembryonic nuclear division


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