ALIMENTARY SYSTEM - (Part III ) The Rectum & Anus

General description - Rectal valve- Rectal gland - Rectal epithelium - Enteric muscles - Defecation Behavior- DMP neurons- Cell List - Back to Contents

General Description

The passage from the posterior intestine, from the rectal valve to the opening to the exterior is called the rectum. The opening of the rectum to outside is a wide cuticle-lined passage called the anus. This portion of the posterior alimentary tract contains some large epithelial cells, gland cells, and various muscles (RectFIG1).

Transverse sections from anterior to posterior:

Rectal Valve

The intestinal-rectal valve is formed by two small darkly staining epithelial cells (virL and virR) that occlude the lumen of the posterior intestine, with very narrow channels perforating this occlusion to allow digested material to leak into the rectum and then to the anus. It is not apparent whether these small openings are flexible enough to constitute a true valve, or whether the rectal valve cell can alter the size or state of these openings since rectal valve cells themselves do not seem to posses any contractile elements.

The valve cells are lineally derived from the ABprpapppp cell. Through ablation studies it has been shown that both ABprpapppp and ABplpapppp have the potential to produce valve cells during development. However, in the wild type animal, ABprpapppp cell is specified for this function whereas ABplpapppp gives rise to the rectal epithelial D cell and PVT neuron when the descendants of ABprpa and ABplpa contact each other at the midline after gastrulation. If this cell-cell interaction is blocked, such as in animals with gastrulation defects, both cells then give rise to valve cells (Bucher E. A. and Seydoux G., 1994; Bowerman B. et al, 1992).

Rectal Gland

A ring of three large rectal gland cells (rectD, rectVL, rectVR) connect to the intestinal lumen just posterior to the rectal valve (rectglFIG1 and rectglFIG2). It is possible that these cells secrete digestive enzymes into the caudal lumen of the intestine, which is slightly inflated compared to lumen in the midbody. These three cells lie at the same level or just behind the rectal valve, and their apical specialization facing the lumen produces both microvilli (similar to intestinal cells) and cuticle (similar to transitional epithelia) in discrete patches (Hall, unpublished data).

Rectal Epithelium

Rectum is formed by a number of cells some of which continue to divide during larval stages to produce other cell types (AlimFIG1-3). Some of these cells may also have different fates in the male. Each produces some portion of the cuticular lining of the rectum, and they all contain secretory membrane stacks along this region facing the lumenal cuticle. Their cytoplasmic contents otherwise resemble that of typical hypodermal cells. These cells connect the intestinal-rectal valve to the main body syncytium (hyp 7). The rectal epithelial cells include:
B: rectal epithelial cell in hermaphrodite; acts as postembryonic blast cell in male (see B lineage)
F: rectal epithelial cell in hermaphrodite; acts as postembryonic blast cell in male (see F lineage)
Y (formerly called postembryonic C): acts as rectal epithelial cell in embryonic and early larval stages, later becomes PDA neuron in hermaphrodite; acts as postembryonic blast cell in male.
U (formerly called postembryonic E): rectal epithelial cell in hermaphrodite; acts as postembryonic blast cell in male (see U lineage)
K'
K: until late L1. At late L1, K cell divides. K.a daughter then becomes the rectal epithelial cell. K.p daughter becomes DVB neuron.
(For embryonic locations of these cells, see TailspFIG1 in Specialized epithelial cells Part III; in this figure, the homologue of U is B and of K is K' on the right side of the embryo)

Muscle Cells of the Posterior Gut and Rectum (Enteric muscles)

There are four specialized muscle cells in this region that operate in the defecation cycle; the L/R stomatointestinal muscles (also called the intestinal muscles) , the anal sphincter muscle (also called the anal dilator or rectal muscle) and the anal depressor muscle (also called the depressor ani muscle) (RectFIG5). These three sets of muscles are jointly called enteric muscles and each send an arm to the DVB neuron along dorsal surface of the preanal ganglion. DVB has been shown to make synapses onto the stomatointestinal muscle and the anal depressor muscle and all three sets of muscles are coupled to each other via gap junctions (White et al 1986). Their couple contractions control the enteric muscle contraction (EMC) step of defecation.

The valve is encircled by a single, saddle-shaped muscle, the anal sphincter muscle , which is suggested to open and close the valve (RectFIG 12-13). Its nucleus is located slightly left of the central midline. The sphincter muscle is dilated before the expulsion step of defecation. It contracts almost simultaneously with the other two types of enteric muscles to either further squeeze the intestine during expulsion of gut contents to outside or to restrict the outflow. The anal depressor muscle is a single, H-shaped cell which lies just above the anus and connects the roof of the anal canal to the dorsal bodywall; its contractions act to increase the size of the anal opening by lifting the roof of the rectum and hence facilitate expulsion of intestinal contents (RectFIG 6-8).

Anal depressor muscle:

The stomatointestinal muscles are two cells which encircle the posterior intestine and attach to it and to the thin layer of hypodermis just ventral to the lateral cords (RectFIG 9-10) (Sulston J. E. and Horvitz H. R. 1977). Usually, the somata lie on the right and left sides of the intestine and thin, arm-like extensions go around the posterior part of the intestine and reach towards the hypodermis ( See a 3-D reconstruction of (L) stomatointestinal muscle cell by R. Newbury & Moerman lab. Cell labels are shown in RectFIG9. 3-D movie was created from confocal images of a strain expressing the GFP marker linked to the promoter for D1081.2 using Zeiss LSM 5 Pascal software v. 3.. Their nuclei which look similar to but are smaller than body wall muscle nuclei reside slightly dorsolaterally to the ventral body muscle rows. The cellular anatomy of these muscles can be quite variable, however, where one or two arms may extend beyond the intestine and a cell soma may be located more dorsoventrally in relation to the longitudinal axis of the intestine (see RectFIG11). These cells have longitudinally oriented muscle filaments. Contraction of these muscles promote defecation by pressurizing the intestinal contents near the posterior end of the intestine. These muscles contract simultaneously with the anal depressor and the sphincter muscles during EMC step of Defecation Motor Program (DMP).

Stomatointestinal muscle:


Anal sphincter muscle:

Defecation Behavior

In C. elegans, defecation is achieved through rhythmic activation of a stereotyped cycle of muscle contractions (Liu D. W. C. and Thomas J. H. 1994). It consistently occurs approximately every 50 seconds at variable temperatures and has 5 cycle components: intercycle period , pBoc (posterior body muscle contraction), pBoc relaxation, aBoc (anterior body muscle contraction) and EMC (enteric muscle contraction, also called expulsion (exp) step). In the hermaphrodite, each defecation starts with pBoc which squeezes intestinal contents anteriorly. Approximately 1 sec later, relaxation occurs and intestinal contents flow posteriorly. Next, aBoc is initiated by contraction of the body muscles near the head and gut contents are concentrated near the anus. Finally by contraction of the enteric muscles the expulsion of gut contents to outside is achieved and the intercycle period starts. The motor components of defecation behavior (pBoc, aBoc and EMC) constitute the defecation motor program (DMP). Defects in any of the motor components of DMP leads to constipation. During larval stages the anal structures of the male is identical to those of the hermaphrodite. In the adult male although DMP is similar to the hermaphrodite, the anatomy and control of the anal structures changes drastically.

The steps of the DMP are coordinated in precise temporal and spatial sequences. The ultradian defectation rhythm which can be reset by light touch stimulus to the body is thought to be controlled by an intestinal pacemaker that keeps time and activates the posterior body contraction at the start of each cycle (Dal Santo P. et al, 1999, Siklos S. et al, 2000). An essential element of this pattern generator and time keeper is suggested to be periodic, autonomous calcium release mediated by the inositol trisphosphate (IP3) receptor, ITR-1, in the intestine (Dal Santo P. et al, 1999). The intestinal calcium levels oscillate with the same periodicity as the defecation cycle and they reach their peak levels just prior to the first muscle contraction (pBoc)step. Also, mutations in itr-1 slow down or eliminate the cycle supporting the idea that IP3 receptor activity and calcium release rather than neuronal control is a determinant of defecation cycle frequency. Nevertheless, the GABAergic motor neurons, AVL and DVB are required for the execution of the anterior body contraction and the enteric muscle contractions for expulsion (McIntire et al. 1993, Avery L. and Thomas J. H., 1997). It is suggested that an intercellular signal originating from the calcium oscillations in the intestine may be propagated to initiate the initial posterior body contraction and as well as activating these two neurons for later muscle contractions.

Motor neurons of DMP

Laser ablation of AVL and DVB together eliminate enteric musce contractions. AVL is an interneuron/motorneuron that is located in the head and sends a process to the tail. It carries out its motor function through an excitatory GABAergic signal to the enteric muscles. Killing AVL alone also causes a strong aBoc-defective phenotype. However, this defect is not seen in mutants that lack GABA, hence, AVL is thought to act as an interneuron and utilize a nonGABAergic pathway to activate the motor neurons that control anterior body wall muscle contraction. DVB is a GABAergic motor neuron that is located in the dorsorectal ganglion in tail. It is born postembryonically at late L1 stage and is the daughter of the K rectal epithelial cell. It extends a prominent process anteriorly which passes through a commissure beneath the depressor muscle and extends forward along the top of the ventral hypodermal ridge (In IntFIG4 above enteric muscle processes are seen in the vicinity of DVB). Around this region its large axon is seen to be filled with synaptic vesicles and make periodic synapses to muscle arms from enteric muscles. The excitatory GABAergic signal from AVL and DVB is thought to be mediated by the nonselective cation-channel type GABA receptor EXP-1 since exp-1 mutants lack enteric muscle contractions and are phenotypically constipated (Beg A. A. and Jorgensen E., 2002, Thomas J. H., 1990)

List of cells of the rectum and anus

i. Intestinal-Rectal Valve Cells

virL
virR

ii. Rectal Gland Cells

rectD
rectVL
rectVR

iii. Rectal Epithelial Cells

B
F
Y

U

K'
K
K.a (Rect_hyp)

iv. Enteric muscles

1- Stomatointestinal muscle
mu intL
mu intR
2. Anal Sphincter muscle
mu sph
3. Anal depressor muscle
mu anal

v. Enteric neurons

AVL
DVB


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