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Descending Colon

Related terms:

Cecum

Abdomen

Bladder

Large Intestine

Small Intestine

Artery

Rectum

Ascending Colon

Sigmoid

Transverse Colon

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Abdominal and Pelvic Anatomy

Luis M. Chiva, Javier Magrina, in Principles of Gynecologic Oncology Surgery, 2018

Descending Colon

The descending colon, similar to the ascending colon, is also retroperitoneal; it is covered along its anterior and lateral surfaces by peritoneum. It measures approximately 22 to 30 cm in length and is located within the anterior left pararenal space. The descending colon is predominantly fixed. It is smaller in caliber and more posteriorly situated on the opposite side of the ascending colon. The caudal portion of the descending colon just distal to the iliac crest sits in the left iliac fossa and is often referred to as the iliac colon. Similar to the ascending colon, the descending colon does not have a true mesentery. Its mesocolon attaches medially to connective tissue anterior to the aorta and contains the inferior mesenteric vein and mesocolic marginal vessels.

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Small Bowel and Large Bowel Resection and Anastomosis

Oliver Zivanovic, Yukio Sonoda, in Principles of Gynecologic Oncology Surgery, 2018

Left Hemicolectomy in Conjunction With Omental Disease

The descending colon is retracted medially after incising along the white line of Toldt, and the splenic flexure is mobilized by dividing the phrenocolic and lienocolic ligaments. Care must be taken during splenic flexure mobilization to avoid excessive downward traction, which could result in capsular tear to the spleen. The left colic artery is carefully isolated, ligated, and divided distal to its origin from the IMA. The middle colic artery should be identified and preserved; however, the left-sided branch may be incorporated within the scope of the resection. The intestine is then divided by using two applications of the linear GIA stapling device or between clamps. The anastomosis can be completed by means of one of several techniques with a stapled or hand-sewn closure. An EEA between the transverse colon and sigmoid colon is commonly selected.

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Gastrointestinal Specimens (Including Hepatobiliary and Pancreatic Specimens)

Susan C. Lester MD, PhD, in Manual of Surgical Pathology (Third Edition), 2009

Diverticulosis

of the descending colon and sigmoid is a common disease and is resected after multiple episodes of diverticulitis (Fig. 19-6 and Box 19-1). The muscularis propria becomes markedly hypertrophied (presumably due to long-term straining at stool) resulting in a thickened bowel wall and a narrowed lumen. The increased intraluminal pressure causes herniation of mucosa through weak points in the muscularis propria adjacent to the penetrating vasculature on either side of each taenia coli. These are false diverticula because they lack a complete muscular coat. True diverticula (e.g., Meckel diverticulum or a solitary cecal diverticulum) have a complete muscle coat and are thought to be congenital in origin.

The best demonstration of diverticula requires inflating an intact specimen with formalin. Close off the ends with hemostats or twine and fix overnight. Open along the antimesenteric side or hemisect. The fat should not be stripped from the specimen, as this will also remove the diverticula.

A metal probe can be used to find the ostia of the diverticula. Count the number of diverticula or estimate the number if there are many. Sections of diverticula can be obtained by cutting in the plane of the probe. Sample areas of interdiverticular mucosa (two to three sections) to look for superimposed diverticulosis-associated colitis, inflammatory bowel disease or ischemia.

If the history is of diverticulitis or there is gross evidence of perforation (induration of pericolonic fat, a serosal exudate, hemorrhage, pericolonic necrosis), the perforated diverticulum should be identified. Probe the diverticula in the most inflamed area and cut cross sections. A perforated diverticulum will show effacement of the mucosa associated with necrosis and hemorrhage in the surrounding soft tissue. If the wall of the diverticulum can be seen, then it is not perforated and it is just an adjacent diverticulum surrounded by the inflammation. Submit sections of all diverticula that appear to be inflamed. Peritonitis without a documented site of perforation can be a medical emergency because the perforation is presumably still within the patient.

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Fecal Incontinence

Nick Cave, in Canine and Feline Gastroenterology, 2013

Awareness of the Need to Defecate

As feces form in the descending colon, occasional peristaltic waves push material distally into the rectum. In dogs, feces arriving from the colon are halted at the rectocolonic junction by sphincteric pressure, thus stool stops short of the rectocolonic junction instead of passing directly to the rectum.6 Rectal tone is maintained mostly through sympathetic innervation.7 Expansion of the rectal walls stimulates stretch receptors that send afferent signals through the pelvic nerve to ascending spinal cord tracts that ultimately lead to the frontal cortex. This leads to the awareness of the urge to defecate. Afferent fibers in the pudendal nerve convey the sensation to reinforce urge when fecal material extends to the level of the anus. A reflex arc is present whereby even slight distention of the rectum of humans or dogs results in relaxation of the IAS. This is the rectosphincteric inhibition reflex.8 This arc is suppressed by upper motor neurons (UMNs) until the conscious decision is made to initiate defecation. Thus a disturbance of descending UMNs decreases inhibition. Contraction of the IAS is unlikely to be important in maintaining continence when pressure in the rectum becomes high, but may help guard against incontinence of small amounts of liquid stool. Additionally, reflex relaxation of the IAS in response to rectal distention allows for conscious "sampling" of the rectal contents and helps distinguish flatus from feces.

If the urge is not acted upon, material in the rectum is often returned to the colon where more water is absorbed. This is an act of conscious suppression that is reliant upon intact neural pathways from rectum to frontal cortex.

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Biology and Diseases of Mice

Mark T. Whary DVM, PhD, DACLAM, ... Stephen W. Barthold DVM, PhD, Diplomate ACVP, in Laboratory Animal Medicine (Third Edition), 2015

Pathology

C. rodentium attaches to the mucosa of the descending colon and displaces the normal flora. Attachment is accompanied by effacement of the microvillus border and formation of pedestal-like structures (attaching and effacing lesions) (Schauer and Falkow, 1993; Newman et al., 1999). Colonization results in prominent mucosal hyperplasia, by unknown mechanisms. The characteristic gross finding is severe thickening of the descending colon, which may extend to the transverse colon and lasts for 2–3 weeks in surviving animals (Fig. 3.38). Affected colon segments are rigid and either are empty or contain semiformed feces. Histologically, accelerated mitotic activity results in a markedly hyperplastic mucosa, which may be associated with secondary inflammation and ulceration (Fig. 3.39). Lesions subside after several weeks. Intestinal repair is rapid and complete in adults but slower in sucklings.



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Figure 3.38. Colons of a normal mouse (right) and of a mouse with transmissible murine colonic hyperplasia (left). The descending colon is thickened and opaque because of mucosal hyperplasia.

From Barthold et al. (1978).



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Figure 3.39. Colonic inflammation, edema, mild hyperplasia of the epithelium, and significant development of mucosa-associated lymphoid tissue (MALT) caused by C. rodentium infection.

Courtesy of Suresh Muthupalani.

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Volume 1

B. Florien Westendorp, ... Gijs R. van den Brink, in Physiology of the Gastrointestinal Tract (Sixth Edition), 2018

4.3.4 Hh Signaling and Development of the Hindgut

The hindgut forms the distal transverse colon, descending colon, sigmoid, and anorectum that develop around the inferior mesentery artery. Very low levels of Shh are expressed at the base of the crypts in the colon at 18.5 d.p.c.,113 whereas Ihh mRNA is expressed throughout the epithelial layer from 11.5 d.p.c. until birth.111,113 In Ihh−/− mice, the epithelial cells fail to organize into a polarized monolayer, and the epithelium does not organize into crypt-like structures.151 The colon is partially dilated in Ihh−/− with an abnormally thin wall and lack of neurons in the dilated areas, a phenotype reminiscent of Hirschprung's disease.113 These data suggest that signaling by Ihh plays a role in the conversion of the pseudostratified endodermal layer to an epithelial monolayer in the hindgut and regulates the survival of neural crest cells that have migrated into the intestinal mesoderm. Signaling by Shh is important in the distal hindgut as Shh null mice were shown to have anorectal malformations.113,152 The anorectum develops with a complex interplay of all three germ layers as it forms at the junction of the endoderm and ectoderm.153,154 Ramalho-Santos et al. find an imperforate anus in their Shh−/− mice, the colon ends in a blind sack and does not form a connection with the perineum.113 Shh−/− mice maintained by Mo et al. display a failure to separate the anorectum from the lower urinary tract, that drain in a common cavity or outlet.152 Gli2−/− Gli3+/− and Gli2+/− Gli3−/− compound mutant mice also have a persistent cloaca and a common outlet for the digestive and urogenital tract but a phenotype that is less severe than Shh−/− mice. Gli2−/− mice have an imperforate anus with a recto-vesical fistula (between the distal intestine and the bladder) and a single urethral opening in the perineum, see also Kim et al.155 The phenotype of Gli3−/− mice is very mild with slight anal stenosis.152 These phenotypes indicate that although Gli3 does play a role in the transduction of the Shh signal to the anorectal mesenchyme, the contribution of Gli2 is much more important similar to the role of Gli signaling in foregut development.

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Diagnostic Imaging of the Gastrointestinal Tract

In Canine and Feline Gastroenterology, 2013

Anatomic Considerations

The large intestine is organized anatomically into the cecum, colon (ascending, transverse, and descending portions), rectum, and anal canal. The large intestine is most readily thought of as a distensible, thin-walled (2- to 4-mm) tube (see Chapters 1 and 58Chapter 1Chapter 58). The cecum is located to the right of the midline at approximately the level of L3 and is a gas-filled sigmoid or corkscrew-shaped structure in the dog, and a conically shaped end of the ascending colon in the cat. In most cases the ascending colon is to the right of midline. The transverse colon passes from right to left cranial to the root of the mesentery. The descending colon is located to the left of the midline and then courses toward the midline when entering the pelvic canal, where it may be displaced toward the right by a distended urinary bladder. The colon is held relatively loosely by the mesocolon and its position in the dorsal- or mid-abdomen can therefore be variable. In the normal canine and feline patient, the colon contains a variable amount of gas and formed fecal material and is generally visible and traceable on survey radiographs. The rectum is located midway between the ventral surface of the sacrum and the floor of the pelvis on the lateral view, and on the midline on the VD view. The colonic and rectal diameters can be quite variable, including regional areas of narrowing and dilation, depending upon the amount of feces. However the colon is approximately twice the diameter of the small intestine. The upper limit of normal colonic diameter should be considered to be less than 1.5 times the length of the body of L7. Reproducible focal narrowing or dilation should always be noted, particularly when fecal material accumulates proximal to the narrowing. This finding would be suspicious for a tumor or stricture and should be further evaluated, especially if persistent over time.

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Differential Diagnosis and Clinical Reasoning

F.D. Pociask PT, PhD, OCS, J.R. Krauss PT, PhD, OCS, in Orthopaedic Physical Therapy Secrets (Third Edition), 2017

47 List the structures contained in each of the four abdominal quadrants.

Right Upper QuadrantLeft Upper QuadrantAscending colon (superior portion)Descending colon (superior portion)DuodenumJejunum and proximal ileumGallbladderLeft colic (hepatic) flexureLiver (right lobe)Left kidneyPancreas (head)Left suprarenal glandRight colic (hepatic) flexureLiver (left lobe)Right kidneyPancreas (body and tail portions)Right suprarenal glandSpleenStomach (pylorus)StomachTransverse colon (right half)Transverse colon (left half)Right Lower QuadrantLeft Lower QuadrantAscending colon (inferior portion)Descending colon (inferior portion)CecumLeft ovaryIleumLeft spermatic cord (abdominal portion)Right ovaryLeft ureter (abdominal portion)Right spermatic cord (abdominal portion)Left uterine tubeRight ureter (abdominal portion)Sigmoid colonRight uterine tubeUrinary bladder (only when full)Urinary bladder (only when full)Uterus (if enlarged)Uterus (only when enlarged)Vermiform appendixRenal

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Organogenesis of the Gastrointestinal Tract

Maxime M. Mahe, ... Noah F. Shroyer, in Fetal and Neonatal Physiology (Fifth Edition), 2017

Organogenesis

The hindgut gives rise to the distal third of the transverse colon, descending colon, sigmoid colon, and rectum, all of which receive blood from the inferior mesenteric artery.

The hindgut originates as the caudal intestinal portal grows both in length and circumference. By the fourth week the portion of the hindgut lying adjacent to the cloacal membrane forms a cavity lined by endoderm and is surrounded by mesenchyme called the cloaca. The cloaca is common to the anorectal and urogenital canals. Between the fourth week and the sixth week the anorectal and urogenital canals arise by septation of the cloaca; the cloaca is partitioned into a posterior rectum and an anterior primitive urogenital sinus by the growth of the urorectal septum. The distal edge of the urorectal septum fuses with the cloacal membrane, dividing the membrane into an anterior urogenital membrane and a posterior anal membrane. The zone of fusion between the urorectal septum and the cloacal membrane becomes the perineum. By the twelfth week the anal, vaginal, and urethral canals have formed.67-69

After the large intestine returns to the abdominal cavity, the dorsal mesenteries of the ascending colon and descending colon shorten and fold, bringing these organs into contact with the dorsal body wall, where they adhere and become secondarily retroperitoneal. The cecum is suspended from the dorsal body wall by a shortened mesentery soon after it returns to the abdominal cavity. The transverse colon does not become fixed to the body wall but remains an intraperitoneal organ suspended by the mesentery. Pressure from this organ may help to fix the underlying duodenum to the body wall. The most inferior portion of the colon, the sigmoid colon, also remains suspended by the mesentery. The last part of the hindgut forms the rectum.3

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