Colon, Cecum, and Appendix

The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. The colon also contains many mucus-secreting glands to lubricate the stool passing through it. The colon receives its blood supply from colic branches of the superior and inferior mesenteric arteries, which form an important anastomosis along the transverse colon. The colon is regulated by the ANS and receives both sympathetic (inhibitory) and parasympathetic (stimulatory) input.

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Development

The large intestines develop from the primitive midgut and hindgut:

  • Midgut: cecum to distal transverse colon
  • Hindgut: distal transverse colon to dentate line in anorectum
  • The primitive gut tube grows and develops:
    • Connected to the posterior abdominal wall by the dorsal mesentery
    • Blood supply located within mesentery
Development of the dorsal mesentery with the primitive gut tube

Development of the gut tube

Image by Lecturio.

Gross Anatomy

Parts of the large intestine

  • Cecum
  • Vermiform appendix
  • Ascending colon
  • Transverse colon
  • Descending colon
  • Sigmoid colon
  • Rectum
  • Anal canal

Mesenteries and intraperitoneal versus retroperitoneal location

  • Intestinal mesenteries are folds of peritoneum that connect these organs to the posterior abdominal wall.
  • Some parts of the large intestine have a mesentery → intraperitoneal
  • Some parts do not have a mesentery and are located beneath parietal peritoneum → retroperitoneal
  • Secondarily retroperitoneal: organs that had a mesentery in utero and that regressed early in development, leaving the organ retroperitoneal in its mature position

General characteristics of the colon

  • Forms an inverted U shape in the abdomen
  • About 1.5–2 m long
  • At the inferior margins of stomach and liver
  • Surrounds the small intestine
  • Begins at the ileum
  • Receives chyme from the ileum through the ileocecal valves
  • Ends at the anus
  • Greater omentum hangs over the transverse colon.
  • Includes ascending, transverse, descending, and sigmoid parts
Colon blood supply, anterior view

Image demonstrating the anatomy of the large intestine in situ

Image by BioDigital, edited by Lecturio

Cecum

  • 1st part of the large intestine
  • Approximately 7.5–9 cm in length and breadth
  • Intestinal pouch between the terminal ileum (at the ileocecal junction) and the ascending colon
  • Located in the iliac fossa of the RLQ of the abdomen
  • Covered on all sides by peritoneum (= intraperitoneal), though has no mesentery (↓ mobility)
  • Contains the opening into the appendix (appendiceal orifice)
Posterior view of the cecum

Posterior view of the cecum:
Location of the vermiform appendix at the taenia confluence

Image by Lecturio. License: CC BY-NC-SA 4.0

Appendix

  • Appendix: blind intestinal diverticulum off the cecum
  • Approximately 6–10 cm in length
  • Contains significant lymphoid tissue
  • Base arises from the posteromedial aspect of the cecum, inferior to the ileocecal junction.
  • Location of tail may be:
    • Retrocecal (65%)
    • Pelvic (30%)
    • Subcecal (2%)
    • Preileal (2%)
    • Postileal (1%)
  • Mesoappendix: 
    • Short triangular mesentery that contains the appendicular vessels
    • Mesentery: intraperitoneal and higher mobility
  • McBurney point: 
    • Surface projection of the base of the appendix 
    • About ⅓ of the way between the anterior superior iliac spine and the umbilicus (closer to the anterior superior iliac spine)

Ascending colon

  • 2nd part of the large intestine
  • Narrower than the cecum
  • Passes superiorly from the cecum to the right lobe of the liver 
  • Right colic flexure (also known as the hepatic flexure): end of the ascending colon where it makes a 90-degree turn toward the midline and becomes the transverse colon
  • Secondarily retroperitoneal organ → no mesentery → less mobility

Transverse colon

  • The 3rd, longest, and most mobile part of the large intestine
  • Runs transversely across the mid-upper abdomen
  • Begins at the liver/right colic flexure (hepatic flexure) 
  • Ends at the spleen/left colic flexure (also known as the splenic flexure), where it makes a 90-degree turn inferiorly and becomes the descending colon
  • Covered by the greater omentum
  • Divides the greater sac of the peritoneal cavity into supracolic and infracolic compartments
  • Transverse mesocolon: 
    • Mesentery for the transverse colon 
    • Mesentery: intraperitoneal structure, ↑ mobility

Descending colon

  • Runs between the left colic (splenic) flexure and the left iliac fossa, where it is continuous with the sigmoid colon 
  • Secondarily retroperitoneal organ → no mesentery → less mobility

Sigmoid colon

  • S-shaped loop of variable lengths 
  • Runs between the descending colon and the rectum
  • Sigmoid mesocolon: 
    • Mesentery of the sigmoid colon 
    • Mesentery: intraperitoneal structure, ↑ mobility

Gross anatomic features unique to the colon

Several anatomic features distinguish the large intestine from the small intestine and rectum, including:

  • Taenia coli: 
    • 3 discrete bands of longitudinal muscle in the colonic wall (rather than a continuous longitudinal layer in the muscularis as seen in the small intestine): 
      • Omental taenia: attaches to the greater omentum
      • Mesocolic taenia: attaches to the transverse mesocolon (which anchors the transverse colon to the posterior abdominal wall)
      • Free taenia coli: not attached to other structures
    • Contract to form the haustra
  • Haustra: 
    • Sacculations in the colon created by contraction of the longitudinal taenia coli 
    • Internally, these sacculations are separated by semilunar folds (protrusions of mucosa into the lumen)
  • Omental appendices (also called epiploic appendages): small, fatty, omentum-like projections
Diagram depicting taenia coli, haustra, and the omental appendices

Diagram depicting taenia coli, haustra, and the omental appendices:
Note how the transverse mesocolon attaches to the mesocolic taenia.

Image by Lecturio.

Microscopic Anatomy

Overview of the wall structure

  • The walls of the large intestine are thinner than those of the small intestine.
  • The general layers are the same: mucosa → submucosa → muscularis propria → serosa
Layers of the colon wall

Layers of the colon wall

Image by Lecturio.

Mucosa

  • 3 sublayers:
    • Columnar epithelium
    • Lamina propria
    • Muscularis mucosae
  • Villi are absent.
  • Microvilli (also known as the brush border): present, but less abundant than in the small intestine 
  • Intestinal glands (also called intestinal crypts):
    • Pores in the wall, which open into tube-like glands.
    • Contain a large number of goblet cells: 
      • Mucin-producing epithelial cells
      • More prominent in large intestine than in small intestine
    • Produce copious amounts of mucus for lubrication of feces
  • MALT: 
    • Significant lymphoid tissue present in the lamina propria 
    • Most abundant in the appendix
Histology of the colon

Histology of the colon:
Note the many tube-like glands within the lamina propria.

Image: “The histologies of the large intestine and small intestine” by OpenStax College. License: CC BY 4.0

Submucosa

  • Loose connective tissue
  • Contains:
    • Larger vessels
    • Meissner nerve plexus (ganglia of the ANS): controls the muscularis mucosa (independent of the muscularis propria)

Muscularis propria

The primary muscular layer of the colon wall.

  • Inner circular layer
  • Outer longitudinal muscles:
    • Not a continuous layer
    • Present in only the 3 bands of tissue, known as taenia coli:
      • Omental taenia coli
      • Mesocolic taenia coli
      • Free taenia coli
  • Contains the myenteric (Auerbach) plexus:
    • Ganglia of the ANS, which controls the muscular layer.
    • Located between 2 layers of smooth muscle

Serosa

  • Made up of connective tissue
  • Continuous with visceral peritoneum 
  • Connected to mesentery or peritoneum

Neurovasculature

Arterial supply

The arterial supply is via the superior mesenteric artery (SMA) and the inferior mesenteric artery (IMA).

  • SMA branches (starting in the RLQ and moving clockwise):
    • Ileocolic artery:
      • Colic branch → cecum
      • Appendicular artery (off the colic branch) → appendix
      • Ileal branch → terminal ileum
    • Right colic artery → ascending colon
    • Middle colic artery:
      • Supplies the ascending and transverse colons
      • Continuous with the marginal artery
    • Marginal artery (of Drummond):
      • Supplies the transverse colon
      • Forms an important anastomosis between the middle colic artery (off the SMA) and the left colic artery (off the IMA)
    • Jejunal and ileal arteries → jejunum and ileum
  • IMA branches (starting in the LUQ and moving clockwise):
    • Left colic artery:
      • Supplies the descending colon
      • Anastomoses with the marginal artery, connecting circulation from the SMA and IMA
    • Sigmoid arteries → sigmoid colon
    • Superior rectal artery → upper rectum
  • Vasa recta: small vessels off the major vessel that directly supply the bowel wall.
Blood supply of the large intestine

Blood supply of the large intestine

Image by Lecturio.

Venous drainage

The cecum, appendix, and colon drain via named veins that run parallel to their arteries, which ultimately drain into the superior mesenteric vein (SMV) and the inferior mesenteric vein (IMV).

  • SMV:
    • Receives drainage from:
      • Right ½ of the large intestines
      • Via the ileocolic and right and middle colic veins
    • Joins with the splenic vein to form the hepatic portal vein
  • IMV:
    • Receives drainage from:
      • Left ½ of the large intestines
      • Via the superior rectal, sigmoid, and left colic veins
    • Drains into the splenic vein → hepatic portal vein
Venous drainage of the colon

Venous drainage of the colon

Image by Lecturio.

Lymphatic drainage

Via multiple abdominal lymph nodes:

  • Epicolic
  • Paracolic
  • Ileocolic
  • Right, middle, and left colic lymph nodes

Innervation

The colon is innervated by the ANS, which has parasympathetic and sympathetic divisions.

  • Parasympathetic innervation:
    • Stimulatory
    • Via branches of the:
      • Vagus nerve
      • Pelvic splanchnic nerves
  • Sympathetic innervation:
    • Inhibitory
    • Via the superior and inferior mesenteric plexuses

Function

  • Water absorption
  • Vitamin production by bacterial flora (and absorption):
    • Vitamin K
    • Several B vitamins
  • Compaction of feces
  • Storage of feces
  • Moving waste materials toward the rectum for expulsion

Clinical Relevance

  • Appendicitis: acute inflammation of the appendix caused by obstruction of the lumen, typically due to calcified feces, tumors, or foreign bodies. Appendicitis is the most common abdominal surgical emergency worldwide; it classically presents with periumbilical abdominal pain that migrates to the RLQ, fever, anorexia, nausea, and vomiting. Management is usually surgical.
  • Diverticulosis: An intestinal diverticulum is a sac-like bulge of the intestinal wall protruding outward. Diverticula arise because of the increased intestinal pressures associated with a low-fiber diet and when the transport of intestinal content is slower. The condition is typically asymptomatic.
  • Diverticulitis: inflammation of diverticula that occurs when the diverticula become occluded. Diverticulitis often presents with lower abdominal pain and changes in bowel habits and may become complicated by abscess, perforation, fistula, and bowel obstruction.
  • Ischemic colitis: ischemia of the colon due to hypoperfusion that may result from vessel occlusion (e.g., arterial thromboembolic events), hypovolemic shock, sepsis, or traumatic vessel laceration. The SMA is most commonly affected. Classically, presentation is with sudden onset of pain that is out of proportion to physical findings. The primary goal of management is to restore blood flow.
  • Colon polyps: A polyp is an overgrowth of mucosal tissue in the large intestine. A polyp may be sessile (wide and flat), branched, or pedunculated. Polyps are usually < 1 cm and do not cause any symptoms, though they may cause constipation, diarrhea, or blood in the stool. Polyps can be classified as neoplastic or nonneoplastic. Hyperplastic polyps are nonneoplastic, while adenomas are the most common type of neoplastic polyp and have the potential to progress to cancer. Polyps may be associated with genetic syndromes.
  • Colorectal cancer (CRC): malignant tumor of the colon and/or rectum. Almost all cases of CRC are adenocarcinoma, and the majority of lesions come from the malignant transformation of an adenomatous polyp. Colorectal cancer is usually asymptomatic, with symptoms such as occult bleeding typically developing late. The prognosis depends on the stage of cancer at discovery.
  • Malrotation of the gut: congenital anomaly resulting from abnormal rotation of the primitive gut tube during development. There are a number of variants of this condition, but typically the entire large intestine is located in the left ½ of the abdomen and the small intestines are located in the right ½. Malrotation of the gut may be asymptomatic, or it may result in significant complications, such as midgut volvulus.
  • Large bowel obstruction: interruption in the normal flow of intestinal contents through the colon and rectum. This obstruction may be mechanical (due to actual physical occlusion of the lumen) or functional (due to a loss of normal peristalsis, also known as pseudo-obstruction). Malignancy and volvulus are the most common causes of mechanical large bowel obstruction. The typical symptoms include intermittent lower abdominal pain, abdominal distention, and obstipation.
  • Irritable bowel syndrome (IBS): functional bowel disease characterized by chronic abdominal pain and altered bowel habits without an identifiable organic cause. Symptoms of IBS may include digestive problems along with abdominal pain, diarrhea, or constipation. This syndrome is associated with gluten sensitivity and psychological factors and is a diagnosis of exclusion.

References

  1. Kapoor, V. (2016). Large intestine anatomy. Medscape. Retrieved September 3, 2021, from https://emedicine.medscape.com/article/1948929-overview#a1 
  2. Kahai, P., Mandiga P., Wehrle C. (2021). Anatomy, abdomen and pelvis, large intestine. StatPearls. Retrieved September 2, 2021, from https://www.statpearls.com/ArticleLibrary/viewarticle/32116
  3. Richard, L., et al. (2019). Abdominal Viscera – Organs. In Gray’s Anatomy for Students, 4th ed. Churchill Livingstone/Elsevier, pp. Pages 316–327.
  4. Moore, K. L., Dalley, A. F., Agur, A. M. R. (2014). Clinically Oriented Anatomy, 7th ed. Lippincott Williams & Wilkins.

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