Embryology

The primitive intestine begins to form in the third week of gestation. It arises secondary to ventral folding of the embryonic yolk sac and results in a tubular structure, lined with endoderm (ultimately forming the colonic mucosa) and covered with mesoderm (from which arises the surrounding muscle and serosa) [1]. This subsequently develops into foregut, midgut and hindgut regions, an understanding of which allows an appreciation of each section’s resultant blood supply, lymphatic drainage and neuronal innervation. The colon is derived from the midgut and hindgut regions (the midgut spanning from the second part of the duodenum to the middle third of the transverse colon and the hindgut extending from the middle third of the transverse colon to the rectum) [2].

Structure

The colon begins at the caecum and terminates with the rectum. It comprises six sections: caecum, ascending colon, transverse colon, descending colon, sigmoid colon and rectum. The junction of the ascending and transverse colon is commonly referred to as the hepatic flexure and the junction of the transverse and descending colon as the splenic flexure.

The caecum lies in the right iliac fossa, is completely covered by peritoneum and is therefore intraperitoneal. It is approximately 6 cm in length, without mesentery and is relatively mobile. Longitudinal muscle bands called teniae coli (which continue throughout the colon) converge at the base of the appendix, a vestigial organ that originates from its posterior surface.

The ascending colon is a continuation of the caecum. This extends upwards along the lateral side of the abdominal wall towards the right upper quadrant of the abdomen. It is approximately 15 cm in length and is covered on its anterior and lateral surfaces by peritoneum (therefore considered retroperitoneal). Once it has reached the inferior surface of the right lobe of the liver, it turns to form the transverse colon, which passes in front of the second part of the duodenum and the head of the pancreas. Following a further turn beneath the spleen, it carries on to become the descending colon, which lies retroperitoneally on the left lateral side of the abdominal wall and is approximately 30 cm long. As the colon continues into the pelvis, it becomes known as the sigmoid colon, which finally terminates as the rectum [3].

Aside from its location at the periphery of the abdominal cavity, the colon may be characterised by the presence of teniae coli and appendices epiploicae (small fatty tags attached to the serosa surface). It is thrown into concertina-like saccular folds referred to as haustra, which are thought to be important for mixing of intestinal contents.

Vascular supply

The vascular supply of the colon is determined by its embryological origin. Structures derived from the midgut receive arterial supply from branches of the superior mesenteric artery and are drained by tributaries of the superior mesenteric vein and thence into the portal system. Distal to the middle third of the transverse colon (hindgut in origin), tissue receives arterial supply derived from the inferior mesenteric artery. Similarly, venous drainage is via tributaries of the inferior mesenteric vein (which also subsequently drains into the portal system).

Neuronal innervation

Colonic innervation is derived from four sources: the enteric, extrinsic afferent, sympathetic and parasympathetic nervous systems.

The enteric nervous system is composed of a number of nerve plexi within the GI wall and is principally responsibly for regulation of colonic motility. The two major plexi are the myenteric plexus and the submucosal plexus. The interstitial cells of Cajal provide the functional link between the nerve processes of the plexi and the muscle cells.

The extrinsic afferent nerves provide sensory innervation of the colon and rectum. The proximal colon receives this supply from the vagus nerve and the distal colon and rectum receive this supply from S1 and S2. It is thought that this innervation is primarily responsible for the conscious perception of rectal filling as well as the initiation of propulsion required for defaecation [4].

Sympathetic and parasympathetic supplies also act to modulate sensory and motor activity. Midgut structures derive this innervation from the superior mesenteric plexus and hindgut structures from the pelvic splanchnic nerves via the inferior mesenteric plexus. Generally speaking, parasympathetic activity exhibits an excitatory effect on colonic function, increasing colonic motility and secretory activity. By contrast, sympathetic activity inhibits colonic tone and motility [5].

Propulsion of intestinal contents

The term ‘colonic motility’ is used to describe the mixing and propulsive movements of the colon that allow for digestion, absorption and transit of intraluminal contents.

The mechanisms responsible for absorption in the colon are slow and the colonic microbiota are facilitated by the speed and orientation of mixing movements. Distal propulsion of contents is therefore gradual to allow for mixing and uniform contact with the colonic mucosa. Contents take roughly 12–30 h to traverse the length of the colon, compared to 2–4 h in the small intestine (which is four or five times greater in length).

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