Let us examine the histological makeup of the colon first in the context of other tubular systems and then review its makeup with a focus on how it functions.

Like many other tubes in the body its wall has 4 layers.  The inner layer that lines the lumen is called the mucosa which rests on its basement membrane. The mucosa consists of a single layer of rectangular cells and the layer is called an epithelium.  Because of the rectangular shape of the cells they are called columnar cells and so the layer is called a columnar epithelium.  The second layer is the submucosa which contains blood vessels, lymphatics, nerves and loose connective tissue.  The third layer is the muscular layer called the muscularis and the fourth layer is called the serosa or adventitia which acts as a protective outer “skin” for the colon.  This layer is called a serosa when it is surrounded by peritoneum (transverse colon and sigmoid colon) and called an adventitia when it is retroperitoneal.  (ascending colon, descending colon and part of the rectum)



The 4 basic layers of the colon
This diagram illustrates the 4 basic layers of the colon.  The inner pink layer is the mucosa, the yellow layer beneath the mucosa is called the submucosa, while the red layer is the muscular layer (muscularis) and the 4th layer is called the serosa or adventitia.

Courtesy Ashley Davidoff MD




Ultrasound of normal large bowel
This ultrasound image of the colon demonstrates the 4 layers of the large bowel quite beautifully.  The luminal contents are heterogeneous and are bordered by an echogenic line that is the combination of the mucosa (pink) and submucosa. (cream) The lucent line we see is the muscular layer (red/orange) while the outer echogenic line (blue) represents the serosa or adventitia.

Courtesy Philips Medical Systems

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The mucosa
The mucosa consists of a simple columnar epithelium which means it is a single layer of epithelial cells that are each shaped like a column – similar to a tall rectangular block of apartments.

Courtesy Ashley Davidoff MD





An artistic rendition of columnar epithelium

An artistic rendition of a simple columnar epithelium (left) is compared to the columnar appearance of a block of apartments. The photograph taken from the Kennedy Museum in Boston frames the columnar shaped apartments. (right)

Courtesy Ashley Davidoff MD

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The submucosa contains the vital lifelines including the terminal branches of the arteries, the capillary network, the smallest and earliest venous branches and the lymphatics.  There are nerve endings in the submucosa as well as the muscular layer.  Circulating white cells that perform the vital local defense functions police the submucosa for potentially harmful organisms and substances.  The muscularis contains spindle shaped smooth muscle cells while the serosa and adventitia consist of relatively strong connective tissue, and in the case of the serosa a layer of epithelial cells derived from the peritoneal lining.



Mucosa submucosa, muscularis and serosa (adventitia)
The lifelines together with the small rounded lymphoid cells are illustrated in submucosa. A capillary network between arteries and veins forms creating the end of one part of the circulation and the beginning of the next.  The spindle shaped smooth muscle cells are illustrated in the red muscular layer and the outer layer in white is made of connective tissue and in the case of the serosa a layer of flat epithelial cells derived from the peritoneum.

Courtesy Ashley Davidoff MD



In order to optimize water absorption the mucosa of the large bowel is thrown into innumerable folds creating the crypts of Lieberkuhn which look like epithelial lined test tubes.  Unlike the small bowel the mucosa does not contain villous projections, but rather these mucosal infolds called crypts which contain mucus secreting and water absorbing cells.

By creating tightly packed infoldings, the surface of the colon is increased significantly.  Macroscopically the mucosal surface of the colon is mostly smooth except for folds called the plicae circulares which are formed by the pleating effect of the taenia coli.


Histology of the mucosa, submucosa and muscularis
The histological section shows three of the four layers of the colon – mucosa on the luminal side, submucosa of loose connective tissue and the muscularis.  Note how the mucosa is thrown into folds of tightly packed crypts resembling a line of test tubes.

Courtesy Barbara Banner MD




Normal Colonic Mucosa
This is a low power photo micrograph of normal colon mucosa illustrating extremely orderly, rigid, “test tube”-like arrangement of the straight tubular glands which comprise the colonic crypts.

Courtesy Barbara Banner MD



The wall of the colon requires a very substantial vascular supply and effective nerve and lymphatic system in order to coordinate its function with other physiological events occurring within the gastrointestinal system specifically and within the body at large.  The neural, lymphatic, and vascular channels enter through supporting mesentery and penetrate the wall (just like the cable systems, water systems, electrical and sewerage enter and leave our homes).  These penetrations produce weakening of the wall and remain the potential site for diverticuli to develop.  The vessels terminate in the submucosa.



The submucosa
Mesenteric arteries, (red) mesenteric veins, (blue) mesenteric nerves (yellow) and lymphatics (not shown) penetrate the wall of the colon and terminate or connect in the submucosal layer.  Lymphoid cells acting as protective policemen scour the submucosa for unwanted elements that get absorbed through the bowel wall.

Courtesy Ashley Davidoff MD



Vascular entry into the transverse colon
The coronal reformat of an abdominal CT shows branches of the middle colic artery in the transverse mesocolon just about to penetrate the wall of the transverse colon.  There is a hint in the middle of the section of transverse colon of smaller vessels running along the mucosal folds.

Courtesy Ashley Davidoff MD



The mucosa and wall of the bowel are not isolated structures.  John Donne a clergyman and poet from the renaissance period stated that “… No man is an island …”  In the same way no organ is an island

Each is connected and dependant on the other organs and the colon and its substructures are no different.  It is not surprising therefore that disease of the colon for example is associated with disease in other parts of the body.  The association of ulcerative colitis and sclerosing cholangitis is a prime example.  The psyche and the bowel are also intimately related.  Acute anxiety is notorious for inducing diarrhea.


Colonic connections
No man or woman is an island as we all are in some way connected and dependant on one another.  In the same way the colon is connected to the other organs as well as to hormonal, neural, and psychological influences.  The colon should always be reviewed in the context of the whole.

This diagram shows the colon in relation to some of the organs of the body

Courtesy Ashley Davidoff MD



The proximal large bowel is presented with the sludge like by products of small bowel digestion consisting of mostly water, and undigested complex carbohydrates. Colonic mucosa absorbs most of the water and together with churning action and bacterial digestion, the nature of the sludgy semi liquid stool is transformed into more solid and compacted stool.


Mucosal function

In the above image the fluid filled chyme represented in the left part of image, is transformed into the gas filled compacted solid stool seen on the right side of image.

Courtesy Ashley Davidoff MD