The colon is responsible for the recovery of water and electrolytes (predominately salts) from the faeces, and the propulsion of increasingly solid matter to the rectum, prior to defecation.   Consequently, the wall of colon is thick and muscular and capable of powerful peristaltic movements.   As in other areas of the intestine, the large bowel wall is composed of four layers.   The contents and functions of the various layers of the bowel wall are as follows:

1.      Mucosa – The colonic mucosa is flat, and has no villi.   The columnar epithelium of the mucosa is arranged in tightly packed, straight, non-branching tubular glands or crypts, which in the normal state, resemble a test tube.   These extend all the way to the muscularis mucosa, which marks the junction between the mucosa and submucosa.  

a.      Epithelium – The epithelium consists predominately of two cell types – absorptive cells and mucous cells.   Whilst the function of the absorptive cells is obvious, the mucous cells are important in lubricating an increasingly solid faecal mass and protecting the delicate lining of the colon from trauma.   The absorptive cells are as tall and columnar with ovoid basal nuclei, whilst the secretory cell nucleus is smaller and more condensed.   Secretory cells dominate in the base of the gland, whilst the absorptive cells occupy the luminal surface.   Stem cells at the base of the gland continually replace the epithelium;

b.      Lamina propria – The lamina propria fills the gap between crypts and the small space just above the underlying muscularis mucosa.   They contain the extensive lymphatic system of the colonic wall, and consequently have a diffuse infiltrate of lymphocytes and plasma cells.   These form lymphoid aggregates.   The colon wall also contains an extensive vascular network to allow the passive diffusion of water across the membrane;

c.      Muscularis mucosa – The muscularis mucosa delineates the mucosa from the submucosa.   It is a thin layer of smooth muscle that contracts rhythmically to prevent obstruction of the glands and enhance mucous expulsion.

2.      Submucosa – The submucosa is a loose connective tissue layer with little inflammatory cell infiltrate.   It contains the submucosal plexus of Meissner, which is responsible for the secretory function of the gut.;

3.      Muscularis propria – Contains two muscular layers separated by the myenteric plexus of Auerbach, which mediates peristalsis.   The muscular layers of the large bowel are the inner circular layer, and outer longitudinal layer.   Whilst the circular layer is continuous with the small bowel, the longitudinal layer is attenuated into three bands of muscle that help in delineating the small bowel from the large.   These are the tinea coli.   Other features differentiating the large bowel from the small macroscopically are:

i.    Calibre of the lumen;

ii.      Presence of haustra;

iii.       Absence of the plicae circularis.

4.      Serosa – The serosa is the protective outer coating of the bowel, a single layer of flat/low cuboidal mesothelial cells and associated adjacent fibroelastic tissue that should appear shiny and smooth in health.

 

 

 

References

 

•         Underwood JCE.   ‘General and Systematic Pathology’ (4^th Edition).   Elsevier Churchill Livingstone (2004).   Chapter 15: Alimentary System; Pg. 377 – 378;

•         Young B and Heath JW.   ‘Wheater’s Functional Histology’ (4^th Edition).   Elsevier Churchill Livingstone (2000).   Chapter 14: Gastrointestinal Tract; Pg. 270.   

•         Kumar V, Abbas AK, Fausto N.   ‘Robbins and Cotran Pathologic Basis of Disease’ (7^th Edition); Elsevier Saunders (2005).   Chapter 17: The Gastrointestinal Tract; Pg. 828 – 830.

•         McLean C.   ‘Clinical Pathology Case Studies’.   Monash University (2004).   Semester 5, Case 7.