An understanding of the anatomy of the large intestine is useful for individuals with intestinal disorders, students of biology or medicine, and those persons with risk factors that can result in life-threating problems of the intestines. Examples of these risk factors include a hypercoaguable state or a family history of colon cancer. Basic understanding of the embryological origin of the large bowel makes it much simpler to learn the anatomy of the large intestine.
While reading this document, you may find that referring to images and diagrams helps with visualization and memorization (just google the terms below).
Embryology of the large intestine:
1. Midgut
The embryonic midgut forms the small intestines up to the level of the mid-transverse colon. Technically, the intestine is not considered to begin as the large intestine until passing the ileocecal valve that is located at the junction between the ileum of the small intestine and the cecum.
2. Hindgut
The embryonic hindgut forms the rest of the colon, proximal anus, as well as the lower genitourinary tract). The hindgut forms in the 6th week of fetal development. This is when the cloaca (a single tube for poo and pee – like birds!) splits apart due to the anorectal septum, forming the genitourinary sinus and the rectum.
3. Distal anus
The distal anus forms differently than the rest of the gut because it forms from ectodermal tissue. This differentiation between the proximal and distal anus is marked by an anatomical line called the dentate line.
4. Embryological abnormalities
In development, the midgut loop rotates 270 degrees counterclockwise around the axis of the superior mesenteric artery. This is the artery that supplies blood to the midgut. A developmental problem that occurs in the fetus is malrotation or failure of the right colon to elongate. This often becomes symptomatic in the newborn period.
Blood supply to the large intestine:
The blood supply to all the parts of the large intestine and rectum can be understood by remembering the blood supply to the three embryological origins. As mentioned above, the superior mesenteric artery SMA supplies the body parts that form from the midgut. The hindgut is supplied by the inferior mesenteric artery IMA, which means the colon and proximal anus, derived from the hindgut, are supplied by the IMA.
Once again the distal anus is different than the rest of the large intesting. Instead of a mesenteric artery supply, the distal anus is supplied by the internal pudendal artery branches.
