Editing Colon and rectum (section)

== '''Embryology''' ==

** Primitive gut tube is formed from the endodermal roof of the yolk sac. In the third week, it divides into foregut, midgut, and hindgut.
*** Midgut - includes distal duodenum, small bowel, right colon, and proximal two-thirds of the transverse colon - supplied by SMA
*** Hindgut - distal third of transverse colon, descending and sigmoid colons, and rectum, down to upper anal canal - supplied by IMA
** Caecal bud appears at about the 6th week
*** Last part of the bowel to re-enter the abdomen after physiological herniation
** Appendix - mid-gut organ - first identified at 8-20 weeks as a small outpouching of the caecum. Becomes more elongated and tubular as the caecum rotates medially and becomes fixed in the RLQ.
** Rectum
*** Proximal - develops similar to the colon
*** Distal - develops from the terminal hindgut that enters into the cloaca (an endoderm-lined cavity in contact with the surface ectoderm at the cloacal membrane). The cloaca separates into an anterior urogenital sinus and a posterior distal rectum and anal sinus at about 5 weeks.
*** External anal sphincter formed by the posterior part of the cloacal sphincter, and the internal anal sphincter is formed from enlarging circular fibres of the rectum.
** Anal canal
*** See separate topic