Hindgut
From IKE
The hindgut is the posterior part of the alimentary canal, including the rectum, and sometimes the large intestine (1).
Development
- The proximal part of the hindgut (distal transverse colon), together with its dorsal mesentery, falls to the left, resulting in the distal transverse colon lying horizontally and the descending colon occupying the left side of the peritoneal cavity
- The descending colon gives way, in the lower, left quadrant, to the sigmoid colon
- The sigmoid colon pursues an S-shaped course that returns it to the midline
- The rectum and anal canal, representing the terminal parts of the hindgut, remain in the midline
- Initially, the distal hindgut opens into the cloaca, a sac which also has the allantois entering it
- During the 5th week, a mesodermal septum (urorectal septum) begins to grow between the allantois and the hindgut, dividing the cloaca into an anterior urogenital sinus and a posterior anorectal canal
- The cloacal membrane also divides, forming a urogenital membrane and an anal membrane
- The ectoderm immediately adjacent to the anal membrane sinks inwards to form the anal pit, so that, following breakdown of the anal membrane (about 8 weeks), the distal part of the anal canal, lined by ectoderm (somatic origin) has a different nerve and blood supply from the proximal part, lined by endoderm (visceral origin - supplied by the inferior mesenteric artery).
