University:
Stanford University
Course:
MED 101 | Human Anatomy
Academic year:

2023
Views:

48

Pages:

1
Author:

larbi43100

EVOLUTION OF THE POSTERIOR PRIMITIVE INTESTINE The primitive hindgut is the portion of the digestive tract that follows the midgut and ends at the cloacal membrane. This part of the digestive tract is irrigated by the inferior mesenteric artery. Upstream of the cloacal membrane, the hindgut is in communication with the allantois, their common segment forming the cloaca. The evolution of the intestine posterior consists of a moderate elongation of its proximal part at the origin of the distal third of the transverse colon, the descending colon, and the sigmoid (distal part of the colon, before the rectum) and in a partitioning of the cloaca separating the digestive tracts (rectum and anal canal) and urogenital (primary urogenital sinus). 1. Cloaca partitioning This septation results from the appearance of a uro-rectal septum in the angle formed by the allantois and the posterior intestine. This partition results from a proliferation of the mesenchyme which is inserted between the two structures derived from the endoderm and progresses in a caudal direction until it comes into contact with the cloacal membrane. This septum is reinforced by two lateral bulges. At the 7th week, this septum divides the cloaca into the primary urogenital sinus anteriorly and the anorectal canal posteriorly. The area where the urorectal septum meets the cloacal membrane forms the perineum and the initial cloacal membrane then gives rise to two distinct areas: the urogenital membrane in front and the anal membrane behind. As soon as the anal membrane is individualized, a mesenchymal rim appears which surrounds it and leads to the formation of a depression: the anal depression (or proctodeum). At the 8th week, the anal membrane disappears and puts the posterior intestine in communication with the outside. In this way, the anal canal has a dual origin, endodermal for its proximal 2/3 and ectodermal for its distal 1/3. This embryological duality explains the double vascularization of the anal canal, branches of the inferior mesenteric artery for its upper part and branches of the iliac arteries for its terminal part. 2. Evolution of the Allantoid Around the 6th week, when the urorectal septum is individualized, the middle part of the allantois expands to form the bladder, its distal portion regresses leaving a ligament remaining which connects the top of the bladder and the umbilicus: the urachus. Its most proximal part stretches to form the pelvic and membranous urethra.