It is the transverse folding during the 4th week which will form the gut tube along the ventral midline
The gut tube is lined with endoderm (which gives rise to the GI epithelium) and is closed by bilaminar membranes:
stomoderm (at the cranial end) and proctoderm (at the caudal end) // bilaminar = ectoderm + endoderm
Lateral mesoderm gives rise to the walls of the cavities and muscles of the GI tract
The gut tube initially has an open lumen. Proliferation of epithelium occludes opening, later recanalization reopens.
If not fully reopened (recanalized) stenosis (a narrowing of the lumen) can occur, where food can get caught and occlude.
If there is an interruption of blood supply to developing parts in the GI tract they may not form.
Foregut
arterial supply from the CELIAC TRUNK
esophagus, stomach, duodenum(1st and 2nd), liver, biliary apparatus, and pancreas (also pharynx and lower respiratory system)
The spleen is not an outgrowth of foregut. It develops in the dorsal mesentery from mesenchyme.
Rotation
due to differential growth: the spleen migrates to the left, the stomach rotates so that the left side faces anteriorly and the right side posteriorly (e.g. left vagus innervates anterior wall) // lesser curve faces right, greater faces left
The gut tube seperates the ventral mesogastrium from the dorsal mesogastrium.
From the liver to the stomach = lesser omentum
From the liver to the anterior wall = falciform ligament
From the stomach to the spleen = gastolienal (gastrosplenic) ligament
From the spleen to the posterior wall = lienorenal (splenorenal) ligament
Pancreas
2 buds (one dorsal one ventral) form. Ventral bud migrates dorsally (behind duodenum) bringing its duct (and the bile duct) into their proper position and forming the uncinate process (Latin for "hook"). The ventral duct contributes the Main duodenal papilla, to which the dorsal pancreatic buds duct joins in addition to its accessory pancreatic duct.
Annular pancreas
when the pancreas surrounds the duodenum causing stenosis, due to a bifid ventral bud (one wrapping anteriorly, the other posteriorly, forming a ring around the duodenum)
Secondarily retroperitoneal
the pancreas and duodenum (2nd, 3rd, and 4th) fuse to the posterior wall.
Midgut
arterial supply via the Superior Mesenteric artery
development of the midgut is characterized by the rapid elongation of the gut and its mesentery resulting in the formation of the primary intestinal loop. At its apex the loop remains in open connection with the yolk sac via the Vitalline duct. As a result of the rapid growth and expansion of the liver the abdominal cavity is temporarily too small to contain all the intestine. The excess enters the extraembryonic coelom during the 6th week and returns during the 10th week rotating counterclockwise to accommodate the extensive elongation.
Omphalocele
- when the developing gut is not pulled back into the body all the way. Distinguished from an umbilical hernia, in that an omphalocele is covered only with amnion, an umbilical hernia is covered by skin.
The Cephalic limb of the primary loop
Þ distal duodenum, jejunum, and some ileum
The Caudal limb of the primary loop
Þ lower ileum, cecum, appendix, ascending colon, and 2/3rds of the transverse colon.
The Vitalline Duct exits at the umbilicus. If it does not completely regress
Þ Meckels diverticulum, this is a problem because the retained ligament may interfere with gut twisting or the ectopic gastric epithelium in diverticulum bleeds.
Also Vitalline cyst (cyst in the retained ligament) or Vitalline fistula (open communication of GI with the umbilicus)
The caudal loop rotates 180
° counterclockwise to end up on top of the cephalic loop. (this puts the transverse colon above the small intestine) rotates another 90° once inside; 270° total
Problems non-rotation, colon on left side, small bowel on right; clockwise rotation, duodenum is over the colon.
Fusion of the duodenum to the posterior wall will compress the colon and reduce motility of the colon.
Hindgut
arterial supply from the inferior mesenteric artery
Distal 1/3rd of transverse colon, descending colon, sigmoid colon, rectum, and superior portion of the rectal canal.
Develops from the Caudal loop, it is the last part of the bowel to reenter the body following rotation
Problems:
Imperforate anus, irregular location, etc: from incomplete separation of cloaca to urogenital and anorectal parts
Umbilical folds
Lateral peritoneal folds: raised by the inferior epigastric vessels.(indirect hernias are lateral to the inf. epigastric)
Medial peritoneal folds: remnants of umbilical arteries
Median peritoneal fold: remnant of the fetal urachus. (alternative source of urine in fetal life)
Testis
form high in the body attached to the Gubernaculum and migrates down behind peritoneum
vaginal process gets brought down with the testis communication between tunica vaginalis and peritoneum fuses
scrotum retains abdominal wall layers. (indirect hernias travel down this passage if fusion is incomplete)