this lecture describes the development of the gut tube starting from the infolding of the yolk sac in the development and positioning of the different sections of the intestines and stomach. early development: the epithelium and mucosa are derived from the endoderm layer while the muscularis is derived from the mesoderm layer. the gut tube is surrounded by two peritoneal sacs that form the visceral and parietal peritoneum. the dorsal mesogastrium is the section of the peritoneum early in development that connects the gut tube to the posterior abdominal wall. the greater omentum is part of the peritoneal layer that folds down over the intestines and serves as a repository for visceral fat (as well as having an active role in the immune system). the mesentary is the portion of the dorsal mesogastrium that attaches to the posterior gut tube and contains neurovascular bundles within its layers.
the gut tube itself undergoes several revolutions and many convolutions during its development, starting with a 90 degree rotation that positions the stomach to the left and the liver to the right of the abdominal cavity. the midgut then herniates and forms a U shaped loop in the 6th week, within which the small intestine develops. the formation of this long section of gut tube causes a bulging of the tube into the vitelline duct (?) and a subsequent entry into the abdominal cavity. the order of re-entry determines whether the section of the gut tube is classified as retroperitoneal or intraperitoneal-- retroperitoneal sections are affixed to the posterior abdominal wall and includes the duodenum, ascending, and descending colons. intraperitoneal are loosely suspended by mesentary and includes the small intestine and transverse colon. if the abdominal cavity closes before reentry of the gut tube is complete, this can result in a persistent vitelline duct, which might lead to such pathologies as vitelline cyst, vitelline fistula, or diverticulosis.
the gut tube is divided into three sections: foregut, midgut, hindgut. each section corresponds to multiple sections of the GI tract as we know it and also corresponds to a different major artery. for example, the foregut includes the stomach and duodenum and has blood supplied by the celiac artery. the midgut includes the jejunum, ileum, and ascending/transverse large intestine, and has blood supplied by the superior mesenteric artery. the hindgut includes the transverse and descending large intestine, rectum, and anal canal. as mentioned before, the mesentery that suspends the small intestine contains neurovascular bundles within its folds; there are two types of arteries that supply blood to the gut- vasa recta and arcade arteries. vasa recta arteries are more prevalent in the jejunum, arcades in the ileum.
sympathetic activity can constrict arteries, either in response to stress or a drop in blood pressure and activation of the RAAS system, decreasing blood and oxygen flow to the intestinal mucosa. eventually, the autoregulatory escape mechanism kicks in and brings the blood pressure in the intestines back to normal even with continued sympathetic activity. the sympathetic response to stress can ultimately result in toxemia due to the weakened epithelial wall (because of reduced oxygen flow) allowing in more microorganisms and toxins. this same result can come about due to ischemia caused by decreased cardiac output or decreased blood pressure as well.
questions
basic structures...
1. what is the gut tube made from? when does it start to develop?
2. the epithelium and mucosa of the gut tube are derived from...
3. smooth muscle is derived from...
4. describe the origin of the peritoneum.
5. what is the dorsal mesogastrium? what does it form?
6. what is the greater omentum? what are some of its properties?
7. spleen splits the greater omentum into...
8. describe the formation of the mesentery.
9. what travels between the layers of mesentery?
rotation and differentiation...
10. describe what happens in peritoneal rotation.
11. how does the large intestine come to surround the small intestine?
12. what does it mean for a section of the gut tube to be retroperitoneal and what is an example?
13. what does it mean for a section of the gut tube to be intraperitoneal and what is an example?
14. what determines whether a section of the gut tube will be retroperitoneal vs. intraperitoneal?
15. what is a persistent vitelline duct and what pathologies can it result in?
16. what is omphalocoele?
gut divisions and blood supply...
17. which arteries define the three sections of the GI tract?
18. what does the foregut form?
19. what does the midgut form?
20. what does the hindgut form?
21. which branches of the superior mesenteric artery supply the jejunum and ileum?
22. which branches of the superior mesenteric artery supply the ascending and transverse colon?
23. what are the two types of arteries that branch off and anastamose in the jejunum and ileum?
24. sympathetics constrict arteries in response to...
25. what is autoregulatory escape?
26. what are the two ways in which ischemia of the gut can occur?
27. how can toxemia result from ischemia of the gut?
answers
1. from the yolk sac during the 4th week.
2. endoderm
3. mesoderm
4. during the 5th and 6th weeks, two peritoneal sacs press against either side of the liver and stomach and form the visceral and parietal peritoneum.
5. the dorsal section of the peritoneal fold- forms the greater omentum and mesentery.
6. part of the dorsal mesogastrium that folds down over the intestine and is a repository for visceral fat. also has strong immune stimulating properties.
7. gastrolienal and lienorenal ligaments.
8. mesentary is formed from the portion of the dorsal mesogastrium that is attached to the posterior wall of the stomach.
9. neurovascular bundles to the visceral organs.
10. due to the 90 degree rotation of the peritoneal cavity, the stomach ends up to the left and the liver to the right (whereas they were both in the center before)
11. the midgut "herniates" and forms a U shaped loop during the 6th week, rotating 270 degrees around the superior mesenteric artery.
12. the portions that are affixed to the posterior wall of the abdominal wall and partially covered by peritoneum: examples are the ascending and descending large instestine and the duodenum.
13. the portion that is surrounded by the visceral peritoneum and is relatively mobile, suspended by mesentary. example is the small intestine and transverse colon.
14. the order of "reentry" into the abdominal cavity.
15. the vestiges of the vitelline duct which can result in a connection between the ileum and abdominal wall- potentially causing meckel's diverticulum, vitelline cyst, or a vitelline fistula.
16. a section of intestine trapped by early closing of the abdominal cavity before full retraction.
17. celiac artery: foregut, superior mesenteric: midgut, inferior mesenteric: hindgut.
18. stomach, duodenum
19. jejunum, ileum, proximal large intestine (ascending and transverse)
20. transverse, descending, sigmoid large intestine, rectum, anal canal.
21. sequential branches
22. ileocolic, right and middle colic arteries.
23. vasa recta in jejunum, arcades in ileum.
24. exercise or a drop in blood pressure
25. a compensatory mechanism which will allow vasodilation to offset excess sympathetic activity.
26. either by occlusion of the mesenteric arteries or decreased cardiac output/continuous vasoconstriction.
27. toxemia can develop by bacterial / toxin entry into intestinal epithelium which might occur when oxygen flow to the mucosa is decreased because blood flow is decreased (oxygen diffuses from arterioles to venules rather than to mucosa)
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