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Development of the GI System (Part II) Lane Anatomy Block II Unit II


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What does the midgut give rise to? (6)
Distal part of the duodenum, jejunum, ileum, and the large intestine up to and including the proximal 2/3rs of the transverse colon. Also gives rise to the appendix.
What artery supplies all derivatives of the midgut?
The superior mesenteric artery.
Describe the formation of the midgut loop. What part of the umbilical cord does it involve?
The midgut elongation exceeds the capacity of the peritoneal cavity (intraembryonic coelom). This produces a physiological iumbilical hernaition of the midgut into the extraembryonic coelom in the proximal part of the umbilical cord.
What does the cranial limb of the midgut loop form?
The small intestine.
WHat does the caudal limb of the midgut form?
The terminal part of the ileum and the large intestine.
What is an ilial diverticulum?
A remnant of the proximal part of the yolk sac (2-4% of the population).
What is the first rotation that the midgut makes and in what direction?
90 degrees counter clockwise. (from an anterior view)
What part of the midgut loop first returns to the abdomen?
The small intestine thus occupying the central part of the abdomen.
After the initial 90 degree turn, to what degree and direction as well as when does the next rotation occur? What is the new placement of the structures of the foregut?
As the large intestine returns, the midgut rotates 180 degrees counterclockwise. This rotation positions the cecum and ascending colon on the right side of the abdomen and the transverse colon in a horizontal orientation just inferior to the liver.
What does the cecum arise from?
The cecal diverticulum.
What dues the appendix arise from?
A diverticulum at the base of the cecum.
In what structures of the large intestine do the visceral and parietal peritoneom fuse to the abdominal wall?
In the ascending and descending colon. Also cecum.
What is directly to the left and right of the dexcending colon?
The left paracolic gutters.
What develops 90 degrees from the terminal ileum on the cecum?
THe appendix
To where do the tenia coli extend proximally?
To the base of the appendix
What is malrotation of the gut?
Any anomaly with improper or incomplete rotation and fixation of the primary intestinal loop.
What is reverse rotation? What is the net rotation?
Where does the transverse colon end up?
The primary intentinal loop rotates clockwise (instead of counter) resulting in the trnasverse colon being located posterior to the duodenum and superior mesenteric vessels.
What is non rotation of the gut? Which rotation does not occur?
The 180 degree rotation does not occur resulting in the large intestines on the left and the small intestines on the right side of the abdominal cavity (AKA left side colon).
WHat is mixed rotation and volvulus? What anchors the cecum and in what manner?
Incomplete rotation results in the cecum being located just inferior to the pylorus of the stomach. CT bands passing over the duodenum anchor the cecum. This also occurs frequently with volvus of the intestine.
What is midgut volvus and how does it occur?
The small intestine fails to reenter the abdominal cavity properly. Only the duodenum and proximal part of the colon become fixed to the posterior abdominal wall.
What happens during subhepatic cecum and appendix?
The cecum becomes fixed to the liver.
WHat is mobil cecum result from? What are the symptoms?
Results form incompete fixation of the ascending colon. Increased risk volvus.
WHat is omphalocele?
Failure of a portion of the intestines to return to the abdominal cavity on week 10. The hernaiated masss issurrounded by the epithelium of the umbilical cord.
What are the 2 kinds of midline herniation?
Umbilical and Internal hernias.
What is umbilical hernia? When does it develop?
Develops after the successful return of the intestine to the abdominal cavity. It is due to a defect in closure of the umbilicus. The herniated mass is surounded by skin.
What occurs during an internal hernia?
A loop of small intestine pushes into the mesentery.
What is gastroschisis? Why does it develop?
A hernia of abdominal viscera through the lateral wall of the abdomen. Due to a defect in closure of the lateral folds during week 4 when the lateral walls are formed.
What is duplication of the intestine? What are the 2 kinds?
Either cystic or tubular in form. Due to a defect in recanalization.
WHat is a merkel diverticulum? Why does it occur?
A merkel diverticulum is a 3-6 cm long outpocketing of the ileum. IT may be attached to the umbilicus. It is due to a persistant yolk stalk. It may contain ectopic gastric or pancreatic tissue.

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Ileal diverticulum with a cord

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Diverticulum with patent yolk stalk (vitelline duct)

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Ileal (Meckel) diverticulum with ulcer.

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Internal hernia

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Left side colon or nonrotation

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Meckel diverticulum

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Midgut volvulus.

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Mixed rotation and volvulus.

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Mixed rotation and volvulus.

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Persistent vitelline artery

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Reverse rotation

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Subhepatic cecum

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Vitelline and umbilical cysts

Identify the defect.
Volvulus of diverticulum
What compresses the transverse colon during reverse rotation?
Duodenum and superior mesenteric artery.
During cystic duplication of the small intestine does the cyst communicate with the small intestine? How about during tubular diverticulum?
During cystic duplication there is no communication but there is/can be with the tubular type.
What does the hindgut give rise to?
The left 1/3rd of the transverse colon, the descending colon, sigmoid colon, rectum, and superior portion of the anal canal. Also epithelium of the bladder and most of the urethra.
What are all derivatives of the hindgut supplied by?
The inferior mesenteric artery.
What is the cloaca? What is in contact with? What overlies it?
The cloaca is the terminal part of the hindgut lined by endoderm. THe cloacal membrane is in contact with the surface ectoderm. THe proctodeum (anal pit) is the region of the surface ectoderm overlying the cloacal membrane.
What does the cloaca recieve? What is this structure?
The allantois is an extension of the yolk sac that is continuous with the region of the urogenital that will become the urinary bladder.
What does the allantoes become?
The median umbilical fold or urachus.
What does the urorectal septum divide the cloaca into?
A ventral urogenital region and a dorsal anal rectal region.
What does the ventral part of the cloacal sphincter become?
All the muscles of the urogenital triangle.
What does the dorsal part of the cloacal sphincter become?
The external anal sphincter.
Describe the 2 sourcess of the anal canal.
Upper 2/3 from hindgut
Lower 1/3 derived from the proctodeum.
What represents the border between the upper 2/3 of the anal canal and the lower 1/3 of the canal?
The pectinate line (also divides hindgut and proctodeum).
What fuses with the cloacal membrane?
The urorectal septum.
Describe the features (nervous, arterial supply, pathologies) for the anal canal above the pectinate line.
Above the pectinate line - autonomic nerves, pain via sympathetics, and inferior mesenteric artery branches. Internal hemorroids can occur here.
Describe the features (nervous, arterial supply, pathologies) for the anal canal below the pectinate line.
Pain fibers via spinal nerves, internal iliac artery supplies, below pectinate line, external hemorroids occur.
What is megacolon? What is it due to?
MEga colon is an excessively dilated large intestine. It is frequenly associated with hirshprung disease, an absence of myenteric ganglia.
What is an imperforate anus?
No orifice/anal canal (absence of normal anal opening.
What is anal stenosis?
Narrowing of the anus and anal canal due to a dorsal deviation of the urorectal septum.
What is membranous atresia of the anus due to?
A failure of the anal membrane to perforate on week 8 (imperforate anus).
What is rectal atresia? What is it due to? Compare it to anal atresia?
Further up from anus atresia. It is due to recanalization error or defective blood supply.

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Anal agenesis with a perineal fistula

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ANal agenesis with rectovaginal fistula.

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Anal stenosis

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Anorectal agenesis with rectourethral fistula

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Membranous anal atresia (imperforate anus)

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Persistent cloaca.

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Rectal atresia.

Identify the defect.
Rectal atresia (higher up).

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