• After folding, endoderm is still connected to yolk sac via yolk stalk..

yolk stalk = vitelline duct = omphalomesenteric duct

Yolk stalk-marks the place known as MIDGUT

NOTE: pharynx is the most rostral portion of foregut

Foregut-supplied by the celiac artery

Midgut-supplied by the superior mesenteric artery

Hindgut-supplied by the inferior mesenteric artery

•          We like to think that the rotation of the gut induces some of the rotation of the caudal part of the gut
•          The foregut goes from the orapharyngeal membrane to a portion of the duodenum-there’s a specialization in the area-area of the stomach

Mesentery-the same tissue that lines that the body-like peritoneum-at two places on the foregut, the posterior midline and anterior midline, peritoneum leaves body and goes out the viscera, folding it, then it goes to the other side body wall

•          there’s anterior & posterior mesentery
•          Gut rotates so that its posterior midline faces left and it’s anterior midline faces right-puts the left vagus anterior and the right vagus posterior

SMA-directed towards the stump of the yolk stalk-as the yolk stalk regress, this artery gets shorter and shorter

 

MIDGUT ROTATION-Occurs in two phases:

1) Gut herniates into the umbilical cord-around 6^th week, start of the 7^th.

•          As this happens, it is arranged in two cords, one is above the SMA, and the other is below the SMA
•          ABOVE SMA-PROXIMAL LOOP
•          BELOW SMA-DISTAL LOOP
•          obviously, the SMA is the axis around which the gut is rotating 270 degrees
•          As the gut goes out, the proximal rotates right 90 degrees and the distal loop rotates left 90 degrees-describes twisting

2) Gut Retracts-At the end of the period of herniation, when gut comes back into the body, the distal loop starts coming in first on the right side of body, causing a 180 degree rotation-total rotation is 270 degrees

•          Looking at person from front, rotation is counter-clockwise

Omphalocoele-persistent herniation of the gut-the gut fails to retract into the abdomen

•          The proximal loop of the gut ends up posterior to the transverse colon
•          SMA will pass anterior to a portion of the duodenum
•          future duodenum is posterior to the superior mesenteric artery

Ø       At the same time as this rotation, other foregut structures are being formed

 

FOREGUT STRUCTURES = digestive tract and abdominal portions of the digestive tract

•          foregut includes the esophagus, stomach, half of duodenum, liver, pancreas, and the biliary apparatus
•          these structures are attached to the 2^nd part of the duodenum-their site of “emptying” into the duodenum is in the foregut
•          Their opening into the duodenum is via the major duodenal papilla-yeah, it’s the same thing as Vater
•          LAST thing in the foregut is the MAJOR DUODENAL PAPILLA

MIDGUT-the 3^rd and 4^th portions of the duodenum + jejunum, ileum, ascending colon, and part of the transverse

HINDGUT-begins roughly at splenic flexure of the colon

•          That portion which DID NOT HERNIATE during the midgut herniation

Spleen is in the abdomen but it’s MESODERMAL derived-it’s out on mesentery but it’s NOT a derivative of the abdomen

Pancreas-it can begin to encircle the duodenum

•          there’s a dorsal and ventral pancreatic bud-as the gut rotates, the two buds are brought together so that they fuse-usually there is one dominant duct remaining
•          after the stomach rotates 90 degrees, the two buds fuse-the ventral bud becomes the uncinate process and part of the head while the dorsal bud becomes the rest of the pancreas

CLEFT PALATES

VENTRAL PANCREATIC BUD-uncinate process and part of head

POSTERIOR PANCREATIC BUD-body of pancreas

•          Main duct is Wirsung-opens into the 2^nd part of the duodenum in the major duodenal papilla, along with the common bile duct
•          The dorsal duct is the duct of Santorini-this is the accessory duct

VENTRAL DUCT-Wirsung                          

DORSAL DUCT-Santorini

•          The entire major pancreatic duct is called “Wirsung”
•          Santorini is proximal to Wirsung

 

HEPATIC PLATE-thickening that grows anteriorly from the duodenal wall towards the septum transversum

Ø       Liver is simply an outgrowth from the distal part of the foregut-wall of the endothelium of the tube

Ø       Biliary apparatus-it grows into the mesentery & invades the septum transversum

Ø       Livers grows into the mesentery and grows between two leves,

•          goes out toward inferior surface of the diaphragm
•          then pushes through the septum transversum, creating the bare area of the liver

 

Bare Area-is such that endodermal tissue is touching mesodermal tissue without any intervening border-this is where the coronary ligament forms-this ligament is the combination of all the peritoneums that surround the bare area

 

CLINICAL CORRELATION: Annular Pancreas-rotation of pancreas doesn’t occur as it should and the pancreas surrounds duodenum

•          annular pancreas can cause problems if it compresses the duodenal lumen

Philo jumps to UG for a minute, then leaves

Descent of the testis

•          testis develop posterior to peritoneal membrane & need to get out into scrotum b/c sperm don’t like it hot, hot, hot
•          as testis grows out it pulls a little bit of peritoneal cavity, forming the parietal and visceral layers of the tunica vaginalis

 

PERITONEUM: As gut grOWS into the peritoneal cavity, it is covered by peritoneum and is separated out into ventral & dorsal mesentery.

Peritoneal Tissue-there’s a parietal and a visceral peritoneum-visceral covers gut….please say you knew that

Mesentery-twin-walled leaf of peritoneum, containing within it fat

•          there is a dorsal/posterior mesentery and ventral/anterior mesentery

Dorsal Mesentery-CONTAINS THE ARTERIES going to the gut from the aorta

Ventral Mesentery-is lost except for that of the foregut-liver & biliary apparatus, part of duodenum, stomach, and the most terminal portion of the esophagus retain it

•          jejunum & ileum have none

Peritoneal-organ suspended by mesentery-it’s covered by peritoneum and suspended by mesentery

•          stomach, cecum, jejunum, ileum, transverse colon, and sigmoid colon

 

RETROPERITONEAL-not peritoneal-abdominal organ NOT suspended by mesentery

•          Retroperitoneal = Preperitoneal = Extraperitoneal……….terminology can be mixed but it’s really all the same
•          ascending and descending colon, rectum-retroperitoneal

Primary Retroperitoneal-those organs that NEVER had a mesentery suspending it, i.e., kidney

Secondarily Retroperitoneal-organs that once had a mesentery and then lost it, i.e., duodenum or pancreas

 

Ø       Stomach begins to rotate as gut is herniating and rotating

Ø       Gut rotates to left such that LT VAGUS GOES ANTERIOR and RT VAGUS GOES POSTERIOR to the esophagus

Ø       So, the old midline of the stomach ends up to the left side-mesentery has grown out to the side

 

OMENTA-specialization of mesentery at the level of the stomach and the liver(foregut)

 

GREATER OMENTUM-a derivative of dorsal mesentery seen as the mesentery grows as a loop over the stomach

•          greater omentum forms a small sac w/in a sac and binds the lesser sac/lesser peritoneal cavity-remainder of the abdominal cavity is the greater sac-again, greater omentum forms the lesser sac
•          Lesser sac is subdivision of the peritoneal cavity and it’s only entrance is the epiploic foramen

 

LESSER OMENTUM-a derivative of the ventral mesentery-lesser omenta is the mesentery b/t stomach and liver

Dorsal MesenteryàGreater Omentum-forms lesser sac  //   Ventral MesenteryàLesser Omentum

GUT ROTATION-about 270 degrees clockwise w/ the SMA as the axis-SMA is artery of midgut

•          Liver, biliary apparatus, and pancreas develop at the very caudal end of the foregut
•          CELIAC ARTERY-artery of the foregut
•          SMA-artery of the midgut
•          IMA-artery of the hindgut

 

PANCREAS develops from ANTERIOR to POSTERIOR bud

•          buds sprout from duodenal foregut into ventral and dorsal mesenteries-the ventral migrates posteriorly to fuse w/ dorsal
•          Dorsal Bud-bud and duct of Santorini
•          Ventral Bud-bud and duct of Wirsung
•          These two buds rotate with the rotation and come together in much harmony
•          Major duct is that of Wirsung and it opens into Major Duodenal Papilla-of Vater
•          Stomach rotates so that it’s old left is facing forward-rotating so that its posterior midline points directly to the left
•          Lesser curvature-see the ventral mesentery-forms the lesser omentum
•          Greater curvature-see the dorsal mesentery-forms the greater omentum
•          Liver grows into ventral mesentery, pushing peritoneal membrane apart, and then grows to the septum transversum-this pushing away at the peritoneal membrane forms the bare area of the liver
•          Coronary ligament-portion of peritoneal membrane pushed aside
•          Falciform Ligament-ventral mesentery between the liver and the anterior body wall
•          There’s no ventral mesentery inferior to the falciform ligament, or inferior to the portion of the lesser omentum betweem the liver and the stomach
•          gut starts out as hollow tube, then becomes filled w/ epithelium growing from the side walls
•          as gut grows in diameter, there’s not sufficient delivery of nutrients to the central portion of the gut-thus, it recanalizes to form the definitive hollow gut present at birth

 

OMPHALOCOELE–DUE TO FAILURE OF COMPLETE ENFOLDING OF THE ABDOMINAL WALL

•          these are also seen in Prune Belly Syndrome
•          GIANT omphaloceole-case where liver is present sac-affected babies have relatively small abdominal wall

CLINICAL CORRELATION: At first these were simply cured with mercuritone(local antiseptic)-problematic b/c not sufficient w/ large omphaloceoles; it left the children with ventral hernia; and it could cause mercury poisoning in some

•          Using silon mesh, the “stuff” can pretty much be gradually forced back into place
•          Child can suffer pulmonary hypoplasia or develop infection
•          Thus, a new silver-based antiseptic, silvadene, is used and kid is closed up gradually after abdomen enlarges

 

Pentalogy of Cantrell-epigastric omphaloceole, ectopia cordis, and other problems combined-mostly incompatible w/ life

•          problem w/ this is its difficult to fix w/o shutting down blood supply-also, the heart’s exposed-ectopia cordis

 

GASTROSCHISIS-defect in abdominal wall resulting from amniotic membrane rupture during physiologic gut-loop herniation or delayed umbilical ring closure-ONLY INTESTINES ARE EXTRUDED

•          gastroschisis as a defect is smaller than omphaloceoles-there’s no liver involved, it’s the bowel that’s exposed
•          may be caused by the fact that as the umbilical vein regresses, there is a weak spot in the right abdominal wall-this is convincing b/c most of reported cases are on the RIGHT

Two problems:            1) Like omphaloceoles, you have to get the intestines back in the abdominal cavity

2) The intestines are shortened, thickened, and matted-they have been exposed to amniotic fluid during the entirety of gestation-an irritant effect that leads to the bowels’ thickening

•          GI tract has blood flow, but lots of SA that was not covered w/ skin-this means fluid and heat loss
•          This is a surgical emergency-children are closed w/ silicon patch gradually-like in 6-7 days post-delivery

 

ESOPHAGEAL ATRESIA-name’s indicative-there’s a gradual separation of the esophagus from air tract over time from lateral enfolding-can be associated w/ polyhydramnios; and is very often associated w/ tracheoesophageal fistula

•          most common type-distal fistula-i.e., proximal esophageal atresia w/ a fistula distal from trachea to stomach
•          variant-long gap esophageal atresia-there’s blind ending of upper esophagus w/ no connections
•          w/ the long gap situation, you hope for an associated tracheoesophageal fistula-w/ treatment, closure of the gap, baby can suffer small leaks in the make-shift esophagus and gastroesophageal reflux

 

Gastric Atresia-unknown to exist-however, there is such thing as microgastrula, or pyloric atresia-both are rare as well

 

DUODENAL ATRESIA-atresia-may arise from annular atresia, or annular pancreas

•          baby-neonatal-with bubble of gas in the stomach
•          Using ultrasound-baby in utero seen with amniotic fluid-filled stomach and proximal duodenum
•          upper GI tract atresia means polyhydramnios
•          1/3 of these atresias are seen assoc. w/ Downs-note: this is not the case alternately
•          mom’s w/ this prenatal diagnosis are given an amniocentesis to check for abnormal chromosomes
•          Theory on duodenal atresia-failure of recanalization of the duodenum

 The lower down the atresia, the lower the incidence of polyhydramnios-b/c amniotic fluid is allowed to circulate more

•          Theory on jejunal and ileal atresias-has to do w/ vascular accidents
•          Jejunal & Ileal atresias-problematic b/c baby may have no gut since blood supply has been cut-thus severe short gut syndrome is present

Jejunal Atresia-could be fatal-if large portion of GI tract is missing-or could be a simple web

•          difficult to deal w/ b/c you are working near Ligament of Treitz and consequently, the pancreatic blood supply

Ø       With GI tract atresias you have the problem of a shortened gut post-operation

Ø       These atresias are seen easily on X-ray-you see dilated bowel-could in stomach, duodenum, etc. but it’s indicative of where the atresia is at-you can see several dilated segments as well

Ø       In lot’s of these cases you see incorrect innervation; w/ lost supply from SMA, you get severe short gut syndrome

Ø       Also, these are all endoderm derived

 

IMPERFORATE ANUS/ cloacal anomalies, etc…

•          problem is that the uro-rectal septum comes down and meets the cloacal membrane and becomes the anal plate
•          anomaly occurs at hindgut-there is thus spearation of urinary and rectal structures

MALES: incidence of fistulas to urinary tract is extremely high

Low lesion imperforate anus-occurs b/t coccyx and pubis-above this is a High imperforate anus

•          Assoc. problems: renal anomalies, spinal cord and sacral anomalies
•          In treatment, muscles must be preserved-also, though many of the babies are repaired anatomically, many suffer incontinence due to faulty innervation-only 50% end up w/ true continence

 

MECKEL’S DIVERTICULUM-vitelline duct persists as remnant of variable length and disposition

•          often presents w/ bleeding b/c they have ectopic gastric muscosa that produces acid & this acid leads to ulceration
•          post-op, you can get persistent tethering & resultant volvulus, or twisting of intestine
•          the duct may remain open to the umbilicus or form an enterocyst of fibrous omphalomesenteric ligament
•          symptoms not unlike appendicitis
•          w/ volvulus, you can lose as much as 2/3 of ileum and still be alright-so long as the ileocecal valve is intact

 

MALROTATION

Remember-we go from a single-endoderm-lined tube to an absorptive surface that has grown and grown and grown

•          w/ that growth, there’s rotation-and this is rotation around the SMA
•          w/ bowel elongation, there’s a duodenal jejunal limb and a cecal ileocolic limb
•          the duodenal-jejunal limb rotates 90, then 180, for 270 total-around the SMA
•          w/ this rotation, you get cecum in lower right quadrant and duodenal-jejunal junction underneath stomach, giving a broad base for the SMA
•          w/ malrotation, you get a short base for the SMA-it can rotate on itself and volvulus occurs
•          patient presents w/ GI tract obstruction distal to Vater ampulla and bilious vomiting
•          side note-ileal atresia also presents w/ bilious vomiting
•          Radiograph shows gas in bowel-in stomach-and then volvulus, or twisted bowel
•          Problem here-infant otherwise normal & if this not all picked up quickly, you got dead bowel due to insufficient blood
•          In some cases, operation means putting the cecum in lower left quadrant

Duplications-they’re infrequent, can occur at any level, and can tend to be asymptomatic

Tags: annular pancreas, esophageal atresia, foregut, gastric atresia, gastroschisis, hepatic plate, hindgut, malrotation, Meckel's diverticulum, midgut rotation, omentum, omphalocele, omphalomesenteric duct, pancreas, pentology of cantrell, santorini dorsal duct, septum transversum, spleen, vitelline duct, wirsung ventral duct, yolk stalk

This entry was posted on Monday, January 12th, 2009 at 12:36 pm and is filed under Embryology. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.