HE 31-32 GI Development

Question 1

Dorsal Mesentery

Answer 1

Formed by the invagination of the gut tube into the peritoneal cavity

Question 2

Three germ layers of GI and what they contribute to

Answer 2

Endoderm: mucosal epithelium (and parenchyma of submucosal and accessory glands)

Visceral mesoderm: Lamina Propria, muscularis mucosa, muscularis externa, serosa, adventitia, stroma of accessory digestive glands

Neural Crest: submucosal and myenteric plexuses (enteric nervous system)

Question 3

Movement of Septum Transversum during Body Folding

Answer 3

Wk 4 the Septum transversum forms at C3-C5 between pericardial sac and yolk sac

(pericardial and peritoneal canals still exist

By week seven it is pushed down with the pleuralperitoneal membranes creating thoracic/abdominal diaphragm

Question 4

Four major components of septum transversum (adult)

Answer 4

Central Tendon (visceral mesoderm)

Crura: esophageal dorsal mesentery

pleuroperitoneal membranes (dorsal)

muscular ingrowth from body wall

Question 5

migration of the gut tube (weeks)

Answer 5

week four: gut tube in broad contact with body wall, parietal and visceral peritoneum form their respective mesoderm

week 5: invagination of the gut tube to form Dorsal Mesentary
-Neurovascular connection runs THOUGH dorsal mesentery for the gut tube.

Question 6

Primary and Secondary Retroperitoneal

Answer 6

primary: never invaginated ie Kidney
secondary: portion of the gut tube invaginated but later will be fused to the wall

Question 7

Ventral Mesentary location and timing

Answer 7

forms at week four, is the contact between the septum transversum and gut tube

-only at distal esophagus, stomach and proximal duodenum

Question 8

Regionalization of gut tube at week four

Answer 8

Retinoic acid gradient secreted by visceral mesoderm
-gradient drives SHH expression in Endoderm

-epithelial to mesechymal interaction creates an upregulation in HOX expression in the mesoderm

Nested HOX expression causes endoderm to express region specific genes

SOX 2-esophagus and stomach

PDX1 - pancreas

CDX2 small intestine

CDX1 large intesting

Question 9

during gut tube formation which two portions of the endodermally lines cavities remain extraembryonic

Answer 9

allantois: urinary bladder and medial umbilical ligament

yolk sac: temporarily connected by vitteline duct
will be obliterated at week 10

Question 10

Arterial development of vitteline arteries and veins to yolk sac

Answer 10

Vitelline arteries branch off dorsal aorta to supply yolk sac
-pairs gradually fuse to form the celiac trunk, superior mesenteric, and inferior mesenteric

Paired vitelline veins drain blood from yolk sac to sinus venosus

Question 11

Forgut (overview)

Answer 11

Common blood supply: celiac trunk

abdominal esophagus, spleen, stomach, pancreas, liver, gall bladder, duodenum (superior to major papilla)

Question 12

Formation of abdominal esophagus

Answer 12

week four respitory diverticulum buds off ventral aspect of the foregut
-tracheoesophageal ridges partition the developing lungs

esophagus initially short, rapid growth from the esophagus of the heart and lungs lengthen it

Question 13

Stomach rotation

Answer 13

Week four,

Differential growth causes rotation in two axis simultaneously

1. longitudinal axis: 90 degree CW rotation
2. AP axis rotation CW

• up-rotates pyloric region, down rotates cardiac region
• former anterior side grows slower than former posterior side for greater and lesser curvatures

Dorsal and ventral mesentery are dragged in unison

-ventral megastrium and dorsal megastrium lined in AP plane origionally and dorsal is pulled left

Question 14

Creation of Greater and lesser omentum and omental foramen

Answer 14

Free edge of ventral megastrium becomes omental foramen (borders?)

ventral megastrium becomes lesser omentum

Dorsal megastrium becomes greater omentum

Omental Bursa: cavity between lesser and greater omentum that is formed by week five. The gastrocolic ligament basically is an extension of the greater omentum and bulges inferiorly extending the omental bursa
-anterior and posterior parts FUSE with eachother and transverse colon reducing the size of the omental bursa

Question 15

Formation and rotation of duodenum in week four to six

Answer 15

NOTE DUAL ORIGIN (midgut and foregut) separated by major papilla

Follows stomach rotation

• begins at midline extending straight forward
• swings right and makes a C curve on the right side
• most becomes retroperitoneal by rotation pressing duodenum and mesentery to wall (secondary retroperitoneal), only very first part remains intraperitoneal

Week 5-6 rapid growth of mucosal epithelium fills and block lumen with epithelial plug. Soon after apoptosis begins causing recanalization of the canal ( complete week 9)

Question 16

Gut Atresia and Stenosis

Answer 16

Atresia: ending Stenosis: narrowing

Atretic duodenum: never recanalized (not common)

Stenotic Duodenum: incomplete recanalization (common)
-occurs inferior to the major duodenal papilla, backup of chime reults in sever vomiting (emesis) that most likely contains green bile

Pyloric stenosis: Hypertrophy of the pyloric sphincter of the stomach. In severe cases becomes atretic. NO bile in vomit.

Question 17

Molecular regulation of Accessory Glands

Answer 17

during week four liver gall bladder and pancreas bud off second part of duodenum

-surrounding tissue especially notochord, express inhibitors to prevent differentiation of foregut endoderm into accessory glands

these inhibitors are blocked by

• FGF2 from heart and blood vessels
• BMP from septum transversum

HOX expression then upregulated in visceral mesoderm to form ventral Hepatic Diverticulum

PDX1 expression then upregulated in endoderm for ventral and dorsal pancreatic buds

Question 18

Draw formation of liver buds at week four

Answer 18

include liver bud, gall bladder, pancreas buds, in relation to stomach

(hepatic diverticulum)-grows INTO septum transversum

note where bile duct and cystic duct and gall bladder form from

Question 19

Draw liver formation and vitelline veins in week four along with formation of liver and know blood flow development in liver

Answer 19

include anastomosis for vitelline veins, formation of hepatic sinusoids, blood flow (left to right for larger right lobe) and THREE major changes towards birth.

hepatic chords (endoderm)-hepatocyte perenchyma

sinusoids: mesoderm from septum transversum, sinusoidal capillaries, hematopoetic cells, connective tissue cells (stromal)

Question 20

derivative of ductus venosus and left umbilical vein

Answer 20

ligamentum venosum and round ligament of liver

Question 21

Vitelline veins form which five major adult viens

Answer 21

R and L hepatic veins, hepatic portion of IVC, portal vien, superior mesenteric vein

Question 22

Later growth of liver and omental formation

Answer 22

Gwoth of liver IN ventral mesentery- liver initially inside septum transversum, eventually grows to large it bulges into the ventral mesentery

-bare area: remains in contact with the septum transversum

• bulging divides ventral mesentery into LESSER OMENTUM (houses hepatic portal vein proper hepatic artery and common bile duct) between stomach and liver and Falciform ligament (contains umb vein -> round lig of liver) between diaphragm and liver
• WK 12 bile formed by liver can enter duodenum

Question 23

Development of Pancreas

Answer 23

Dorsal Pancreatic Bud: endodermal diverticulum, grows into dorsal mesoduodenum

• goes 90 degrees to the left from the back
• more cranial
• uncinate process and inferior portion of head

Ventral Pancreatic Bud: (endodermal diverticulum) grows from bile duct into ventral mesoduodenum

• goes ALL the way around and under
• longer portion
• neck body and tail of panc

Question 24

pancreatic duct drainage

Answer 24

Main pancreatic duct: ventral and distal part of dorsal pancreatic ducts merges with common bile duct at major duodenal papilla

accessory pancreatic duct: proximal part of dorsal pancreatic duct, empties into minor duodenal papilla

Question 25

Annular Pancreas

Answer 25

ventral bud splits around duodenum, it can sometimes constrict the duodenum and lead to low birth waight and vomiting

Question 26

Spleen formation

Answer 26

NOT endodermally derived

wk 5

• mesodermal growth within the dorsal mesogatrium
• longitudinal rotation swings spleen to the left by week 11
• also good for omentum and falciform

Spleen is intraperitoneal!!

Question 27

Midgut overview and limbs

Answer 27

Primary blood supply through superior mesenteric artery

duodenum (inferior to maj papilla) jejunum, ileum
-cephalic limb of primary loop

cecum, appendix, ascending colon, transverse colon (proximal 2/3
-caudal limb of primary loop

Question 28

Weeks five and six to ten of primary intestinal loop and rotation

Answer 28

rapid growth of liver and cephalic limb of primary intestinal loop limits space in the abdomen

wk 6- primary intestinal loop enters extraembryonic (chorionic) cavity in umbilical cord

90 degree rotation CCW about the superior mesenteric artery
-during this time the cephalic loop Is also elongating to form for loops explaining kinkiness of ileum and jejunum

-cecal bud appears in caudal limb

WK 10 with enough space in the abdomen the loop retracts from the umb chord

• cecal bud is a knot at this point and comes out last
• during retraction another 180 degrees CCW
• 270 degrees total

Question 29

Omphalocele and Gastroschesis

Answer 29

Omphalocele: failure of complete gut tube retraction during weeks 10-12, gut tube remains in umbilical chord.
HIGH mortality rate, associated with other abnormalities such as cardiac and NT

Gastroschesis: protrusion of the gut tube through the abdominal wall

• related to defect in body folding
• exposed gut tube may be damaged by amniotic fluid BUT has high survival rate

Question 30

Left Sided Colon

Answer 30

only 90 degree complete rotation or reverse rotation

-usually asymptomatic, but can allow twisting of the gut tube (volvulus) and blockage

Question 31

Descent of Cecum

Answer 31

After retraction, cecum lies in upper right quadrant
-elongates down to lower right quadrant to form cecum and ascending colon

• sprouts diverticulum, appendix
• 50% of the time descent drags appendix to retrocecal, or retrocolic positions

Question 32

Ileum diverticulum

Answer 32

Vitelline duct usually obligterated by WK 6

• for some vitelline duct persists
• Meckels ileal diverticulum (rule of 2’s)
• 2% of pop
• 2ft prox to ileocecal valve
• 2 in long
• 2 types of tissues (panc and gastric)
• 2 yrs diagnosed
• 2x more likely in men
• open and attached to vitelline ligament

other cases

• vitelline cyst
• vitelline fistula

Question 33

Colic Mesentaries

Answer 33

When gut tube regions obtain definitive positions some of their mesenteries press against post abdomen wall
-ascending colon, descending colon, rectum all become secondary retroperitoneal

Question 34

persistence of fused mesocolon of the acending or descending colon

Answer 34

abnormal movements of the gut tube, twist and volvulus or become trapped behind the mesocolon, extreme can block portions of gut t ube or lead to necrosis.

Question 35

Anal Canal

Answer 35

Developed in Week 7 (LOOK AT PICS)

• allantois: urogenital sinus -> urinary bladder and urogenital membrane
• yolk sac: anal membrane and anal rectal canal (hindgut)