Entodermal derivatives:formation of the gut,liver, and pancreas

Mike Gershon

Folding formsthe gut

Primitive gut extends frombuccopharyngeal to cloacalmembrane.

Move toward each other

Cardiogenic mesenchyme isoriginally rostral, but foldingbrings it caudal to buccalmembrane.

Foregut and hindgut becomerecognizable

Portion of yolk sac isincoporated into the embro asbowel.

Midgut remains open.

Cephalocaudal and lateralfolding occur simultaneously

Meeting and fusion of cranial, lateral, and caudal edges of the embryocreate the primordial foregut and hindgut

Slow fusion of midgut-due to presence of yolk sac. Midgut remains open untilweek 6-connects to yolk sac via vitelline duct.

Buccopharyngeal membrane opens at 4 and cloacal membrane at 7 weeks

Flexion delimits the bowel

After the gut forms, it is attached to the body wall bydorsal and ventral mesenteries; ventral is lost except inregion of liver. Vetelline duct remains in umbilical cord.

Anterior-posterior and lateralfolding form the primitive gut

Embryonic disc growsfaster in length than theyolk sac causing theembryo to bend.

Dorsal surface grows morerapidly than the ventral

Lateral foldingFusion with apposing sideexcept in the region of theyolk sac, and allantois

Folding brings the heartand septum transversumcaudal to bucco-pharyngeal membrane.

The dorsalmesentery

thins toallow the gutto be flexiblysuspended

The foregut has manyderivatives

Pharynx and its derivatives

Lower Respiratory tract

Esophagus

Stomach

Duodenum proximal to ampulla of Vater

Liver

Biliary Apparatus

Pancreas

From stomach to biliary apparatus, all are suppliedby the celiac artery, “the artery of the foregut.”

Esophaguselongates rapidly

Appears to grow faster at its cranialthan caudal end.

Stomach does not descend butarises from a region just caudal toseptum transversum that has beenfated to be stomach.

Epithelium obliterates lumen ofesophagus and is recanalized byapoptosis (week 8).

Failure causes polyhydramnios

Esophageal atresia or tracheo-esophageal fistula.

Stomach enlarges and rotates

Obliteration of the lumen andrecanalization occurs

The stomach rotates 90° in aclockwise direction

Dorsal surface grows faster than the ventral to create thegreater and lesser curvature. Acquires a transverse position

Rotation ofthe stomachcreates thelesser sacDorsal mesogastriummoves to left.

Ventral mesogastriumattaches to liver andbody wall.

Inferior recess formthe greater omentum

Layers fuse toobliterate the lessersac

Rotation of the stomachforms the omental bursa

Movements of the mesenteryand stomach are made

possible by vacuolization dueto selective apoptosis

Liver, biliary system andpancreas arise from the

duodenum

Hepatic diverticulum growsfrom the duodenum into the

ventral mesenteryBegins ~ week 4

Divides into cranialand caudal buds.

Cranial bud growsfaster and becomesthe hepaticparenchyma;

Hematopoieticcolonists arrive ~week 6

Caudal bud givesrise to the biliarysystem.

Ventral mesentery forms falciformligament, hepatic peritoneum, and

lesser omentum

Ventral mesogastriumsupports liver and stomach

Rotation of the stomachshapes the pancreas

Pancreas arises from dorsal and ventralbuds.

Rotation brings ventral to dorsal bud.

Buds fuse.Ventral duct becomes the main pancreatic ductbut the dorsal bud forms most of the pancreas

Ventral bud forms only the uncinate processand inferior part of the head of the pancreas.

Aberrant rotation causes anannular pancreas

Review of the Gut Tube

Derivatives of the midgut

Small intestine (except for the proximalduodenum.

Cecum

Appendix

Ascending colon

Right 1/2 to 2/3 of the proximal transversecolon

All are supplied by the superior mesentericartery (“the artery of the midgut”)

The midgutgrows

rapidly andherniatesinto the

umbilicalcord

Week 6

The midgutrotates

around anaxis of thesuperior

mesentericartery:

1. 90°2. 180°

Midgut hernia reduced atweek 10.

Rotation of the midgut

1. Cranial and caudal loop form.

2. Cranial growth >>> caudal growth.

3. Apex of loop is vitelline duct.

4. Cranial loop moves to right and caudal loop toleft (90° counterclockwise).

4. Reduction of midgut hernia with rotation afurther 180°.

Brings cecum to right

Moves down

Becomes secondarily retroperitoneal.

Loops of bowel fuse with thebody wall and become

secondarily retroperitoneal

Volvulus is a seriouscomplication of excessive

flexibility

Derivatives of the hindgut

Left 1/3 to 1/2 of the distal transverse colon

Descending colon

Sigmoid colon

Rectum

Superior part of anal canal

Epithelium of unrinary bladder and most of theurethra

All are supplied by the inferior mesentericartery, “the artery of the”. hindgut

The hindgut is originally acloaca-partioned to form

rectum and urogenital sinus

Urorectal septum divides thecloaca

Hindgutforms

superior 2/3of rectalcanal;

proctodeumforms lower1/3; dividedat pectinate

line

Never forgetthe pectinate

line

If anything can go wrong itwill; anorectal malformations

The END

Have a nice day!