EMBRYOLOGY: Development of Gut and Mesenteries (II)

Recommended reading: Larsen (3rd ed.), pp. 239-259 Syllabus, 10 pages Animations: http://www.med.uc.edu/embryology/chapter9/animations/contents.htm

OBJECTIVES: Following this lecture and having read the assigned text, the student should be able to:

1. Explain establishment of the gastric curvatures, rotation of the stomach, and establishment of the dorsal and ventral mesogastria.

2. Account for the anatomic relationships that are established between the liver and the ventral mesogastrium.

3. Explain development of the liver, biliary system, and pancreas.

4. Correlate the dorsal and ventral pancreatic diverticula with the various regions of the definitive pancreas.

5. Explain herniation, rotation, and reduction of the midgut.

6. Explain fixation of the gut.

7. Account for partitioning of the hindgut.

I. Development of FOREGUT (excluding pharynx & respiratory system)

A. Esophagus

1. That segment of the gut between the pharynx and the stomach

2. The respiratory or tracheobronchial diverticulum grows from the ventral foregut just distal to the pharynx at the end of week 3.

a. The dorsal part of the foregut, i.e. the esophagus, is separated from the respiratory diverticulum by a partition, the esophagotracheal septum, during week 4.

b. The developing trachea and lung buds lie ventral to the esophagus.

c. The esophagus lengthens during descent of the heart and lungs.

d. Esophageal musculature develops from mesenchyme.

(1) Striated muscle-approximately upper 2/3

(2) Smooth muscle-approximately lower 1/3

B. Stomach

1. Origin (during week 4)

a. The stomach first appears as a spindle-shaped dilatation of the foregut.

b. The dilatation is suspended in the body cavity by dorsal and ventral mesogastria (mesenteries of the stomach).

2. Rotation of the stomach

a. Rotation is caused by different rates of growth in various regions of its wall.

b. The stomach rotates 90o clockwise (viewed from above) around a longitudinal axis.

(1) The left vagus ends up on the anterior wall.

(2) The right vagus ends up on the posterior wall.

(3) The original dorsal surface (now at the left) grows faster than the original ventral surface (now at the right).

(a) The faster growing left side becomes the greater curvature.

(b) The slower growing right side becomes the lesser curvature.

(4) The dorsal mesogastrium is pulled to the left and becomes the greater omentum (attached to the greater curvature).

(a) The omental bursa forms behind the stomach.

(b) The dorsal mesogastrium walls off the left side of the omental bursa.

(c) The spleen develops within the dorsal mesogastrium in the left wall of the omental bursa.

(d) The greater omentum hangs down from the greater curvature as a double-leaved apron, which later fuses together and also fuses with the transverse mesocolon.

(5) The ventral mesogastrium is pulled to the right and becomes the lesser omentum (attached to the lesser curvature and liver) and falciform ligament.

A, B, and C show in anterior views the progression of clockwise rotation of the stomach along its longitudinal axis.

D, E, and F show in transverse sections the progression of clockwise rotation of the stomach and its mesenteries along its longitudinal axis.

G and H show in anterior views the progression of clockwise rotation of the stomach along its anteroposterior axis.

Adapted from: Langman's Medical Embryology

c. The differential growth also causes the stomach to rotate clockwise (viewed from the front) around an anterior-posterior axis.

(1) The caudal (pyloric) end is displaced to the right and cephalad.

2) The cephalic (cardiac) end is displaced to the left and caudad.

(3) The long axis then runs from upper left to lower right.

C. Duodenum

1. Formed by the distal part of the foregut and the proximal part of the midgut

a. Junction of the two parts is just distal to the origin of the hepatic diverticulum.

b. Thus, the duodenum is supplied by branches of the celiac (foregut) artery and the superior mesenteric (midgut) artery.

2. As the stomach rotates to the right, the duodenum becomes C-shaped and also rotates to the right.

a. The duodenum is pressed against and the mesoduodenum fuses with the dorsal body wall.

b. Thus, most of the duodenum (but not its proximal end) becomes secondarily retroperitoneal.

D. Liver and gall bladder

1. Origin of the hepatic diverticulum (middle of week 3)

a. A ventral outgrowth of endodermal epithelium at the caudal end of the foregut (middle of future duodenum)

Adapted from: Langman's Medical Embryology

b. Penetrates the septum transversum and breaks up into strands of cells

2. Relationship with umbilical and vitelline blood vessels

a. Diverticular strands invade veins within the septum transversum.

b. Liver cords become intermingled with hepatic blood sinuses.

3. Biliary system

a. The connection between the hepatic diverticulum and the foregut persists to become the bile duct.

b. A secondary diverticulum on the ventral side of the bile duct becomes the cystic duct and the gall bladder.

4. Subsequent development of the liver

a The enlarging liver expands caudally from the septum transversum into the ventral mesogastrium.

b. The ventral mesogastrium between the ventral body wall and the liver becomes the falciform ligament.

c. The ventral mesogastrium between the liver and the lesser curvature of the stomach becomes the lesser omentum.

d. The liver takes over hemopoietic function from the yolk sac for a period between the 2nd and 7th months of development.

E. Pancreas

1. Originates from two pancreatic diverticula, both evaginations of distal foregut endoderm

a. A dorsal pancreatic diverticulum appears opposite the hepatic diverticulum.

b. A ventral pancreatic diverticulum

(1) Appears in the caudal angle of the hepatic diverticulum.

(2) It migrates dorsally with the lower end of the common bile duct.

Adapted from: Langman's Medical Embryology 2. Fusion of pancreatic diverticula

a. Fusion of parenchyma

(1) Dorsal diverticulum becomes the upper half of the head, the isthmus, the body, and the tail.

(2) Ventral diverticulum becomes the lower half of the head of the pancreas.

b. Fusion of ducts

(1) Distal portion of the dorsal duct fuses with the ventral duct and becomes the common pancreatic duct (of Wirsung).

(2) Proximal portion of the dorsal duct either is obliterated, or persists as an accessory pancreatic duct (of Santorini).

(3) Pancreatic islets (of Langerhans)

(a) Develop from pancreatic parenchyma in 3rd month.

(b) Insulin secretion begins during the 5th month.

II. Development of MIDGUT

A. Primary intestinal loop

1. Formed by rapid elongation of the midgut

a. Cephalic limb of loop extends from the foregut junction to the vitelline duct.

b. Caudal limb of loop extends from the vitelline duct to the hindgut junction.

Unrotated

Adapted from: Langman's Medical Embryology

2. Rapid elongation of the cephalic limb forms the secondary intestinal loops.

a. The abdominal cavity is too small to accommodate the secondary loops.

b. At week 6, the midgut herniates into the extraembryonic coelom in the umbilical cord (physiological umbilical herniation).

3. The midgut rotates around an axis formed by the superior mesenteric artery.

a. Counterclockwise rotation (viewed from the front) of 180o puts the caudal limb superior to the cephalic limb.

b. The rotation continues for an additional 90o in the same direction.

Adapted from: Langman's Medical Embryology

4. Herniated intestinal loops return to the abdominal cavity at the end of the 3rd month.

B. Result of midgut development

1. Cephalic limb of intestinal loop becomes:

a. Distal half of duodenum

b. Jejunum

c. Major proximal part of ileum

2. Caudal limb of intestinal loop becomes:

a. Distal ileum

b. Cecum

c. Appendix

d. Ascending colon

e. Proximal 2/3 of transverse colon

Adapted from: Langman's Medical Embryology C. Remnants of vitelline duct

1. In 2-4% of people, a portion of the vitelline duct persists, forming an outpocketing of the ileum known as Meckel's Diverticulum.

a. It will be located about 18 inches proximal to the ileocecal junction.

b. It usually does not cause a problem, but may become inflamed and cause the same problems as appendicitis.

2. The entire vitelline duct may persist, forming a direct communication between the umbilicus and the ileum.

a. Known as umbilical or vitelline fistula

Adapted from: b. A fecal discharge can occur at the umbilicus. Langman's Medical Embryology 3. The vitelline duct may persist as a fibrous cord, which may contain a vitelline cyst.

D. The midgut is suspended by the dorsal mesentery, but some of the midgut derivatives later become fixed to the body wall.

1. The mesoduodenum fuses with the dorsal body wall (i.e. most of the duodenum becomes secondarily retroperitoneal).

2. The mesentery proper of the jejunum and ileum persists throughout life.

a. The superior mesenteric artery branches within the mesentery.

b. The mesentery runs diagonally from upper left to lower right.

3. The mesentery of the ascending colon fuses with the dorsal body wall.

The dark cross-hatched areas show the parts of the dorsal mesentery which have fused with the peritoneum of the posterior abdominal wall, making the associated organs secondarily retroperitoneal.

Adapted from: Langman's Medical Embryology

4. The transverse mesocolon persists.

5. The mesoappendix persists.

III. Development of the HINDGUT

A. Originates from endoderm that extends from the posterior intestinal portal to the cloacal membrane.

B. The descending colon becomes fixed (i.e. secondarily retroperitoneal).

C. The sigmoid colon retains its dorsal mesentery.

D. Partitioning of hindgut

1. The allantois is a diverticulation from the ventral floor of the hindgut and extends into the umbilicus.

2. The superior angle between the hindgut and allantois (urorectal septum) grows caudally toward the cloacal membrane.

3. The urorectal septum divides the terminal hindgut (i.e. the cloaca) into ventral and dorsal portions.

a. The ventral portion becomes the urogenital sinus.

b. The dorsal portion becomes the rectum.

c. The urorectal septum reaches the cloacal membrane at week 7, thereby dividing it into:

(1) An anal membrane

(2) A urethral membrane

d. The anal membrane comes to lie at the bottom of an ectodermal depression, known as the proctodeum.

(1) The surrounding swellings are known as anal folds.

(2) The anal membrane ruptures during week 9.

Adapted from: Langman's Medical Embryology