Embryology: Development of Gut and Mesenteries (II) M1 Gross and Developmental Anatomy 11:00-11:50 AM, January 9, 2009 Dr. Milton M. Sholley Professor of Anatomy and Neurobiology

The web address listed on syllabus page 38 has changed. ANIMATIONS: The web address listed on syllabus page 38 has changed. The new address is: http://cna.uc.edu/embryology/chapter9/animations/contents.htm

The primitive gut regions (Foregut, Midgut, and Hindgut) develop from the endoderm by folding during the 4th week as shown in the animation. Endodermal sheet Starting at ~20 days Yolk sac Ending at ~26 days Larsen Website 9-1.avi

The Respiratory Diverticulum buds from the ventral side of the proximal foregut at the end of week 3. (Respiratory diverticulum=Tracheobronchial diverticulum=Laryngotracheal diverticulum) Adapted from: Langman's Medical Embryology

Rotation of the Stomach (Syllabus page 40) 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

Rotation of the Stomach Dorsal mesogastrium Ventral mesogastrium Starting at ~27 days Ending at ~56 days Left vagus nerve Larsen Website stomrot.avi

The 90o Clockwise Rotation of the Stomach drags the dorsal mesogastrium to the left and forms the omental bursa. Dorsal mesogastrium Right Left S Ventral mesogastrium Liver Larsen Website lessersacA.avi Ventral

Rotation of the stomach drags the dorsal mesogastrium inferiorly to form the omental apron (the redundant part of greater omentum). Larsen Website lessersacB.avi

Let’s vent our spleens. Q: Does the spleen develop from a diverticulum of the foregut? A: No. Q: So it doesn’t develop from the endoderm? A: That’s correct. Q: Does the spleen have any association with the foregut? A: Yes. It develops within the dorsal mesentery of the stomach (dorsal mesogastrium) and is supplied by a branch of the artery that supplies the foregut (celiac artery). Q: Well, what germ layer does form the spleen? A: The mesoderm in the dorsal mesogastrium.

The duodenum is formed by the most distal part of the During the 6th week, the endoderm proliferates excessively and the lumen of the duodenum becomes occluded. The lumen is recanalized over the next two weeks and the definitive mucosal epithelium is completed in the 9th week. This happens along the entire gut tube, but the duodenum is the gut segment most prone to failure of recanalization. The duodenum is formed by the most distal part of the foregut and the most proximal part of the midgut Larsen Website tube.avi

The originally intraperitoneal DUODENUM and PANCREAS rotate concurrently with the stomach and become secondarily retroperitoneal (except for the 1st part of the duodenum). Before rotation After rotation Adapted from: Langman's Medical Embryology

Correction to printed embryology syllabus (page 41). D. Liver and gall bladder 1. Origin of the hepatic diverticulum (middle of week 3) Cross out “week 3” and replace it with “week 4”.

Bile ducts and Gall Bladder, Formation of Liver, Bile ducts and Gall Bladder, and Pancreas (Syllabus page 41) Adapted from: Langman's Medical Embryology

Bile ducts and Gall Bladder, Formation of Liver, Bile ducts and Gall Bladder, and Pancreas (Syllabus page 41) Adapted from: Langman's Medical Embryology

The expanding liver divides the ventral mesogastrium into the lesser omentum and the falciform ligament. From: Langman's Medical Embryology

Fusion of Pancreatic Diverticula (Syllabus page 43) Adapted from: Langman's Medical Embryology

Primary Intestinal (midgut) Loop before rotation and after 180o rotation (Syllabus page 44) Adapted from: Langman's Medical Embryology

showing the elongation of the original cephalic limb Midgut at 180o of Rotation showing the elongation of the original cephalic limb and the resulting physiological umbilical herniation. (Syllabus page 44) Adapted from: Langman's Medical Embryology

Midgut after 270o of Rotation and in Its Final Position (Syllabus page 45) Adapted from: Langman's Medical Embryology

Student question: The Larsen book says that the midgut rotation is first 90 degrees and then 180 degrees. Dr. Sholley’s syllabus handout describes a 180 degree rotation and then a 90 degree rotation. Which is correct for the test? Answer: I find it best not to talk about two distinct phases of the midgut rotation, since it is a continuous process that spans weeks (starting at week 6 and being completed between weeks 10-12). It is important to remember the direction of midgut rotation. Viewed from the front, the midgut rotates counterclockwise around an axis formed by the superior mesenteric artery. It is also important to remember that the total rotation encompasses 270 degrees. The reason for noting the position of 180 degrees (syllabus page 44) is because that amount of rotation reverses the positioning of the original cephalic and caudal limbs of the midgut loop. The original cephalic limb elongates much more than the original caudal limb, but it has assumed a caudal position as the length increase is nearing completion. When the final 270 degree position is reached, the jejunal loops (from the original cephalic limb) will have been positioned in the upper left quadrant of the abdomen.

Remember the vitelline duct (or yolk stalk)? It usually involutes completely. What if it doesn’t? Vitelline duct Adapted from: Langman's Medical Embryology

Meckel’s Diverticulum (Syllabus page 45-46) Rule of 2’s ~2% of the population has one ~2 feet proximal to the ileocecal junction ~2 inches long ~2 years is the usual age of trouble, if any Adapted from: Langman's Medical Embryology

Fixation of the Gut i.e. fusion of mesenteries with the posterior parietal peritoneum renders some segments of the gut secondarily retroperitoneal. (Syllabus page 46-47) 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

 A retroperitoneal organ is located behind the parietal peritoneum and does NOT have a mesentery. –A secondarily retroperitoneal organ had a mesentery early in embryological life but lost it during further development. –A primarily retroperitoneal organ develops in a retroperitoneal position and never had a mesentery.  An intraperitoneal organ is invested by visceral peritoneum and has a mesentery.

Development of the Hindgut (Syllabus page 47) Adapted from: Langman's Medical Embryology