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Development of the GI System I Lane Anatomy Block II Unit II


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Describe the foregut.
A blind pouch
Describe the hindgut.
A blind pouch.
What does the splanchnic mesoderm give rise to?
The CT and smooth muscle of the gut.
What is the stomodeum?
An inpocketing that becomes part of the oral cavity.
What is the proctodeum?
The anal pit.
What artery supplies the hind gut?
The inferior mesenteric artery.
Describe the primordial gut.
A tube that is closed at the cranial and caudal ends.
What forms the primordial gut?
Folding of the head, tail, and lateral walls incorporating the yolk sac into the embryo.
What is the primary lining of the primordial gut?
What does the endoderm of the primordial gut form?
Epithelium and glands of the digestive tract.
What is the stomodeum? Where is it? What is it lined by and where does it end?
The stomodeum is at the cranial end and it is lined by ectoderm. It is closed by the oropharyngeal membrane.
What is the proctodeum? Where is it? What is it lined by and where does it end?
The proctodeum is at the caudal end of the gut and it is lined by ectoderm and it is closed by the cloacal plate or membrane.
What does splanchnic mesenchyme surrounding the primordial gut form?
CT and muscle of the digestive tract.
Where do the accessory organs of the primordial gut arise from?
The epithelium.
What are the 3 parts of the primitive gut?
The foregut, midgut and hindgut.
What does the foregut give rise to?
Esophagus, stomach, proximal half of the duodenum, liver, and pancreas.
What does the midgut give rise to?
The distal half of the duodenum, jejunum, ileum, cecum, appendix, ascending colon and the right 2/3 of the transverse colon.
What does the hindgut develop into?
The left 1/3 of the transverse colon, the descending colon, sigmoid colon and the rectum down to the ano-rectal line in the endoderm-ectoderm junction.
What regulates regional differentiation of the primordial gut? What expresses these?
Hox genes expressed by the endoderm and splanchnic mesoderm regulate the regional differentiation of the primordial gut.
What is the main artery of the foregut?
The celiac artery.
What is the main veins of the the foregut?
Portal vein, splenic vein, gastric veins.
What are the lymphatics of the foregut?
Celiac nodes.
What is the sympathetic innervation of the foregut?
The celiac ganglia.
What is the parasympathetic innervation of the foregut?
What is the artery of the midgut?
Superior mesenteric artery.
What is the main vein of the midgut?
The superior mesenteric vein.
What are the lymphatics of the midgut?
The superior mesenteric nodes.
What is the sympathetic innervation of the midgut?
Superior mesenteric ganglia.
What is the parasympathetic innervation of the midgut?
What is the main artery of the hindgut?
The inferior mesenteric artery.
What is the main vein of the hindgut?
The inferior mesenteric vein.
What is the lymphatics of the hindgut?
The inferior mesenteric nodes.
What is the sympathetic innervation of the hindgut?
Inferior mesenteric ganglia and hypogastric plexus.
What is the parasympathetic innervation of the hindgut?
Pelvic splanchnic nerves.
What attaches the digestive system to the body wall?
The mesentery.
What are the 2 aspects of the primitive mesentery?
The dorsal and the ventral aspects.
What forms the mesentery?
A double layer of mesothelium that suspends the gut from the dorsal wall of the foregut to the hindgut.
What does the layer of mesothelium that lines the entire coelomic cavity form?
The parietal peritoneum and the visceral peritoneum.
What provides the pathway for blood supply in the primitive digestive tract?
The mesentery.
What happens to the ventral mesentery during development?
It degenerates during development with the exception of the foregut ventral mesentery which develops into specialized structures.
What is the gut in its primitive stage almost entirely, retro or intraperitioneal?
What suspends the gut in the peritoneal cavity and allows it to move left and right from the midline?
The dorsal mesentery.
What are secondarily retroperitoneal organs?
Organs that start out free and become attached to the posterior body wall.
Describe the path of a sympathetic nerve to the primitive gut parenchyma.
A neuronal axon exits the spinal cord via the ventral root and travels along a mixed spinal nerve. IT exits the spinal nerve via the white ramus at the level of the sympathetic trunk and passes through a paravertebral ganglion without synapsing. It then travels via a splanchnic nerve to reach a pre-vertebral ganglion where it synapses. Axons from this ganglion travel to synapse with the target tissues in the gut.
HOw do all of the blood vessels, nerves and lymphatics reach the gut?
Via the dorsal mesentery.
What is the origin of the sympathtic postganglionic cells that form the pre-and para-vertebral ganglia?
Migrating neural crest cells.
Describe how para sympathetic innervation travels to innervate the gut wall.
Parasympathetic innervation travels via the vagus and innervates neural crest cells that differentiate into postganglionic neurons named Aurbach's plexus and meissner's plexus that are found within the gut wall.
What is Hirschsprung's Disease? What causes it? What is the result? Where is it mainly occuring?
Hirschsprung's disease is failure of the neural crest cells to reach a particular area of the gut. It is mostly large intestine. The vagus cannot cause parasympathetic contraction and the colon fills with waste materials. Therefore you have a aganglionic megacolon with the absence of parasympathetic ganglia in the wall of the colon.
What are the derivitives of the foregut?
The oral cavity to the duodenum, salivary glands (from endoderm), tonsils, lower respiratory track. Also includes liver, gall bladder and pancreas.
Describe the develpment of the esophagus.
Initially short but elongates quickly. The endoderm proliferates so much that the lumen is obliterated for a while, then recannulates.
What is the striated musclularis externa of the upper 1/3 of the esophagus derived from?
The mesenchyme of the caudal pharyngeal arches.
What is the smooth muscle of the lower 1/3 of the esophagus derived from?
Splanchnic mesenchyme.
What is esophageal atresia?
Esophagus becomes a blind end or incomplete during development.
What is polyhydramnios? What does it frequently occur with?
Excess amniotic fluid. Usually occurs with atresia.
What is esophageal stenosis?
Narrowing of the lumen (variablility of recannulation or reopening of the esophagus).
What will a congenital short esophagus lead to?
Congenital hiatal hernia.
Describe the development of the stomach. What forms the greater/lesser curvatures. Which way does it rotate and end up?
The stomach starts as a dorsal enlargement of the caudal part of the foregut during the fourth week. Differential growth leads to the formation of the greater and lesser curvatures of the stomach. As the stomach enlarges it rotates clockwise 90 degrees causing the greater curvature to be on the left side of the stomach.
Describe the formation of the lesser sac. Which directions does the sac expand? What other important structure is formed via this process?
As the stomach rotates the dorsal mesogastrium is pulled to the left. This produces a cavity behind the stomach (lesser sac or omental bursa). The lesser sac expands superiorly and inferiorly. The dorsal mesogastrium continues inferior elongation forming the greater omentum.
What is another name for the lesser sac?
Omental bursa
What is the artery of the foregut?
Celiac artery.
What is the omental foramen?
Entrance to the lesser sac.
What does the omental bursa form from?
Clefts in dorsal mesogastrium of the stomach.
What artery stays attached to the stomach through out its rotation?
The gastric artery.
Where does the duodenum form?
Caudal end of the foregut and cranial end of the midgut.
What is similar between the esophagus and duodenum during development?
Both have endoderm that proliferates so much that the lumen is obliterated and then recannulates.
Give some examples of duodenum developmental disorders. (3)
Doudenal stenosis
Duodenal atresia
What do the liver gallbladder and biliary apparatus arise from?
AN evagination from the ventral surface of the caudal foregut called the hepatic diverticulum.
What forms the ventral mesentery of the liver gallbladder and biliary apparatus?
The hepatic diverticulum growing into the septum transversum (splanchnic mesoderm).
What forms the cords of hepatic cells and the epithelia cells of the biliary system within the liver?
Endodermal cells.
What portions of the liver, gallbladder, and biliary apparatus arise from the splanchnic mesenchyme of the ventral mesentery? (4)
CT, endothelial cells, hemopoietic tissue, and kupffer cells.
What do the cystic duct and gallbladder arise from?
The small caudal part of the hepatic diverticulum.
What does the main bile duct initially start out as?
A solid cord of epithelial cells that is later canalized.
Where is the bile duct initially attached? What causes it to move to its final postion? What is its final position?
THe bile duct is initially attached to the ventral surface of the duodenal loop but rotation of the duodenum moves it to the posterior surface.
Give an example of a developmental defect that can occur during gallbladder development.
Biliary atresia.
What are the three things the ventral mesentery forms?
The lesser omentum
The falciform ligament
The visceral peritoneum of the liver.
What do hepatocytes arise from?
Where is the lesser sac?
Behind the greater omentum and the stomach
Where is the superior recess of the stomach?
Behind the stomach.
What does the pancreas arise from?
2 evaginations of the caudal foregut called pancreatic buds.
Which bud does most of the pancreas arise from? What happens to the other one?
The larger dorsal pancreatic bud. The smaller ventral bud forms the uncinate process and part of the head of the pancreas.
What does the main pancreatic duct form from?
The duct of the ventral bud and the distal part of the dorsal bud duct.
What does the accessory pancreatic duct represent?
The proximal end of the duct of the dorsal bud.
What forms both the exocrine and the endocrine pancreatic tissues? (Langerhans etc.)
The endodermal cells
What forms the vascular system and CT elements of the pancreas?
The splanchnic mesoderm.
Give 2 examples of pancreatic developmental abnormalities. How does each occur?
Accessory pancreatic tissue- drain into small intestine is ok but blocked is bad. Can also go into wall of stomach, duodenum, or the diverticulum.

Anular pancreas- the two buds rotate opposite ways (ventrally) forming a ring around the duodenum. The bile duct ends up passing dorsal to the duodenum and pancreas.
What does the spleen arise from?
Mesenchyme of the dorsal mesogastrium.
What forms the capsule, connective tissue, and parenchyma of the spleen?
Mesenchymal cells.
What components does the spleen lack?
No endodermal components.
Describe the initial and final appearance of the spleen.
Initially starts out lobulated. IT takes on its adult smooth shape by the time of birth.
What is the spleen's bridge to the celiac trunk? (vessel)
Splenic artery.
During development the liver, aorta, spleen, kidneys, dorsal mesogastrium, stomach, form a sort of line. Put them in order from dorsal to ventral.

Also name the ligaments from dorsal to ventral that connect them.
Kidney, Aorta, dorsal mesogastrium, spleen, stomach, liver.

Splenorenal ligament, gastrosplenic ligament, hepatic ligament, falciform ligament.
What are the 2 ligaments that the ventral mesentery forms?
The hepatogastric and the falciform.
What are the borders of the lesser sac?
Right lesser omentum, behind liver (coronary ligaments), left bordered by spleen, spleno renal ligament, gastro splenic ligament. In the middle is behind the stomach.
What is the origin of the spleen? What cells does it lack?
Mesodermal, no endodermal cells!

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