Development of the Digestive System Flashcards
Describe how body folding plays a role in forming the primitive gut.
- body folding occurs during week 4
- it encorporates endoderm from the dorsal portion of the yolk sac into the embryo
- endoderm forms the inner most lining of the primative gut tube
- epithelial lining
- glands of the GI tract
- endoderm forms the inner most lining of the primative gut tube
Note: cranio-caudal folding
- foregut forms during head fold
- hindgut forms during tail fold
- midgut forms during lateral folding
Identify components of the foregut and include the main artery that supplies this region.
components:
- Pharynx
- Lower respiratory system
- Esophagus
- Stomach
- Proximal ½ of duodenum (up until just after entrance of pancreatic & bile ducts)
- Associated organs (liver, gall bladder, biliary system, pancreas)
main artery:
- celiac trunk a.
Identify the components of the midgut and include the main artery that supplies this region
Components:
- Distal ½ duodenum (just after major duodenal papilla)
- Jejunum
- Ileum
- Cecum & appendix
- Ascending colon
- Proximal 2/3 transverse colon
Main artery:
- Superior mesenteric a. (SMA)
Identify the components of the hindgut and include the main artery that supplies this region
Components:
- Distal 1/3 transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Superior portion of anal canal
Note: rectum and superior portion of anal canal are derived from posterior portion of the cloaca after it is partitioned by the urorectal septum
Main artery:
- Inferior mesenteric a. (IMA
List the contributions of endoderm to the gastrointestinal tract.
- GI epithelial lining
- GI glands
List the contributions of mesoderm to the gastrointestinal tract. (Include the role of mesoderm in forming peritoneum and mesenteries.)
splanchnic mesoderm :
- smooth muscle and CT of GI tract
- mesentary
- visceral peritoneum
somatic mesoderm:
- parietal peritoneum
List the contributions of neural crest to the gastrointestinal tract.
- aka endoderm contribution
- Neural crest cells migrate into walls of G.I. tract to form the enteric nervous system
What condition may result from the failure of neural crest cells to migrate to the walls of the GI tract? How will aganglionic segments of the bowel appear vs. normal segments?
- Hirschsprung’s Disease (Aganglionic Megacolon)
- Affects variable portions of the bowel (usually sigmoid colon and/or rectum)
- Aganglionic portion constricts, bowel distends proximal to constriction, can lead to severe constipation / failure to thrive
- Aganglionic segments are more constricted than normal segments
Describe how peritoneum and mesentery are organized in the body.
- mesentery suspends organs from the body wall
- peritoneum lines the abdominal cavity and organs
Define peritoneum
- the serous membrane lining the abdominal cavity (parietal) & organs (visceral)
- arises from somatic and splanchnic mesoderm
Define mesentery
- a double-layer of peritoneum that suspends an organ from the body wall (can contain CT, blood vessels, lymphatics, nerves)
- created by lateral folding
- from splanchic mesoderm
- ventral mesentery
- reabsorbed inferior to foregut
- dorsal mesentery
Describe the formation of the lower respiratory tract.
- occurs in the foregut
- Esophagus develops as the portion of the foregut immediately caudal to the pharynx
-
Week 4: a respiratory diverticulum appears on ventral side of foregut
- becomes the trachea, respiratory tree & lungs
- slide 23
Describe the growth and rotation of the stomach.
Growth:
- in foregut
-
Week 4
- A portion of foregut caudal to esophagus begins to dilate uniformly.
- As the stomach enlarges, it’s dorsal side expands faster than the other sides
- becomes the greater curvature.
- The ventral face of stomach will become the lesser curvature.
Rotation:
- Starts on day 35 ends around day 56
- Stomach undergoes 90° clockwise rotation (if looking from superior view)
- Ventral side (lesser curvature) ends up on the right
- Dorsal side ends up on the left
- Right vagus n. now supplies dorsal stomach
- Left vagus n. now supplies ventral stomach
How does stomach rotation reposition the right and left vagus nerves?
After rotation:
- Right vagus n. supplies dorsal stomach,
- left vagus n. supplies ventral stomach
How does stomach rotation reposition the dorsal mesogastrium?
- As stomach rotates, the dorsal mesogastrium is carried to the left.
- This enlarges the omental bursa into a sizeable recess between the stomach & posterior abdominal wall.
- Spleen forms between 2 leaves of dorsal mesogastrium (week 5)
- spleen is carried to the left with the dorsal mesogastrium
What is the omental bursa?
- lesser sac of peritoneum
- becomes a sizeable recess between the stomach & posterior abdominal wall as stomach rotates
Describe the development of the liver, gall bladder and bile ducts as diverticula of the foregut
- Hepatic diverticulum emerges ventrally from endoderm of distal foregut (week 4),
- grows superiorly toward septum transversum
- gives rise to liver, gall bladder, and bile ducts
- The liver, gall bladder, biliary tree, and pancreas all develop as endodermal diverticula from the distal foregut.
How does an annular pancreas develop, and around what structure does it form?
- If the 2 ventral buds of the pancrease fail to fuse prior to rotation
- If they fail to fuse, each portion may wrap oppositely around the duodenum and fuse with the other, forming an obstructive ring.
Explain what it means for an organ to be retroperitoneal vs. intraperitoneal.
Retroperitoneal: Behind the peritoneum. Organs aren’t supported by mesentery and lie between the parietal peritoneum(anterior side) and abdominal wall
Intraperitoneal: surrounded by peritoneum, within the intraperitoneal space, within the confines of the parietal peritoneum
Be able to identify portions of the GI tract and associated organs that are retroperitoneal
primarily retroperitoneal:
- Kidneys (midgut)
secondarily retroperitoneal
- Distal duodenum (midgut)
- Pancreas (foregut)
- Ascending colon (midgut)
- Descending colon (hindgut)
What structure serves as the axis of rotation for the midgut?
- Superior mesenteric artery (SMA)
Describe normal midgut rotation
Normal rotation:
- 1st rotation occurs at week 6
- 90 degree counter clockwise rotation
- loop is in proximal umbilical cord
- 2nd rotation occurs at week 10
- intestines return to abdomen
- cranial limb returns first
- week 11 cecum descends
- 180 degree counter clockwise rotation
- brings intestines to normal postion
- intestines return to abdomen
Note: direction of rotation is assuming an anterior view and the axis of rotation is SMA
When and why does a physiological umbilical herniation occur?
- Occurs during weeks 6-10
- Occurs because of insufficient room in the abdominal cavity
- it’s a temporary herniation of midgut loop into the proximal umbilical cord
Describe an omphalocoele, and explain how this condition differs from gastrochisis.
- Congenital Omphalocoele
- Persisting umbilical hernia
- Viscera covered by amnion & within proximal umbilical cord
- Often associated with chromosomal abnormalities and other severe malformations, so mortality rate is high (25%)
- Gastrochisis
- Doesn’t involve umbilical cord
- Hernia of small intestine and occasionally other abdominal viscera through anterior body wall
- Exposure to amniotic fluid can damage viscera
