Development of GI Flashcards
What germ layer is the epithelium derived from, and what are the exceptions?
Endoderm
Except the Mouth and Anal canal
What germ layer is the mesenchyme layer derived from?
Splanchnic mesoderm
Also makes the smooth muscle and connective tissue
What tissue are derived from Neural Crest Cells?
The autonomic ganglia
What germ layer are the mouth and anal canal derived from?
Ectoderm
What are the divisions of the GI tract and what structure determines this layout?
Foregut
Midgut
Hindgut
Determined by where the Yolk sac attaches.
Above Yolk sac = Foregut
At Yolk Sac = Midgut
Below Yolk sac = Hindgut
What structures are part of the Foregut?
Pharynx Respiratory System Esophagus Stomach Liver Pancreas Biliary Apparatus Proximal Duodenum
What structures are part of the Midgut?
Small intestine Cecum Appendix Ascending Colon Right Half of Transverse Colon
What structures are part of the Hindgut?
Left Half of Transverse Colon Descending Colon Sigmoid Colon Rectum Superior Anal Canal Epithelium of Urinary Bladder/Urethra
What structure separates the Foregut and Midgut?
The Major Duodenal Papilla (the opening in the 2nd part of the duodenum where the common bile duct and pancreatic duct meet the intestine)
What are the 3 blood vessels that supply blood to the 3 divisions of the GI tract respectively?
Celiac Trunk supplies the Foregut
Superior Mesenteric A. supplies the Midgut
Inferior Mesenteric A. supplies the Hindgut
What germ layer derives the Mucosa of the Esophagus?
Foregut endoderm
What’s special about the esophageal mesenchyme and what germ layers cause this?
Esophagus has skeletal and smooth muscle
Skeletal primarily in the superior portion
Smooth primarily in the inferior portion
Smooth Muscle is derived from Splanchnic Mesoderm
Skeletal Muscle is derived from Pharyngeal Arch Mesoderm (specifically arch 4 and 6)
What is a tracheal esophageal fistula?
An abnormal passage between the trachea and the esophagus (due to an failure of tracheoesophageal folds to properly form the tracheoesophageal septum; occurs in week 5)
Presenting signs and symptoms:
- Prenatally: Polyhydramnios (too much amniotic fluid)
- Birth: Coughing, gagging, cyanosis, vomiting, voluminous oral secretions, possible respiratory distress
What is Esophageal Atresia?
Blind end esophagus
When does the stomach begin to form?
4th week
How does the stomach form?
Starts as a swelling inferior to esophagus
Ventral side grows at a faster rate forming a pocket
Pocket rotates in transverse plane 90 degrees so that ventral pocket is facing left
Then it rotates in the coronal plane 90 degrees so that the greater curvature is below and the lesser curvature is above
What happens to the vagus nerves as the stomach rotations occur?
The Left vagus nerve becomes anterior while the Right Vagus nerve becomes posterior (clockwise rotation)
What is Hypertrophic Pyloric Stenosis?
Circular and some longitudinal muscles of the Pylorus become hypertrophied, constricted the Pylorus
Patient projectile vomits soon after eating, has smaller and fewer stools, failure to gain weight or may lose weight
Cause unknown; very common
What structure are formed from the hepatic diverticulum?
The liver, biliary system, and the ventral pancreatic bud
As the liver grows superiorly, it fuses with what structure?
Septum transversum (diaphragm)
When does bile production begin?
12th week
When does hematopoiesis in the liver begin?
Week 6-30
How is the pancreas formed?
There is a ventral and dorsal primordia of the pancreas
Ventral primordia is fromed from the hepatic diverticulum.
These structures eventually rotate and fuse
The ventral primordia forms some of the head of the pancreas, the uncinate process, and the main pancreatic duct
The dorsal primordia forms some of the head, the body, and the tail of the pancreas
What is an Annular Pancreas?
Annular Pancreas occurs when the ventral pancreatic bud is bifid.
It then rotates in the opposite direction it should be rotating, fusing around the duodenum in a ring, causing a constriction of the gut.
Similar to Hypertrophic Pyloric Stenosus, but the vomiting with have bile in it.
2/3 of patients are asymptomatic
What germ layer is the Spleen derived from?
Mesoderm (it is still foregut though)
When does the spleen form?
Week 5
When does hematopoiesis occur in the Spleen?
Week 9-28
How does the midgut develop?
The mid gut along with the Superior Mesenteric A. herniates into the yolk stalk forming an elongated loop.
The loop rotates 90 degrees clockwise so that the inferior half is on the left, and the superior portion is on the right.
The right half (inferior) forms a bulge near the end that forms into the cecum.
The left half (superior) becomes very convoluted (squiggly).
Left half ultimately becomes the large intestine, while the right half forms the small intestine.
The abdomen grows larger, and the right (superior/small intestine) portion returns to the abdomen first and PASSES UNDER the left (inferior/large intestine portion) to the left side of the abdomen.
This results in the proximal duodenum being in a position posterior to the transverse colon
Why does the midgut herniate into the yolk stalk for development?
To make room for the massive liver (which is producing blood) and the large kidneys
When does the Midgut herniate into the yolk stalk?
Week 6
When does the midgut return to the abdomen from the yolk stalk?
Week 10
Where and how much are the 3 rotations?
Rotates 90 degrees in the yolk stalk
Then the small intestine rotates 180 degrees when it returns to the abdomen
Then the large intestine rotates 180 degrees when it returns to the abdomen
Total rotation = 270 degrees
What is omphalocele?
Gut in umbilicus (with sac of peritoneum and skin surrounding guts
Caused by failure of guts to return to abdomen at week 10
NOT AN UMBILICAL HERNIA
What is Gastroschisis?
Guts outside of abdomen without sac of peritoneum and skin
Failure of body wall closure at week 4
What is Hirschprung Disease?
Also known as mega colon. Most common cause of neonatal obstruction
Failure of neural crest, resulting in lack of autonomic ganglion cells (preventing motility of segment distal to dilated (clogged) section.
Dilated (feces filled section) is normal; only distal part is affected.
What is Midgut Nonrotation?
Nonrotation is when the midgut returns to the abdomen from the yolk stalk, but fails to complete its final rotation
This results in the small intestine lying in the Right side of the abdomen and the large intestine lying in the left side of the abdomen
Fissures of the colon are tighter, and can cause constriction
However, patients can also be asymptomatic
What is Midgut Reversed Rotation?
When the Midgut returns to the abdomen from the yolk stalk, it completes its final rotation in the wrong direction, resulting in the duodenum being anterior to the transverse colon.
Duodenum can potentially cause a constriction of the transverse colon
How is the lumen of the gut tube formed?
At first the tube is hollow
Then tube is occluded by endodermal proliferation
Then Recanalization occurs via apoptosis
What can go wrong with lumen formation
Duplication can occur, where a pocket or pouch forms on the wall of the gut tube, which can collect material and occlude the gut
Stenosis (narrowing of lumen due to failure of apoptosis) can occur, causing a constriction of the gut tube
What is Meckel’s diverticulum?
Abnormailty of the vitelline duct (normally regresses)
Causes a projection of the ileum to abdominal wall (essentially making a fork in the road with a dead end at the belly button)
Leads to intestinal obstruction, gastrointestinal bleeding, bowel sepsis (presents similar to appendicitis)
RULE OF 2’s
- Affects 2% of population
- Approx. 2 feet form ileocecal junction
- Pouch is 2 inches long
- Conatins 2 types of ectopic tissue (pancreatic or gastric tissue)
- Present at 2 years of age
- Males 2x more than females
Some can have Meckel’s without symptoms
What is an Omphalomesenteric fistula?
One form of Meckel’s Diverticulum
Diverticulum leads straight through umbilicus resulting in Ileal contents to drip out of belly button
What is an Omphalomesenteric Cyst?
Bubble of fluid encased in a ligement remnant of the vitelline duct.
Usually asymptomatic; rare
What is a Omphalomesenteric Ligament?
Ligament remnant of the vitelline duct that connects the ileum to the umbilicus
Asymptomatic; rare
What developmental processes occur at the cloaca?
Urorectal septum grows toward the cloacal membrane and divides it into 2
The Urogenital Membrane - which forms into the Bladder
The Anal Membrane - which forms into the anus
Both are derived from ENDODERM AND ECTODERM
What germ layer forms the Urorectal Septum?
Mesoderm
Why does the cloacal membrane not have mesoderm?
Without mesoderm, the cloacal membrane is able to eventually rupture to form the open anus and urethra
How does the Anal Canal Develop?
The Proximal 2/3 of the Anal Canal is formed from Hindgut (endoderm)
The Distal 1/3 of the Anal Canal is formed from the Anal Pit (formed from ectoderm proctodeum)
The Pectinate Line is where the two portions are divided
What is the Blood Supply, Nerve Innervation, and Lymph flow for the Hindgut portion of the Anal Canal?
B: Inferior Mesenteric A.
N: Autonomics
L: Inferior Mesenteric Lymph nodes
Cannot feel pain if hemorrhoid is present above the Pectinate Line
What is the Blood Supply, Nerve innervation, and lymph flow for the Anal Pit portion of the Anal Canal?
B: Middle Rectal Arteries and Inferior Rectal Arteries
N: Pudendal Nerve (somatic; voluntary control)
L: Superficial Inguinal Lymph Nodes
Can feel pain if hemorrhoid is present below Pectinate Line
What are the two processes that must take place in order to correctly form the Anorectal structure?
The Urorectal septum must fully migrate and separate the cloaca
The ENDODERM (hindgut) and the ECTODERM (anal pit) must meet at the correct place
Failure of either of these events can cause a variety of complications
What germ layer is the septum transversum derived from?
Somatic layer of lateral plate mesoderm
How does closure of the Percardioperitoneal Canal Occur?
Septum transversum does not migrate all the way to the posterior wall, leaves the Pericardioperitoneal canals open
The Pleuroperitoneal membrane migrates from the posterior wall to meet the septum transversum
Then finally, tissue from the body wall is pulled into the diaphragm (forming the outermost portions of the diaphragm)
What specific part of the diaphragm is derived from the Septum Transversum?
Central Tendon (white part)
What part of the diaphragm is derived from the dorsal mesentery?
The Right and Left Crus
What is a mesentery?
Double layer of splanchnic mesoderm
What do somatic and splanchnic mesoderm form in the parietal cavity?
Somatic Mesoderm - Parietal Layer of peritoneum
Splanchnic Mesoderm - Visceral Layer of Peritoneum
