3.1 Development of the GI tract Flashcards
In what week does folding occur?
What are the 2 types of folding and what does each achieves
4th week
Lateral folding (transverse folding)
- creates ventral body wall
- primitive gut becomes tubular
Craniocaudal folding
- creates cranial and caudal pockets from yolk sac endoderm (beginning primitive gut development)
When folding of the embryo creates a primitive gut tube (lined with endoderm) how can it be divided into 3 regions?
Which region is continuous with the yolk sack?
Foregut
Mid-gut
Hindgut
Foregut and hindgut begin as blind diverticula, Midgut is continuous with the yolk sac
Why is the importance of foregut, midgut and hindgut + what is the blood supply of each?
These embryonic divisions have implications for blood supply and lymphatic drainage in the adult
Forgut ➞ Celiac trunk
Midgut ➞ SMA
Hundgut ➞ IMA
What week does development of the primitive gut tube begin?
How does it form and anatomically where in the foetus does it start and end anatomically?
There is one opening, where is this?
3rd week
It pinches off from the yolk sac cavity and runs from stomatodeum towards to proctodeum caudally with an opening at umbilicus
What is the stomatodeum and the proctodeum?
stomatodeum = future mouth
proctodeum = future anus
What is the internal and external lining of the primitive gut tube derived from and what does each give rise to?
Internal lining derived from endoderm
- future epithelial linings
External lining derived from splanchnic mesoderm
- future musculature
- visceral peritoneum
What are the 6 adult derivatives of the foregut?
- Oesophagus
- Stomach
- Pancreas
- Liver
- Gall bladder
- Duodenum (proximal to entrance of bile duct)
Where and how is the primitive gut tube suspended in the developing foetus?
Suspended in intraembryonic coelom by a double layer of splanchnic mesoderm
What is a messentry and what is its purpose?
Messentry is a double layer of peritoneum suspending the gut tube from the abdominal wall
Allows a conduit for blood and nerve supply and provides mobility where needed
What are the 2 types of messentry, what does each attach and what division of the developing gut tube is it found?
What do these give rise to in an adult?
1) dorsal mesentery:
- suspends the entire gut tube from the dorsal body wall
- attaches to the roof of the abdominal cavity
- foregut, midgut and hindgut
2) ventral mesentery:
- attaches the developing gut tube to the floor
- foregut ONLY
These mesenteries become the various peritoneal folds and reflections that suspend the gut and give passage to vessels and nerves in the adult
Where is the ventral messentry derived from?
Give one other structure it gives rise to?
Ventral messetry derived from the septum transversum
Gives rise to the thoracic diaphgragm
Note: foregut ONLY
(undifferentiated mesoderm)
What are the 2 layers of lateral plate mesoderm and what does each go on to become?
1) Splanchnic: surrounds the developing gut tube and forms a double fold of peritoneum (messentry)
2) Somatic: lines body wall
What does the intraembryonic coelm become in an adult?
the abdominal cavity
What is the arterial, lymph and nervous innervation to the Foregut?
Arterial: coeliac trunk
Lyphatics follow the arterial supply
Parasympathetic innervation from the vagus nerve
During development how do we get formation of a greater and lesser curvature (as seen in an adult stomach)?
The dorsal wall of the stomach grows FASTER than the ventral wall. This differential growth will form the greater (dorsal wall) and lesser curvatures of the stomach
In the foregut what do the dorsal and ventral messentries divide?
What does each contribute to?
They divide the foregut cavity into R and L sacs (in this region ONLY).
Once development of the omenta and rotations of the foregut have completed, these sacs produce distinct spaces in the peritoneal cavity
- The left sac will become the space anterior and inferior to the stomach = Greater Sac
- The right sac will become the space posterior to the stomach = Lesser Sac
How do the greater and lesser sacs communicate?
Via a small opening located near the hilum of the liver called the epiploic foramen (aka Foramen of Winslow)
How is the developing stomach attached to the body walls?
What is found either side of the developing stomach before rotation and enlargement?
By dorsal and ventral mesentary
The right and left vagus nerves flank the left and right sides of the developing stomach
What are the 2 rotations the developing stomach undergoes simultaneously?
Between what weeks is this occuring?
1) Rotation around longitudinal axis
2) Rotation around anteroposterior axis
Occurs between weeks 4-8
Describe the longitudinal rotation of the stomach
What 2 things does this rotation lead to?
Rotation 1
During week 7 the stomach rotates 90o clockwise about a longitudinal axis.
1) rotation draws the dorsal messentery into a sac which produces a space behind the stomach called the lesser sac
2) the greater curvature now faces to the LEFT of the body and the lesser curvature faces to the right
Describe the anteroposterior rotation of the stomach
What does it lead to?
What week do rotations finish and what is the result?
Rotation 2
The stomach and duodenum rotate about a anterior-posterioraxis which pulls the end of the stomach upwards
By the the 8th week rotations pull the duodenum into a C-shaped positon
At the end of the rotations what has happened to our dorsal and ventral messenteries?
1) The dorsal mesentery has thinned and now hangs from the greater curvature. It now called the Greater Omentum
2) The ventral mesentery is now attached to the developing liver and has formed the Lesser Omentum
What does the lesser peritoneal sac create?
The greater omentum; fold of visceral peritoneum that hangs down from the greater curvature of the stomach
Give 4 things that rotation of the stomach aims to achieve
1) puts the vagus nerves anterior and posterior to the
stomach instead of left and right
2) shifts cardia and pylorus from the midline (stomach lies obliquely)
3) contributes to moving the lesser sac behind the stomach
4) creates the greater omentum
Name 3 functions of the greater omentum
1) Physically limits the spread of intraperitoneal infections
2) Immune contribution
3) Fat deposition
As the stomach rotates what happens to the Vagus nerves?
What are these renamed as a result?
Initially vagus nerves are located on the R and L body walls of the developing stomach. As the stomach rotates as do these.
Results in the L vagus nerve being located on the anterior side of the stomach and the R vagus nerve being located on the posterior side.
As a result the left vagus nerve is renamed the ANTERIOR vagal trunk and the right vagus nerve is renamed the POSTERIOR vagal trunk
Forgut derivatives extend between what 2 developing buds?
Extends from the lung bud to the liver bud
From which messentary does the spleen deveop?
Dorsal messentry stem cells grow into a region to form the spleen
From which messentary does the liver develop and where?
What are the 2 components it divides into during development?
How are the hepatocytes and duct system formed?
The liver develops as a hepatic bud within the ventral mesentery at the foregut-midgut boundary.
As it grows it divides into two parts:
- the falciform ligament
- the lesser omentum
The hepatic bud goes on to form hepatocytes and the duct system of the liver. The ventral mesentery goes on to form the vascular and connective tissue components
As the liver develops at the foregut-midgut boundary, what marks the transition between the 2?
What does its development cause in relation to other closely related orangs?
During development the common bile duct joins the main pancreatic duct to enter the duodenum together at the duodenal papilla.
This point marks the transition (boundary) from foregut to midgut
The liver occupies a large proportion of the abdomen during development hence causes slight movemnet of both the stomach and the spleen
During development of the liver what can failure of the duct system to canalize lead too
What would be observed in an infant soon after birth?
Can cause biliary stenosis or atresia which results in obstruction to outflow of bile.
The infant becomes jaundiced soon after birth
What vein runs along the falciform ligament during development of the liver and what shunt does it travel through?
What happens to these after birth?
The umbilical vein from the placenta runs along the free border of the falciform ligament. It bypasses the developing liver through the ductus venosus (shunt)
After birth these channels close forming the:
- ligamentum teres (remenant of the umbilical vein crossing the falciform ligament)
- ligamentum venosum (remenant of the shunt)
Endodermal cells also give rise to the primordium of lung, where does this grow and how is respiratory system seperated from the GI system?
In the 4th week, a respiratory diverticulum forms in the
ventral wall of the foregut at the junction with the pharyngeal gut:
- Respiratory primordium located ventrally
- Oesophagus located dorsally
Initally these remain connected but as the transoesophageal septum grows across and eventually fuses it seperates the future trachea from the gut tube
Name 2 conditions that can occur as a result of abnormal positioning of the tracheoesophageal septum
1) Proximal blind ended esophagus
2) Tracheoesophageal fistula
Does the pancreas develop from the ventral or dorsal messentery and which region of the developing foregut is it located in?
BOTH!! The pancreas is a foregut derivative
The pancreas has two origins, differential growth and rotation of the duodenum brings the two pancreatic buds togther and they fuse
The ventral bud becomes: the uncinate process and inferior head of the pancreas
The dorsal bud becomes: the superior head, neck, body and tail of pancreas (majority)
What can happen if there is abnormal rotation of the duodenum? (think pancreas)
Abnormalities in the rotation process can lead to an annular (ring-shaped pancreas). This is rare, but important because it can obstruct the duodenum
Which region of primitive gut tube does the duodenum develop from?
Describe its growth
Develops from caudal foregut and cranial midgut
Grows rapidly. Forms a C-shaped loop when stomach rotates which pushes the duodenum to the right, then against posterior abdominal wall (retroperitoneal)
In 5th-6th weeks the lumen is obliterated, then recanalised by the end of the embryonic period
What are the 2 retroperitoneal structures of the foregut?
1) Duodenum (except duodenal cap)
2) Pancreas
What are the 6 adult derivatives of the Midgut?
- Duodenum (distal to entrance of bile duct)
- Jejunum
- Ileum
- Cecum
- Ascending colon
- Proximal 2/3 transverse colon
What is the arterial, lymph and nervous innervation to the midgut?
artery: superior messenteric artery
lymphatics follow the arterial supply
Parasympathetic innervation from the vagus nerve
How does the “Primary intestinal loop” form?
How is it connected to the yolk sac and what structure acts as its axis for rotation?
The midgut elongates enormously and eventually runs out of space. This causes it to make a loop
The apex of the primary loop is connected to the yolk sac by the vitelline duct (which lies within the umbilical cord).
It has the SMA as its axis and it has cranial & caudal limbs
What structures do the cranial and caudal limbs of the primary intestinal loop go on to become?
Cranial Limb of the primary intestinal loop will form most of the small intestine
Caudal Limb of the primary intestinal loop will form the very distal part of the ileum and proximal large intestine
- cecum/appendix/ascending colon, proximal 2/3rds transverse colon
What is Physiological herniation?
When and why does it occur and what happens after it?
During week 6 growth of the primary intestinal loop and abdominal organs (liver) are growing FASTER than the abdominal cavity itself.
The loop continues elongating and the abdominal cavity space becomes too small to accommodate it.
This forces the primary intestinal loop to herniate THROUGH the umbilicus into the umbilical cord ➞ “physiological herniation”
As herniation occurs what else is happening to the primary intestinal loop?
As herniation occurs the loop undergoes the first 90o COUNTERclockwise rotation about the Superior Mesenteric Artery
The primary intestinal loop continues to grow and elongate and the caudal limb also develops an enlarged Cecal Bud which will form the cecum
In which week do the intestines return to the abdominal cavity following “physiological herniation” and what else occurs as it does this?
Which structures re-enter first and last?
By week 10, the midgut loop returns to the abdominal cavity as it has now grown sufficently enough to fit the intestines
As it does this it undergoes its second and third 90o COUNTERclockwise rotations
- The future jejunum and ileum re-enter first
- The cecum decends last, resulting in its adult position in the iliac fossa also (this also causes the ascending colon to elongate)
Abnormalities of rotation are common, give one example that is NOT of concern
If there is an abnormally large opening between the abdominal cavity and the umbilical cord after physiological herniation why may occur?
Abnormal positioning of midgut derivatives
- e.g. appendix on the left
An umbilical hernia may be present at birth
Any malrotation of the gut is associated with a great risk of what?
Volvulus (where intestine twists on itself), which can cause pain/necrosis of the twisted segment
Give 3 congenital malformations
1) Omphalocele
2) Meckel’s Diverticulum
3) Non-rotation
Name 3 things that can occur as a result of a persisting vitelline duct (in order of severity)
1) Vitelline cyst: vitelline duct forms fibrous strands
2) Vitelline fistula: direct communication between the umbilicus
and intestinal tract
3) Meckel’s diverticulum: a persistent yolk sac remnant in the midgut (the most common GI anomaly)
What is the rule of 2’s in Meckel’s diverticulum? (5 things)
2% of the population
2 feet from the ileocecal valve
2 inches long
usually detected in under 2’s
2:1 male:female
What are the potential consequences of having a Meckel’s diverticulum?
The mucosa can contain ectopic gastric or pancreatic tissue, which may contain cells that secrete acid and/or proteolytic enzymes. If the secretions are high enough it can cause an ulceration
What other condition can Meckel’s diverticulum mimic?
Early stages of appendicitis
What is recanalisation and give examples where it may be required
What can happen if it’s unsuccessful?
During development, some structures cell growth becomes so rapid that the lumen is partially or completely obliterated (eg. oesophagus, bile duct, small intestine). Recanalisation must occur to restore the lumen
If recanalisation is wholly or partially unsuccessful then atresia (lumen obliterated) or stenosis (lumen narrowed) of the structure can occur
Where do atresias and stenoses most commonly occur in the GI tract and what are the 2 most common causes?
Most occur in the duodenum, commonly due to:
- incomplete canalisation (most common) ➞ upper duodenum
- “vascular accidents” caused by malrotation, volvulus, body wall defect etc.. ➞ lower duodenum
What is Gastroschisis?
Failure of closure of the abdominal wall during folding of the embryo. Leaves gut tube and derivatives outside the body cavity
What is omphalocele?
Persistence of a physiological herniation
These are a type of Intestinal hernia and occur when the intestines fail to return to the abdominal cavity in the 10-11th week. The herniated loops of bowel produce a large swelling covered only by the amnion outside the body!
Usually associated with other malformations such as a neural tube defect or cardiac malformation
What are the 5 adult derivatives of the hindgut?
- Distal 1/3 transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Upper anal canal
- Internal lining of bladder & urethra
What is the arterial, lymph and nervous innervation to the Hindgut?
artery: inferior mesenteric artery
lymphatic drainage follows the arterial supply
Parasympathetic innervation from the pelvic splanchnic nerves (S2, S3 & S4)
What structure does the Hindgut end in and during what week?
What separats the hindgut and proctodeum + how does this then form the adult anal canal
At 6 weeks the hindgut ends in the cloaca and is separated from the proctodeum by the cloacal membrane
The membrane ruptures resulting in hindgut connected to the exterior (the anal canal)
What structure is responsible for cloacal partitioning, how does this form and what are the end results?
A wedge of mesoderm grows down into the cloaca known as the urorectal septum which provides an anteroposterior division
This divides it into the urogenital sinus anteriorly and the anorectal canal posteriorly
Give 3 Hindgut abnormalities and briefly explain each
What is an imperforate anus and anorectal agenesis?
1) Imperforate anus: failure of anal membrane to rupture
2) Anal / anorectal agenesis: high blind-ending rectum, absence of anus and anal canal
3) Hindgut fistulae
Which structures retain their mesenteries and which becomes fused?
Jejunum, ileum, appendix, transverse colon and sigmoid colon RETAIN their dorsal messenteries ➞ gut remains mobile
- these are structures of the foregut/midgut
Duodenum, caecum, ascending colon, descending colon and rectum adhere to the peritoneum of the posterior abdominal wall so they have “fused messenteries”
- these are structures of the midgut/hindgut
