27-01-23 – Peritoneal and GI Development Flashcards
Learning outcomes
- Describe the embryological formation of the components of the foregut, midgut and hindgut
- Describe the embryological formation of the intra-abdominal accessory organs of the digestive system
- Describe the embryological development of the peritoneal cavity, greater and lesser sacs
- Describe the embryological development of the greater and lesser omenta
- Describe how the omental bursa (Lesser sac) is formed
- Explain how the omental bursa (Lesser sac) can be accessed
- Describe the development of the spleen
- Relate adult structure and position to embryological development
- Explain the mechanisms underlying congenital abnormalities of the gastro-intestinal tract and associated glands
Trilaminar embryo. What does the neural crest form?
What does the paraxial mesoderm form?
What does intermediate mesoderm form?
What does parietal (somatic) lateral plate mesoderm form?
What does the intraembryonic coelom form?
- Neural crest forms the endocardial cushions
- Paraxial mesoderm forms somites
- Intermediate mesoderm forms genitourinal (GU) systems
- Parietal (somatic) lateral plate mesoderm forms the body walls
- The intraembryonic coelom forms body cavities (thoracic, abdominal, pelvic) in between the visceral and parietal lateral plate mesoderm
- The visceral (splanchnic) lateral plate mesoderm forms serous membranes e.g pleural membranes, serous pericardium, peritoneum
What does the lateral folding of the body walls cause?
What occurs when lateral folding is complete?
What is the primitive yolk sac used for?
What does the primitive yolk sac form?
What connection between structures exists?
What does the intraembryonic cavity for?
What is the gut tube suspended by?
What will the layer of visceral lateral plate mesoderm surrounding the gut tube form?
- The lateral folding of the body wall results in the strangulation of the yolk sac
- When the lateral folding is complete, the yolk sac develops into a secondary yolk sac outside of the body
- The primitive yolk sac acts as our nutrient, helping us to develop
- The primitive yolk sac develops into the endodermal digestive tract or the gut tube in week 4
- A connection will remain between the gut tube and the secondary yolk sac.
- This will be called the yolk stalk, which will later contain the Vitelline duct
- Intraembryonic cavity (coelom) develops into the abdominopelvic cavity after lateral folding
- The gut tube is suspended by dorsal mesentery
- Visceral layer of lateral plate mesoderm surrounding the gut tube will develop into visceral peritoneum, which will form the dorsal mesentery
Where does the gut tube extend between?
What are the 3 parts of the gut tube?
What is the arterial supply to each section?
Where does each portion extend between?
Where is the vitelline duct located?
What is the foregut innervated by?
Where is pain referred?
What occurs in the foregut by week 5?
What 6 things does the foregut give rise to?
What are the foregut and hindgut closed over by?
- Some time later, the gut tube is developed and extends from the mouth to the anus
- The gut tube is divided into 3 parts based on arterial supply
1) Cranial foregut
* Supplied by the coeliac trunk/branches of the coeliac trunk
* Part of gut tube extending from the mouth to halfway round the duodenum (small intestine) just distal to the developing liver
* Innervated by T7 to T9 - pain is referred to epigastrium
* By week 5, the thoracic and abdominal portion of the foregut is visibly divided into the pharynx, oesophagus, stomach, and proximal duodenum
- 6 things the foregut gives rise to:
1) Oesophagus (which, in turn gives rise to Tracheo-bronchial tree)
2) Stomach
3) Proximal duodenum
4) Liver
5) Gall bladder
6) Pancreas
2) Midgut
* Supplied by the superior mesenteric artery
* Opens to the yolk sac through the vitelline duct
* Goes from the distal portion of the duodenum to 2/3rds of the way round the transverse colon
3) Caudal hindgut
* Supplied by the inferior mesenteric artery
* Extends between the distal 1/3rd of the transverse colon up to the proximal ½ of the anal canal
- Both ends are closed over initially – the oral cavity (foregut) is closed over by the oropharyngeal membrane and the cloaca (hindgut) is covered by the cloacal membrane
What happens to the foregut by week 4?
What is the gut tube covered by?
What is the role of the mesogastrium?
How is the greater curvature, fundus, and cardiac notch of the stomach formed?
How do the left and right vagus sit on the stomach?
Why is this?
What is the lesser sac?
- By Week 4, part of the foregut which will become the stomach starts to dilate and become fusiform
- The gut tube is covered by a visceral layer of lateral plate mesoderm which will become visceral peritoneum
- The dorsal mesentery attaches the stomach to the posterior abdominal wall, and we call It the mesogastrium
- Differential growth of the stomach walls (the dorsal wall of the stomach growing faster than the ventral wall) results in formation of the greater and lesser curvature, fundus and cardiac notch of the stomach
- The left vagus on the anterior surface of the stomach and the right is on the posterior surface
- This is due to the rotation of the stomach
- The lesser sac may be conceptualized as the space posterior to the lesser omentum, between the posterior wall of the stomach and the surface of the peritoneum that covers the anterior surface of the left kidney
Greater and lesser sac
greater and lesser omentum diagram
Describe the rotation of the stomach in weeks 6-7.
How does this alter the position of the stomach?
Describe what forms the greater and lesser omentum form
- During weeks 6-7, Stomach rotates 90° clockwise around its longitudinal (vertical) axis (while also rotating around its sagittal axis)
- As a result of rotation around the longitudinal axis, left side faces anteriorly, the lesser curvature faces to the right (and upwards) and the greater curvature faces to the left (and downwards)
- As a result of slight rotation around the sagittal axis, lesser curvature faces slightly upwards
- When the stomach starts to rotate, the dorsal mesentery surrounding the stomach and attaching it to the posterior abdominal wall is still there, but it rotates
- The dorsal mesentery (mesogastrium) attaching to the greater curvature will become the greater omentum
- The lesser omentum is a derivative of ventral mesentery, as it is as a derivative of the stomach and extends between the lesser curvature and the liver
Describe where the liver grow.
What forms the falciform ligament?
What structures does it sit between?
What vein sits in the falciform ligament?
What does the visceral mesentery covering the liver form?
What is the bare area?
Where is it attached?
- The liver grows anterior and superior to the stomach, still in the lateral plate mesoderm
- The ventral mesentery and septum transversum forms a ligament that extends between the liver and anterior abdominal wall called the falciform ligament
- At the end of the free end of the falciform ligament, we have the umbilical vein from the umbilicus, which goes in the falciform ligament towards the liver
- The visceral mesentery covering the liver will become the visceral peritoneum of the liver
- The ventral mesentery covers all of the liver, but leaves an area at the top called the bare area, which is attached to the under surface of the diaphragm
Is the duodenum part of the foregut or midgut?
When will the duodenum rotate?
How is its position altered?
What shape does it take on?
Which parts of the duodenum are intraperitoneal and secondary retroperitoneal?
What does secondary retroperitoneal mean?
- The duodenum forms from the foregut & beginning of midgut
- When the stomach rotates, it rotates the duodenum as well
- The duodenum is shifted from the midline to the right of the midline becomes adjacent to the posterior abdominal wall
- The connection with the rest of the small intestine remains, so it takes on a C-shape
- The 1st part of the duodenum is intraperitoneal, because it is like a continuation of the stomach and is covered fully by the peritonuem
- The 2nd, 3rd, and 4th part of the duodenum is secondary retroperitoneal
- The duodenum is part of the foregut and the midgut, so will receive blood supply from both the coeliac trunk and the superior mesenteric artery, both of which will anastomose
- Structures that are secondary retroperitoneal develop intraperitoneally (covered in peritoneum), and are attached to the posterior abdominal wall
- They will then lose their peritoneum and move behind the posterior abdominal wall, becoming secondary retroperitoneal
What occurs inferior to the stomach on day 22?
What does the liver bud form?
Where does the hepatic vierticulum grow towards?
What does the hepatic diverticulum also give rise to?
What does the septum transversum develop into?
What 3 things is the liver formed from?
What 3 things does the septum transversum develop into?
- On day 22, inferior to the stomach, a small endodermal thickening, the liver bud forms on the ventral side of the duodenum (hence in the ventral mesentery)
- The liver bud forms the hepatic diverticulum, which forms hepatoblasts, which forms hepatocytes (parenchyma), which forms lobules of the liver
- Hepatic diverticulum grows towards and into the septum transversum
- The hepative diverticulum also gives rise to bile canaliculi of the liver (hepatic ducts), in addition to hepatocytes (parenchyma)
- Septum transversum develops into Liver sinusoids (stroma)
- 3 things the liver is formed from:
1) Stroma (liver sinusoids)
2) Parenchyma (hepatocytes)
3) Hepatic ducts - 3 things the septum transversum develops into:
1) Diaphragm
2) Ventral mesentery
3) Stroma (liver sinusoids)
What structure does the liver develop within?
Where does the liver move when the stomach rotates?
What forms the falciform ligament?
What is located at the falciform ligament?
Where does the lesser sac develop?
Where is the bare area of the liver located?
Where does it attach?
What is the lesser omentum?
What does the free border of the lesser omentum mark?
- The liver develops within the ventral mesentery, with the mesentery around the liver forming its visceral peritoneum
- With the rotation of the stomach, it also moves the liver to the right of the abdomen
- The ventral mesentery is attached to the liver and anterior abdominal wall above the midline, which forms the falciform ligament
- The falciform ligament is a free border where the ligamentum venosum is located
- Posteriorly and medially to the liver, the lesser sac develops
- The ventral mesentery covers all of the liver, but leaves an area at the top called the bare area, which is attached to the under surface of the diaphragm
- The lesser omentum is part of the ventral mesentery that extends between part of the liver and the lesser curvature of the stomach
- The free border of the lesser omentum marks the epiploic foramen between the greater and lesser sacs of the peritoneal cavity
Greater and lesser sac
Greater and lesser omentum diagram
What occurs on day 26?
Where does the gallbladder develop?
What is it formed from?
- By day 26, an endodermal thickening called the cystic diverticulum develops on the ventral side of the duodenum just caudal to the hepatic diverticulum and grows into the ventral mesentery
- The gallbladder develops immediately below the hepatic bud/liver
- It is formed from the cystic diverticulum, which will form the gallbladder and cystic duct
What does the pancreas develop from?
Where are the ventral and dorsal pancreatic bud located?
What will they each develop into?
How does the rotation of the duodenum affect the position of the pancreas?
How does the pancreas become secondarily retroperitoneal?
- The pancreas develops from the ventral and dorsal pancreatic bud
- The ventral bud is immediately below the cystic diverticulum, and develops in the ventral mesentery as the gallbladder
- The dorsal bud is 180 degrees behind the ventral bud and develops towards the dorsal mesentery just caudal to the developing gallbladder
- The ventral pancreatic bud will give rise to the uncinate process and part of the head of the pancreas
- The dorsal pancreatic bud forms part of the head of the pancreas, the neck, body and the tail of the pancreas
- The rotation of the duodenum causes the ventral bud to migrate around to lie behind & fuse with the dorsal bud so that the head and uncinate process of pancreas lie in the curve of the duodenum
- During this process, the pancreas becomes adjacent to the posterior abdominal wall
- The peritoneum/visceral mesoderm is absorbed, so the pancreas becomes secondarily retroperitoneal
What are the ducts of the dorsal and ventral pancreatic buds?
Why do these buds need ducts?
Describe the 6 steps in the fusion of the ductal system of the pancreatic buds?
- The ventral pancreatic bud contains the ventral pancreatic duct, which connects to the duodenum via the major papilla
- The dorsal pancreatic bud contains the dorsal pancreatic duct and connects to the duodenum via the minor papilla
- These buds need ducts due to the endocrine/exocrine nature of the pancreas, and the need to have a connection to the duodenum in order to release substances
- 6 steps in the fusion of the ductal system of the pancreatic buds:
1) When the ventral and dorsal pancreatic buds fuse, their ductal systems also become interconnected.
2) The proximal portion of the duct of dorsal pancreas degenerates
3) The accessory pancreatic duct is the remnant of the duct of the dorsal bud
4) The ducts of the dorsal & ventral buds unite to form the main pancreatic duct
5) The main pancreatic duct and the common bile duct meet and drain into the 2nd part of duodenum at the major duodenal papilla or ampulla of Vater.
6) Sometimes the accessory pancreatic duct persists and empties into the duodenum at a minor duodenal papilla
What 6 structures does the midgut form?
What occurs in the future ileum at week 5?
What is the midgut connected to?
- 6 structures the midgut forms:
1) Distal duodenum
2) Jejunum
3) Ileum
4) Caecum
5) Ascending colon
6) Proximal two thirds of the transverse colon - The future ileum elongates more rapidly and by the fifth week the midgut makes the primary intestinal loop
- The midgut, especially the ileum, is the connected to the umbilicus and the vitelline duct
Why do midgut organs start to herniate at around week 6?
What rotation/ position changes do we see during this process?
What do the small intestine and caecum form during this?
- Pressure of growing abdominal organs (especially the liver) forces the primary intestinal loop to herniate into the umbilicus during 6th week
- As the primary intestinal loop herniates, it rotates around the axis of the superior mesenteric artery by 90 degrees counter-clockwise (as viewed from the ventral side) so that the future ileum lies in the right abdomen and the future large intestine lies in the left abdomen
- The small intestine keeps on elongating to form jejunal-ileal loops
- The caecum expands and vermiform appendix is also formed
What occurs in the midgut in week 10?
What occurs in week 11?
How do the positionings of the caecum and descending colon change?
What 4 structures in the foregut remain suspended by mesentery?
- During the 10th week, the midgut retracts into the abdomen
- During the 11th week, the retracting midgut completes this rotation
- The caecum is then displaced inferiorly, pulling down the distal midgut to form the ascending colon.
- The descending colon is simultaneously fixed on the left side of the posterior abdominal wall
- 4 structures in the foregut remain suspended by mesentery:
1) Jejunum
2) Ileum
3) Transverse colon
4) Sigmoid colon
What 4 structures does the hindgut form?
What is the last part of the hindgut called?
What is it closed over by?
How is the allantois form?
What will the allantois eventually form?
What occurs in the hindgut from weeks 4 to 6?
What does the urorectal septum do to the cloacal membrane?
How does this affect the anorectal canal?
What is the pectineal line?
When was the cloacal membrane connected to the pectineal line
- 4 structures the hindgut forms:
1) Distal third of the transverse colon
2) Descending colon
3) Sigmoid colon
4) Upper two thirds of the anorectal canal - The last part of the hindgut is called the cloaca, which is closed over by the cloacal membrane
- A slim diverticulum of the cloaca called the allantois extends into the yolk stalk
- The allantois will eventually form the urachus, which forms the posterior surface of the abdominal wall between the bladder and the umbilicus
- During the 4th to 6th weeks, a coronal urorectal septum made from mesoderm divides the cloaca by moving towards the skin surface and forming the perineal body:
1) Anteriorly the cloaca develops into the urogenital sinus, which will form urogenital structures, such as the urethra
2) Posteriorly the cloaca develops into the anorectal canal
- As the tip of the urorectal septum approaches the cloacal membrane, the anal part of the membrane sinks into the anal pit and the cloacal membrane ruptures
- As a result, the anorectal canal open to the exterior
- The pectinate line marks the boundary between hindgut endoderm and ectoderm, hence the boundary between different sources of blood & nerve (somatic vs autonomic)
- The cloacal membrane was also attached to the pectineal line up until week 7
Mesentery derivatives.
What 3 structures develop from the 2 layers of ventral mesentery?
Where is the dorsal mesentery found?
What is the falciform formed by?
What does the free caudal region of the falciform ligament carry?
What forms the lesser omentum?
What is the hepatoduodenal ligament formed from?
What 3 things does the hepatoduodenal ligament contain?
What forms the hepatogastric ligament?
- Mesentery derivatives.
- 3 structures develop from the 2 layers of ventral mesentery:
1) Liver
2) Gall bladder
3) Ventral pancreatic bud - The dorsal mesentery is found posterior to the greater curvature of the stomach and duodenum
- The remnant of ventral mesentery connecting the liver to the anterior body wall together with the septum transversum becomes the falciform ligament
- The free caudal margin of falciform ligament carries the umbilical vein from the body wall to the liver
- The ventral mesentery between the liver and lesser curvature of the stomach forms the lesser omentum.
- The caudal border of the lesser omentum, connecting the liver to the developing duodenum, is called the hepatoduodenal ligament
- 3 things the hepatoduodenal ligament contain (portal triad):
1) Portal vein
2) Proper hepatic artery
3) Common bile duct - The region of the lesser omentum between the liver and the stomach is called the hepatogastric ligament.
MESENTERY DERIVATIVES: Greater omentum.
Where does the greater curvature lie initially?
What allows it to be attached?
How does the positioning of the stomach change after rotation in the sagittal axis?
How is the greater omentum formed?
Where is the greater omentum attached?
Where do peritoneal layers of greater omentum fuse?
What structures form behind the stomach due to rotations?
How can we access the lesser sac from the greater sac?
- MESENTERY DERIVATIVES: Greater omentum
- The greater curvature was initially attached to the posterior abdominal wall
- This is where the dorsal mesentery/mesogastrium is attached to
- After the stomach’s rotation in the sagittal axis, it is separated from the posterior abdominal wall
- The dorsal mesentery maintains its attachment to the greater curvature and will become the greater omentum hanging over the transverse colon with 3 free borders
- The greater omentum is partially attached to the posterior abdominal wall
- The 4 peritoneal layers of the greater omentum will fuse together anterior to the transverse colon
- As a result of the rotations, a pouch called omental bursa or lesser sac is formed behind the stomach
- We can access the lesser sac from the greater sac by cutting through these 4 fused peritoneal layers of the greater omentum (and epiploic foramen)
Is the spleen endodermal?
What does the spleen develop from?
Where does the spleen develop?
What forms the gastrosplenic ligament?
- The spleen is a mesodermal derivative, not a product of the gut tube endoderm, meaning the spleen is not a digestive organ
- The spleen develops from mesenchyme near the body wall
- The spleen develops in the dorsal mesentery, but with rotation of the stomach and growth of the dorsal mesogastrium, the spleen is shifted to the left side of the abdominal cavity
- The portion of the dorsal mesentery between the spleen and the stomach is called the gastrosplenic ligament (not a ligament, actually a peritoneal fold)
What happens normally in the development of the midgut?
What are 3 congenital abnormalities of the midgut?
- In normal development, the midgut herniates through the umbilicus because the abdomen is not big enough
- When it is big enough, the midgut retracts and the umbilicus closes after birth
- 3 congenital abnormalities of the midgut:
1) Gastroschisis
* Baby’s intestines extend through a hole next to the belly button
* Gastroschisis is a defect of the abdominal wall, which results in your baby’s bowel being outside of their abdomin
* This means the bowel is not protected by any membrane or sac.
2) Exomphalos
* Exomphalos is a defect at the base of the umbilical cord.
* It causes the bowel to push through (herniate) into the umbilical cord
3) Umbilical hernia
* If the umbilicus doesn’t close, the abdominal contents can herniate through the naval
What are 2 congenital anomalies of the midgut?
- 2 congenital anomalies of the midgut:
1) Partial rotation
2) Abnormal rotation
Where does the vitelline duct extend between in the embryo?
When does it obliterate?
What are 4 abnormalities associated with the vitelline duct?
What conditions will cause faecal discharge at the umbilicus?
- In the embryo, there is a vitelline duct that extends between the ileum of the gut tube and the secondary yolk sac
- The vitelline duct obliterates at 5-9 weeks of gestation and forms a fibrous cord
- 4 abnormalities associated with the vitelline duct:
1) Meckel’s diverticulum
* There is only partial obliteration of the vitelline duct, still a remnant left
* In the ileum
* In 2% of people
* It may ulcerate causing signs/ symptoms similar to appendicitis
2) Vitelline sinus
* Opposite to Meckel’s diverticulum
3) Vitelline cyst
* Centre of fibrous cord still has remnants of the vitelline duct
4) Vitelline fistula
* Vitelline duct persists
- Patent vitelline duct or vitelline fistula causes faecal discharge at the umbilicus
Describe the formation of an annular pancreas
- The ventral pancreas may consist of two buds, which if they migrate around the duodenum in opposite directions to fuse, they can form an annular pancreas
- This results in constriction around the duodenum
What is Hirschsprung disease?
How does this affect the colon?
- Hirschsprung disease is a Lack of normal development of the colonic innervation
- This leads to a constricted aganglionic segment of bowel with a distended segment proximally the innervation of which is normal
Describe the 4 congenital anomalies of the hindgut.
- 4 congenital anomalies of the hindgut:
1) Urorectal fistula
* Anorectal canal not connected to the anal put
* Fistula between the bladder and anorectal anal
2) Rectovaginal fistula
* Anorectal canal connected to the vagina
3) Rectoperineal fistula
* Anorectal canal not connected to the anal pit
* Faecal contents come out of the perineum
4) Anal membrane doesn’t rupture
* Anorectal canal not open to the exterior
What does the respiratory system develop from?
What is a fistula?
What is the most common tracheooesophageal defect?
What are tracheooesophageal fistulas caused by?
What will happen to the trachea with the most common defect?
What will happen if the baby tries to feed?
What are other common fistulas?
- The respiratory system develops from the gut tube endoderm
- A fistula is an abnormal connection between two body parts, such as an organ or blood vessel and another structure
- The most common tracheooesophageal defect is a tracheooesophageal fistula, where there is no connection to the proximal gut tube (oesophagus), and an abnormal connection between the distal gut tube and the trachea (A on diagram)
- This type of fistula accounts for 90% of tracheooesophageal defects
- Tracheooesophageal fistulas are formed by the failure of the fusion of the tracheooesophageal ridge while separating the trachea from the gut tube
- With the most common defect, contents can move back up into the trachea, causing chemical pneumonitis, as the acid from the stomach will damage the respiratory epithelium lining the trachea
- If the baby tries to feed, it will reflux/vomit the food back up
1) What are 4 other tracheooesophageal fistulas?
* 4 other tracheooesophageal fistulas
1) B on diagram
* The gut tube and trachea are separated, but there is no connection between the proximal and distal oesophagus
2) C on diagram
* Connection between the oesophagus and trachea
3) D on diagram
* Connection between proximal oesophagus and trachea, no connection to distal oesophagus
4) E on diagram
* Connection between proximal oesophagus and trachea, and distal oesophagus and trachea
