Development of the gut Flashcards
What are the divisions of the lateral plate of the mesoderm?
- Somatic - develops into the abdominal wall
- Splanchnic - develops into smooth muscles of gut wall
Which parts of the trilaminar disc are involved in the development of the GI tract?
- Endoderm - forms internal epithelial lining of GI tract
- Mesoderm - splits into 2 layers
- This creates a space known as the coelomic cavity
When does development of the primitive gut tube begin?
- Week 4
Which bits of the GI tract are in the foregut?
- Oesophagus
- Stomach
- Duodenum
Which bits of the GI tract are in the midgut?
- Duodenum
- Jejunum
- Ileum
- Ascending colon
- Distal 2/3 transverse colon
Which bits of the GI tract are in the hindgut?
- Distal 2/3 transverse colon
- Descending colon
- Sigmoid colon
- Rectum
How does the trilaminar disc form in the 4th week of development?
- Lateral folding
- This creates a ventral body wall and a tubular primitive gut
- Cranio-caudal folding
- This creates the cranial and caudal blind end pouches from yolk sac endoderm.
- i.e. there is no hole for the mouth or anus yet
What is the gut tube?
- Lined with endoderm
- Has blind pouches at the head and tail ends
- Opens at umbilicus via Vitelline duct
Where do the different divisions of the gut develop from?
- Foregut + hindgut develop from head and tail blind pouches
- Midgut develops from Vitelline duct
What is the intraembryonic coelom?
- A cavity in the centre of the embryo
- Forms the thoracic and abdominal cavities
- Cavities are lined by serous membrane
- This is the peritoneum in the abdomen
Which embryonic structure forms the viscera of the abdominal cavity?
- Splanchnopleuric mesoderm
What is a mesentery?
- Double fold of peritoneum
- Suspends gut tube within intraembryonic coelom away from abdominal wall
- Allows for gut tube motility
- Provides a path for blood and nerve supply from abdominal wall to gut tube
- Form as a continuation of the mesoderm
Which blood vessels supply the gut?
- Foregut - coeliac artery
- Midgut - superior mesenteric artery
- Hindgut - inferior mesenteric artery
What happens to the gut tube in the 5th week of development?
- Endoderm proliferates
- Occludes lumen of gut tube
- This recanalises in the 9th week
When does the tracheoesophageal septum form?
- Week 4
- Trachea sits anteriorly and oesophagus sits posteriorly
- Lung bud forms ventrally off of foregut
How does the stomach form?
- As a dilation in the foregut
- Rotates longitudinally 90o anticlockwise
- Dorsal edge swings left and becomes greater curve
- Ventral edge swings right and becomes lesser curve
- Greater curve grows faster than lesser curve
What are the two mesenteries attached to the gut tube?
- Dorsal mesentery
- Ventral mesentery
Where is the dorsal mesentery found?
- Attaches the back of the gut tube to the posterior abdominal wall.
- Runs the whole length of the gut tube
Where is the ventral mesentery found?
- Attaches the front of the gut tube to the anterior abdominal wall
- Only attaches the foregut
- Leaves a free edge at the end of the foregut
What forms the greater and lesser sacs of the GI tract?
- Dorsal and ventral mesenteries divide the intraembryonic coelom into 2 sections: the left and right sac.
- Left sac develops into greater sac
- Right sac develops into lesser sac
Where do the liver and spleen grow from?
- Liver grows in ventral mesentery
- develops from anterior bud of foregut
- Spleen grows in dorsal mesentery
Is there a connection between the greater and lesser sac of the abdomen?
- Yes - the epiploic foramen or foramen of Winslow
What are omenta?
- Sheets of visceral peritoneum
- Extend from stomach and duodenum
- Form boundaries of greater and lesser sacs
Describe the structure of the greater omentum
- 4 layers of visceral peritoneum
- attaches to greater curvature of stomach and transverse colon
- prevents adhesion of organs to parietal peritoneum
- covers inflamed organs to protect nearby structures
Describe the structure of the lesser omentum
- 2 layers of visceral peritoneum
- attaches to lesser curvature of stomach and liver
- has a free edge which conducts bile duct, portal vein, and hepatic artery to the liver.
What are the peritoneal ligaments?
From anterior to posterior:
- Falciform ligament (liver)
- Lesser omentum (stomach)
- Gastrosplenic ligament (spleen)
- Spleno-renal ligament (left kidney)
Outline how the pancreas develops
- Develops in 2 buds - one in ventral mesentery and one in dorsal mesentery
- Ventral bud rotates so that it lies inferior to dorsal bud
- Buds then fuse to become one gland
Where does the midgut begin?
- At the midpoint of the second part of the duodenum
- Where common bile duct and major pancreatic duct enter duodenum
When does herniation of the midgut occur?
- During 6th week of development
Why does the midgut herniate?
- Intestinal loop of midgut elongates rapidly
- Liver is also growing rapidly
- Abdominal cavity doesn’t grow at the same rate
- There isn’t enough room for both of these viscera to develop.
- Intestinal loop herniates out through umbilical cord to create space.
What important structure is the axis of the loop of intestine that herniates out of the abdominal cavity?
- Superior mesenteric artery
What happens to the loop of intestine after it herniates out of the abdominal cavity?
- It makes 3x 90o turns anticlockwise
- This moves the small intestine to the left of the large intestine
- Between 2nd and 3rd rotation, caecum descends to normal location in right iliac fossa
- Then intestine returns into abdominal cavity at week 10 of development
What is incomplete rotation of the midgut?
- Only 1x 90o rotation occurs
- Left-sided colon
What is reversed rotation of the midgut?
- 1x 90o rotation occurs clockwise
- Transverse colon sits posterior to small intestine
What is recanalisation of the gut?
- Growth of intestine is so rapid that the lumen can become partially or completely occluded.
- Recanalisation restores the lumen
What is omphalocoele?
- Herniated contents of midgut remain within umbilical cord when baby is born
- Midgut contents still covered with peritoneum so still develop relatively normally
- Mortality is high due to associated genetic defects
What is gastroschisis?
- Abdominal wall fails to form anteriorly
- Intestine and other organs herniate through abdominal wall
- No peritoneal covering
- Amniotic fluid negatively affects gut development
- Mortality rate lower than omphalocoele due to less associated genetic defects
What causes vitelline duct abnormalities?
- When vitelline duct does not fully obliterate
- Can cause vitelline cysts or fistulae
- Meckel’s diverticulum
What is Meckel’s diverticulum?
- Outpouching of gut where vitelline duct hasn’t fully obliterated
- Like an appendix of small bowel
What is the rule of 2s?
Meckel’s diverticulum:
- Affects 2% of population
- Is located 2 feet from the ileocecal valve
- Detected in under 2s
- Affects 2x more men than women
What happens if recanalisation of the midgut is unsuccessful?
- Atresia (complete closure) or stenosis (narrowing) can occur
- Often affects duodenum
What supplies the hindgut?
- Inferior mesenteric artery
- Least splanchnic nerve (T12)
- Pain in the hindgut localises to the suprapubic region
What is the cloacal membrane?
- Covers the hindgut during development
- Ruptures in week 7, opening up anorectal canal to amniotic fluid
What is the cloaca?
- The name given to the hindgut that communicates with the developing genitourinary tract
- Area where ectoderm meets endoderm without any dividing mesoderm
What is the role of the urorectal septum?
- Wedge of mesoderm
- Descends caudally during weeks 4-7
- Separates cloaca into urogenital sinus and anorectal canal
What divides the anal canal into sections?
- Pectinate line
- This is the point where endoderm and ectoderm met suring development
Describe the anal canal above the pectinate line
- Anal canal developed from endoderm
- Supplied by inferior mesenteric artery and least splanchnic nerve (T12)
- Vague pain due to stretch
- Non-keratinised
Describe the anal canal below the pectinate line
- ## Anal canal derived from ectoderm
Describe the anal canal below the pectinate line
- Anal canal derived from ectoderm
- Supplied by pudendal artery and pudendal nerve
- Localised pain
- Keratinised
What are some hindgut abnormalities?
- Imperforate anus - failure to rupture anal membrane
- Anorectal agenesis - anus doesn’t form
- Fistulae between rectum and anus
