Development Of The GI System 1&2 Flashcards
when does embryonic folding take place? How does it fold and what does that create?
in the 4th week
Laterally
-> ventral body wall, primitive gut becomes tubular
Craniocaudally -> cranial and caudal pockets from yolk sac endoderm
What is the primitive gut tube?
Endoderm lined tube which runs the length of the body
Blind pouches at the head and tail ends (not open)
Opening at the umbilicus (->midgut)
Splanchnic mesoderm covering
Formed from definitive yolk sac during folding - blind diverticula cranially (buccopharyngeal membrane) & caudally (cloacal membrane)
Embryonic divisions - different blood supply, innervation, Lymph systems
What do the somatic and splanchnic mesoderm become and which is more sensitive to Pain?
Somatic mesoderm -> parietal pleura - more innervation so more sensitive to pain
Splanchnic mesoderm -> visceral pleura - poorly innervated Ill- defined pain
What does the foregut become and what blood vessel supplies them?
Oesophagus -> duodenum (before bile duct)
Oesophagus Stomach Pancreas Liver Gallbladder Duodenum (proximal)
Celiac trunk branch of abdominal aorta
What does the midgut become and what blood vessel supplies them?
Duodenum (after bile duct) -> last 2/3 transverse colon
Duodenum (distal) Jejunum Ileum Caecum Appendix Ascending colon Proximal 2/3 transverse colon
Superior mesenteric artery branch of abdominal aorta
What does the hindgut become and which blood vessel supplies them?
(Distal 1/3) Transverse colon-> internal lining of bladder and urethra
Transverse colon (distal) Descending colon Rectum Upper anal canal Internal lining of bladder and urethra
Inferior mesenteric artery branch of abdominal aorta
Which structures have mixed blood supply and why?
Structures which develop close to the junction between foregut and midgut (duodenum and pancreas)
Duodenum (proximal to bile duct) gastroduodenal artery and superior pancreaticoduodenal artery (celiac trunk)
Duodenum (distal to bile duct) inferior pancreaticoduodenal artery (superior mesenteric artery)
Pancreas superior pancreaticoduodenal artery (celiac trunk) and inferior pancreaticoduodenal artery (superior mesenteric artery)
what is the intraembryonic coelom? How does the intraembryonic coelom get divided and into what?
Subdivided by the future diaphragm (septum transversum) into abdominal and thoracic cavities
Intraembryonic coelom suspends primitive gut tube and is a double layer of splanchnic mesoderm
What does the membrane lining the intraembryonic cavity become?
Pericardium, pleural membrane of lungs and peritoneum of peritoneal cavity (all have parietal and visceral pleura)
What is a Mesentery and what’s its function?
Double layer of peritoneum suspending the gut tube from the abdominal wall
Allows a conduit for blood and nerve supply and mobility where needed
Formed from a condensation of splanchnic mesoderm which surrounds the gut
What are the two gut mesenteries?
Dorsal - suspends the entire gut tube from the dorsal body wall
Ventral - only in the region of the foregut
What do the dorsal and ventral mesenteries in the foregut create and what do they become?
Divide the cavity into:
Left sac contributes-> greater sac
Right sac becomes -> lesser sac (comes to lie behind the stomach and allows for distension)
Form:
Specialised regions of peritoneum
Dorsal Mesentery -> greater omentum (first structure seen when abdo cavity opened anteriorly)
Ventral Mesentery -> lesser omentum (free edge conducts the portal triad)
How does the stomach rotate and what does this mean for the position of the certain structures? How is the greater curve formed?
The stomach enlarged and tips on its axis
Dorsal Mesentery grows faster and creates the greater curve, it folds to form the greater omentum
The vagus N goes from right and left to anterior and posterior
Cardiac and pylorus (opening to duodenum) shifted from midline
Ventral Mesentery becomes the lesser omentum
What is peritoneal reflection?
The peritoneum changes direction so
parietal peritoneum becomes Mesentery
and then Mesentery becomes visceral peritoneum
and that becomes Mesentery etc.
What are the meaning of the words retroperitoneal and secondarily retroperitoneal? What are examples of structures?
Retroperitoneal - never in the peritoneal cavity and never had a Mesentery e.g. kidneys, aorta, IVC
Secondarily retroperitoneal - began development invested by peritoneum and had a Mesentery but Mesentery is lost through fusion at posterior abdominal wall (replaced by fusion fascia) e.g. pancreas, duodenum bar cap, ascending and descending colon, distal 1/3 rectum
What does the foregut extend to and from in development?
Foregut extends from the lung bud (4th week respiratory diverticula forms in ventral wall at junction with pharyngeal gut) to the liver bud
What separates the developing GI and respiratory tracts? What are some consequences of abnormal positioning of this structure?
Respiratory diverticulum and foregut separated by a tracheoesophageal septum
There are a variety of oesophageal abnormalities e.g. proximal blind-end of oesophagus, tracheoesophageal fistula (connection)
What do the foregut-derived glands develop from (liver, biliary system, pancreas)?
Liver and biliary systems (liver, gallbladder, bile ducts) - ventral Mesentery
Pancreas
Uncinate process and inferior head - ventral Mesentery
Superior head, neck, tail, body - dorsal
Peritoneal reflections of the liver from 6th week
Whole liver covered in peritoneum bar ‘bare area’ on diaphragmatic surface - rough, dull areas
Falciform ligament to left
Lesser omentum to right
What does the duodenum develop from and what determines its shape?
Develops from caudal foregut & cranial midgut
Shape determined by rotation of stomach: pushes duodenum to the right and against posterior abdo wall -> secondarily retroperitoneal bar cap
What is the primary intestinal Loop?
Midgut elongates - runs out of space so makes a loop that has:
Superior mesenteric artery at axis
Connected to yolk sac via vitelline duct
Has cranial and caudal limbs
What is the normal physiological herniation that occurs in the 6th week of development?
Growth of the primary intestinal loop is v rapid, liver is also growing rapidly so abdo cavity too small for both -> intestines herniate into umbilical cord
Midgut rotation
during herniation turns 90d
After herniation cranial limb first returns to abdo cavity by rotating 90d to left (so cranial limb now inferior)
Third 90d rotation creates transverse colon (cranial limb now left, SMA middle, caudal limb right)
How does the ascending gut develop?
Caecal bud descends
What are some malrotations of the midgut and what problems are associated with these?
~ incomplete rotation- midgut loop makes one 90d rotation = left sided colon
~ reversed rotation- midgut loop makes one 90d rotation clockwise = transverse colon passes posterior to duodenum
Most complications in neonatal period -> volvulus (loop of intestine twists around itself and Mesentery that supports it) -> strangulation (bowel obstruction) & ischaemia
What are 3 abnormalities associated with the vitelline duct persisting (remnants of the yolk sac)? whats a main problem that could be as a result of all three?
- vitelline cyst
Forms fibrous strands between midgut and umbilicus
-Vitelline fistula
Communication between umbilicus and intestines (contents can leak)
-Meckel’s diverticulum
A.k.a ileal diverticulum, 2% population, 2:1 Male:Fm, 2 feet from ileocaecal valve (62cm), normally picked up before 2yrs
All could contain ectopic gastric or pancreatic tissue -> can become inflamed
In which structures does the lumen become at least partially obliterated and why? What does the body do to restore the lumen? What occurs if this is unsuccessful?
Gut structures: oesophagus, bile duct, small intestine
Cell growth becomes so rapid
Recanalisation occurs to restore lumen
If wholly/ partially unsuccessful -> atresia (no lumen) /stenosis (narrowed)
What is pyloric stenosis? What’s a characteristic symptom?
Region between stomach and duodenum, hypertrophy of circular muscle in sphincter
Not a recanalisation failure
Common abnormality in stomach of infants
-> projectile vomiting
What is gastroschisis?
Failure of closure of the abdominal wall during embryonic folding
Leaves gut tube and derivatives outside the body cavity, no peritoneal layer
Caesarean section but normally fine
What is an omphalocoele?
Persistence of physiological herniation, have covering of peritoneum in umbilical cord
(An umbilical hernia differs as covered in skin and subcutaneous tissue)
Often associated with other genetic defects so mortality rate is high
What separates the two regions of the anal canal? What is different about these regions?
Divided by the pectinate line into distinct superior (part of hindgut) - Inferior mesenteric artery, S2-4 pelvic parasympathetic, columnar epithelium, internal iliac nodes - sensation of stretch only bc made from endoderm
Pectinate line
and inferior (not part of hindgut), pudendal artery, S2-4 pudendal N, stratified epithelium, superficial inguinal nodes, sensations of touch/ temperature & pain bc made from ectoderm
indicates differences in arterial supply, venous and lymphatic drainage & innervation
What do different locations of visceral pain mean?
Poorly localised bc splanchnic innervation
Epigastric pain (top abdominal cavity) -> foregut and derivatives
Periumbilical (belly button) -> midgut
Suprapubic (pelvic region) -> hindgut
Parietal peritoneum- more localised as somatic innervation
What is imperforate anus?
Failure of anal membrane to rupture
What is anal/ anorectal agenesis?
Never develops
What is a hindgut fistulae?
Connection between:
Bladder and rectum - recto-vesical
Vagina and rectum-
Recto-vaginal
Recto-uretheral
What will the cranial and caudal limb of the primary intestinal loop become?
Cranial: distal duodenum, jejunum, proximal ileum
Caudal: distal ileum, caecum, appendix, ascending colon, proximal 2/3 transverse colon
