Development of GI Tract Flashcards
What is the significance of folic acid during development?
Folic acid helps fusion and closure of spinal cord
When are the primary germ layers formed?
The primary germ layers are formed during the process of gastrulation
How long after does an embryo implant into the uterine wall?
At the beginning of the third week, the embryo has implanted into the uterine wall
Describe the structure of an embryo
The embryo is a flat disc, comprised of two cell layers
Epiblast
Hypoblast
Explain how the embryo initially begins to develop
Epiblast cells in the midline of the embryo begin to ingress, starting from the caudal end
Visible as the primary streak
What do the ingressing cells differentiate into?
Ingressing cells differentiate into Mesoderm
Surrounding muscles, connective tissue & mesenteries and blood vessels
What does the epiblast give rise to?
Epiblast gives rise to Ectoderm (neural crest)
Innervation of the gut (enteric NS), skeletal muscles
What does the hypoblast and epiblast develop into?
Hypoblast (and epiblast) gives rise to Endoderm
Epithelium of the gut tube and glands
When does gastrulation occur?
Gastrulation generates the three primary germ layers within the 3rd week
How is the gut tube formed?
The gut tube is formed by folding of sheets of cells in two directions
Explain the folding that occurs to form the gut tube
Folding towards the midline along the cranial-caudal axis
Folding towards the yolk sac at the cranial and caudal ends
What does the cloacal membrane form?
Cloacal membrane - urogenital sinus; anus and genitals
What does the buccopharyngeal membrane form?
Buccopharyngeal membrane - head and neck
What is the septum transversum?
Septum transversum is the area where the diaphragm forms
What is the mesentery?
Mesentery = folded membrane that anchors on posterior axis (peritoneum)
Where is the GI tract anchored to/?
GI tract is anchored to the posterior abdominal wall
Describe the initial structure of the embryo
The embryo is initially a solid flat disc attached to the hemispherical yolk sac (& similarly to amnion)
How is part of the yolk sac cavity enclosed in the embryo?
Part of the yolk sac cavity is enclosed within the embryo by pinching-off the yolk sac to form a yolk stalk and balloon like yolk sac
How does the gut tube link from the mouth to the anus in the embryo?
Within the embryo, the cranial and caudal intestinal portals extend the tube towards the mouth and anus, delimited by the procordal and cloacal plates
What is the primary gut tube composed of?
Sheet of endoderm which makes the epithelia and glands
Surrounding mesoderm, which makes muscle and connective tissue (including mesentery)
What is the foregut composed of?
Pharynx Oesophagus Stomach Cranial half of duodenum Ampulla of Vater (joining of common bile duct and pancreatic duct)
What is the midgut made up of?
Caudal duodenum (from duodenal papilla) Jejunum Ileum Caecum Appendix Ascending colon Proximal 2/3 of transverse colon
What composes the hindgut?
Distal 1/3 of transverse colon
Descending Colon
Rectum
Describe the atrial blood supply of the gut
Inferior mesenteric artery - hind gut
Superior mesenteric artery - midgut
Celiac artery - foregut & abdominal organs
How well perfused is the gut?
Gut surrounded by plexus of blood vessels, joining vitelline vessels to aorta
What is the significance of the plexus of arteries around the gut?
Plexus resolves to form the arteries that supply the GI tract from the aorta
These define the boundaries of the gut
Describe the arterial blood supply of the GI tract
~ 5 arterial branches to the thoracic oesophagus (from the descending aorta)
Celiac artery to the foregut
Superior mesenteric artery to the midgut
Inferior mesenteric artery to the hindgut
Where do sympathetic ganglia form?
Sympathetic ganglia develop next to major branches of the aorta
Where do the postganglionic sympathetic axons innervate?
Postganglionic sympathetic axons innervate the same tissues that the arteries supply with blood
Explain the sympathetic innervation of the gut
Celiac ganglion - foregut
Superior mesenteric ganglion - midgut
Inferior mesenteric ganglion - hindgut
How does the stomach develop from the gut tube?
Stomach arises by expansion and rotation
During week 4 at the level where the stomach will form the tube begins to dilate, forming an enlarged lumen
Explain the development of the stomach from the gut tube
Initially concave ventral, convex dorsal
90°turn about craniocaudal axis
The dorsal border grows more rapidly than ventral, which establishes the greater curvature of the stomach
What is the sciatic nerve?
Sciatic Nerve - largest nerve in body, supplies bum
How is the stomach held in place?
Dorsal wall of stomach attached to body by mesentery; the dorsal mesogastrium (will form greater omentum)
Ventral wall attached by ventral mesentery, which includes the liver (will form lesser omentum)
What is the significance of the vagus nerve in the GI tract?
Vagus nerve help supply intricate parts of GI tract
What is the role of the peritoneum?
Peritoneum membrane protects
Mesentery attached to stomach liver and abdominal organs
What is the falciform ligament?
Falciform ligament - thin white fibro structure attached to abdominal wall to anchor organs to the front and back
What is the mesogastrium?
the middle region of the abdomen between the epigastrium and the hypogastrium
Explain how the mesogastrium forms the greater omentum
As the stomach rotates, the dorsal mesogastrium is drawn with it
The mesogastrium encloses a space, the omental bursa (lesser arc)
The folded mesogastrium grows to form the greater omentum from the greater curvature, the folds fusing to obliterate the bursa
How is the mature gut folded in place?
Fusion of the mesenteries with the posterior abdominal wall fixes the mature gut in plac
Where are retroperitoneal structures of the GI tract located?
Retroperitoneal structures are sitting on the abdominal wall
What is the role of the Cystic Artery?
Cystic artery allows communication between organs behind the liver
Intraperitoneal
What is pyloric stenosis?
An abnormality of foregut development - pyloric stenosis (narrowing)
Gastric outlet obstruction caused by smooth muscle hypertrophy
What is the incidence of occurence of pyloric stenosis?
3 in 1000 incidence
What are the symptoms of pyloric stenosis?
Projectile vomiting shortly after feeding (not bile stained)
Pyloric channel elongation ‘railroad track’
L >16mm, wall > 4mm, diameter > 14mm
How does the liver bud from the foregut?
Inducing signal:
Heart to ventral gut endoderm
Hepatic diverticulum grows into mesenchyme of septum transversum, expanding into ventral mesentery
How is a blood supply and drainage system formed around the liver?
Cords of hepatic endoderm, bile drainage ducts, and blood vessels proliferate arranged as sinusoids
How does the liver move to its position within the body?
Liver exceeds size of septum transversum (diaphragm area), expands into ventral mesentery
What does the remaining ventral mesentery give rise to?
Remaining ventral mesentery gives rise to:
Falciform ligament between liver and body wall
Lesser omentum between liver and stomach
What is the liver diverticulum?
The hepatic diverticulum (or liver bud) is a primordial cellular extension of the embryonic foregut endoderm that gives rise to the parenchyma of the liver
How does the liver diverticulum form?
The liver diverticulum arises as a bud from the most caudal portion of the foregut
What are the 2 Pancreatic buds?
- Dorsal from duodenal endoderm (induced by
notochord) - Ventral from hepatic diverticulum (induced by hepatic
mesoderm)
How do the pancreatic buds form the pancreas?
As duodenum rotates, ventral and dorsal buds meet and fuse
What is an annular pancreas?
Rare condition where the second part of duodenum is surrounded by a ring of pancreatic tissue continuous with pancreatic head
This portion of pancreas can constrict duodenum and block/impair flow of food to rest of intestines
How does an annular pancreas form?
If ventral bud bifid (bi-lobed), and one rotates around duodenum, annular pancreas forms, which can obstruct duodenum
What kind of gland is the pancreas?
Pancreas is an endocrine and exocrine gland
How are the intestines (midgut) attached to the body?
Attached throughout length by dorsal mesentery (but not ventral mesentery)
What causes the folding of the gut?
The mesentery and gut grow at different rates, leading to stereotypical folding of the gut
Describe the blood supply to the intestines
A ventral branch of the aorta supplies the midgut: superior mesenteric artery (SMA)
Explain the development of the intestines around the superior mesenteric artery
With a very rapid increase in length, the intestines rotate around SMA
Abdomen is too small to accomodate, so herniates into umbilical stalk at 6/7 weeks
By 10 weeks, abdomen is bigger, and intestines return
Describe the midgut development
- 90 degree rotation of midgut
- Elongates into yolk sac and comes out
- Does another 180 rotation
- Starts to form large bowel as it comes back in around
10-12th week (small intestine + main parts of large
intestine forming)
List some abnormalities of intetsinal (midgut) development
- Umbilical hernia
- Omphalocele
- Gastroschisis
- Persistance of yolk duct
- Meckel’s Diverticulum
What is an umbilical hernia?
Intestines return normally, but rectus abdominis fails to fuse around umbilicus: gut covered in skin
What is omphalocele?
Failure of intestinal loops to return into abdomen
Hernia covered in amion (causes unknown, but associated with maternal obesity, alcohol / tobacco, SSRI use)
What is gastroschisis
Failure of ventral body wall to fuse: no covering Increasing incidence (1 in 3000). Marked association with young maternal age, low maternal BMI, recreational drugs (especially cocaine)
Describe features of a persistent yolk duct
Most common intestinal abnormality
Yolk duct attached to ileum, near ileocecal junction - apex of midgut loop (about 2 ft from ileocecal junction)
What is meckel’s diverticulum?
Out-pouching / bulge in lower part of small intestine
Bulge is congenital and is a leftover of umbilical cord
Describe the features of meckel’s diverticulum
(2-4% population) usually asymptomatic
Can get inflamed (clinically indistinguishable from acute appendicitis - pain)
May contain ectopic gastric cells; ulceration + lower GI bleeding
Can be connected to umbilicus by ligament
- Gut rotation causes volvulus
- Can form umbilical fistula
What is Hirschsprung’s disease?
Aganglionic megacolon - not able to pass + digest food due to lack of neural cells
Primarily affects hindgut
Dilatation of sections of colon, with lack of tone + peristalsis, leading to profound constipation
Absence of parasympathetic ganglia
What causes Hirschsprung’s disease?
Caused by lack of neural crest cells
Ganglia present in dilated/hypertrophic region
Aganglionic segment shows contraction
What is the significance of neural crest cells?
Neural crest cells populate the developing gut and give rise to enteric ganglia - parasympathetic supply
What gives rise to the enteric ganglia?
A subpopulation of the neural crest cells which migrate through the embryo from the lateral edges of the neural plate gives rise to the enteric ganglia
Describe the location of different neural crest cells in the gut
Neural crest cells from the occipitocervical (vagal) region populate the entire gut
Neural crest cells from the sacral region populate the distal gut
What are the consequences of failure of enteric gut development ?
Hirschsprung’s disease results from failure of development of the enteric ganglia
normally :
Neural crest cells give rise to enteric ganglia
Normal rectal innervation is inhibitory
What are the effects of Hirschsprung’s disease?
Failure of neural crest cells to migrate to the correct location leads to absence of ganglion cells
Absence of inhibitory innervation results in tonic contraction and colonic obstruction
Explain the significance of the cloaca
The cloaca is the transient common end of digestive and urogenital systems, including the base of the allantois (urogenital sinus)
Describe the development of the anus
Covered by cloacal (proctodeal) membrane over ectoderm depression, the proctodeum
Split by the urorectal septum
Gives rise to urogenital membrane and anal membrane (perforate at 7-8 weeks)
What is an imperforate anus?
defect that is present from birth (congenital)
Describe the effect of an imperforate anus
Imperforate anus can be:
- Persistence of anal membrane
- Atresia of anal canal, rectum or both
What does the cloaca give rise to?
Cloaca gives rise to the rectum and urogenital sinus
